0.1.1 | 0.2.0 | 0.3.0 | 0.4.0 | 0.5.0 | master

# Index

- Introduction
- Hello World
- Comments
  - Doc comments
- Values
  - Primitive Types
  - Primitive Values
  - String Literals and Character Literals
    - Escape Sequences
    - Multiline String Literals
  - Assignment
    - undefined
- Variables
  - Global Variables
  - Thread Local Variables
  - Local Variables
- Integers
  - Integer Literals
  - Runtime Integer Values
- Floats
  - Float Literals
  - Floating Point Operations
- Operators
  - Table of Operators
  - Precedence
- Arrays
  - Anonymous List Literals
  - Multidimensional Arrays
  - Sentinel-Terminated Arrays
- Vectors
  - SIMD
- Pointers
  - volatile
  - Alignment
  - allowzero
  - Sentinel-Terminated Pointers
- Slices
  - Sentinel-Terminated Slices
- struct
  - Default Field Values
  - extern struct
  - packed struct
  - Struct Naming
  - Anonymous Struct Literals
- enum
  - extern enum
  - packed enum
  - Enum Literals
  - Non-exhaustive enum
- union
  - Tagged union
  - extern union
  - packed union
  - Anonymous Union Literals
- blocks
  - Shadowing
- switch
  - Exhaustive Switching
  - Switching with Enum Literals
- while
  - Labeled while
  - while with Optionals
  - while with Error Unions
  - inline while
- for
  - Labeled for
  - inline for
- if
- defer
- unreachable
  - Basics
  - At Compile-Time
- noreturn
- Functions
  - Pass-by-value Parameters
  - Function Parameter Type Inference
  - Function Reflection
- Errors
  - Error Set Type
    - The Global Error Set
  - Error Union Type
    - catch
    - try
    - errdefer
    - Merging Error Sets
    - Inferred Error Sets
  - Error Return Traces
    - Implementation Details
- Optionals
  - Optional Type
  - null
  - Optional Pointers
- Casting
  - Type Coercion
    - Type Coercion: Stricter Qualification
    - Type Coercion: Integer and Float Widening
    - Type Coercion: Arrays and Pointers
    - Type Coercion: Optionals
    - Type Coercion: Error Unions
    - Type Coercion: Compile-Time Known Numbers
    - Type Coercion: unions and enums
    - Type Coercion: Zero Bit Types
    - Type Coercion: undefined
  - Explicit Casts
  - Peer Type Resolution
- Zero Bit Types
  - void
  - Pointers to Zero Bit Types
- Result Location Semantics
- usingnamespace
- comptime
  - Introducing the Compile-Time Concept
    - Compile-Time Parameters
    - Compile-Time Variables
    - Compile-Time Expressions
  - Generic Data Structures
  - Case Study: printf in Zig
- Assembly
  - Output Constraints
  - Input Constraints
  - Clobbers
  - Global Assembly
- Atomics
- Async Functions
  - Suspend and Resume
    - Resuming from Suspend Blocks
  - Async and Await
  - Async Function Example
- Builtin Functions
  - @addWithOverflow
  - @alignCast
  - @alignOf
  - @as
  - @asyncCall
  - @atomicLoad
  - @atomicRmw
  - @atomicStore
  - @bitCast
  - @bitOffsetOf
  - @boolToInt
  - @bitSizeOf
  - @breakpoint
  - @mulAdd
  - @byteSwap
  - @bitReverse
  - @byteOffsetOf
  - @call
  - @cDefine
  - @cImport
  - @cInclude
  - @clz
  - @cmpxchgStrong
  - @cmpxchgWeak
  - @compileError
  - @compileLog
  - @ctz
  - @cUndef
  - @divExact
  - @divFloor
  - @divTrunc
  - @embedFile
  - @enumToInt
  - @errorName
  - @errorReturnTrace
  - @errorToInt
  - @errSetCast
  - @export
  - @fence
  - @field
  - @fieldParentPtr
  - @floatCast
  - @floatToInt
  - @frame
  - @Frame
  - @frameAddress
  - @frameSize
  - @hasDecl
  - @hasField
  - @import
  - @intCast
  - @intToEnum
  - @intToError
  - @intToFloat
  - @intToPtr
  - @memcpy
  - @memset
  - @mod
  - @mulWithOverflow
  - @OpaqueType
  - @panic
  - @popCount
  - @ptrCast
  - @ptrToInt
  - @rem
  - @returnAddress
  - @setAlignStack
  - @setCold
  - @setEvalBranchQuota
  - @setFloatMode
  - @setRuntimeSafety
  - @shlExact
  - @shlWithOverflow
  - @shrExact
  - @shuffle
  - @sizeOf
  - @splat
  - @sqrt
  - @sin
  - @cos
  - @exp
  - @exp2
  - @log
  - @log2
  - @log10
  - @fabs
  - @floor
  - @ceil
  - @trunc
  - @round
  - @subWithOverflow
  - @tagName
  - @TagType
  - @This
  - @truncate
  - @Type
  - @typeInfo
  - @typeName
  - @TypeOf
  - @unionInit
  - @Vector
- Build Mode
  - Debug
  - ReleaseFast
  - ReleaseSafe
  - ReleaseSmall
- Single Threaded Builds
- Undefined Behavior
  - Reaching Unreachable Code
  - Index out of Bounds
  - Cast Negative Number to Unsigned Integer
  - Cast Truncates Data
  - Integer Overflow
    - Default Operations
    - Standard Library Math Functions
    - Builtin Overflow Functions
    - Wrapping Operations
  - Exact Left Shift Overflow
  - Exact Right Shift Overflow
  - Division by Zero
  - Remainder Division by Zero
  - Exact Division Remainder
  - Attempt to Unwrap Null
  - Attempt to Unwrap Error
  - Invalid Error Code
  - Invalid Enum Cast
  - Invalid Error Set Cast
  - Incorrect Pointer Alignment
  - Wrong Union Field Access
  - Out of Bounds Float to Integer Cast
  - Pointer Cast Invalid Null
- Memory
  - Choosing an Allocator
  - Where are the bytes?
  - Implementing an Allocator
  - Heap Allocation Failure
  - Recursion
  - Lifetime and Ownership
- Compile Variables
- Root Source File
- Zig Test
- Zig Build System
- C
  - C Type Primitives
  - Import from C Header File
  - C Pointers
  - Exporting a C Library
  - Mixing Object Files
- WebAssembly
  - Freestanding
  - WASI
- Targets
- Style Guide
  - Whitespace
  - Names
  - Examples
- Source Encoding
- Keyword Reference
  - Keyword: pub
- Grammar
- Zen

# Introduction §

Zig is a general-purpose programming language and toolchain for maintaining robust, optimal, and reusable software.

- Robust - behavior is correct even for edge cases such as out of memory.
- Optimal - write programs the best way they can behave and perform.
- Reusable - the same code works in many environments which have different constraints.
- Maintainable - precisely communicate intent to the compiler and other programmers. The language imposes a low overhead to reading code and is resilient to changing requirements and environments.

Often the most efficient way to learn something new is to see examples, so this documentation shows how to use each of Zig's features. It is all on one page so you can search with your browser's search tool.

If you search for something specific in this documentation and do not find it, please file an issue or say something on IRC.

The code samples in this document are compiled and tested as part of the main test suite of Zig. This HTML document depends on no external files, so you can use it offline.

Where is the documentation for the Zig standard library?

# Hello World §

hello.zig

    const std = @import("std");

    pub fn main() !void {
        const stdout = std.io.getStdOut().outStream();
        try stdout.print("Hello, {}!\n", .{"world"});
    }

    $ zig build-exe hello.zig
    $ ./hello
    Hello, world!

Usually you don't want to write to stdout. You want to write to stderr. And you don't care if it fails. It's more like a warning message that you want to emit. For that you can use a simpler API:

hello.zig

    const warn = @import("std").debug.warn;

    pub fn main() void {
        warn("Hello, world!\n", .{});
    }

    $ zig build-exe hello.zig
    $ ./hello
    Hello, world!

Note that you can leave off the `!` from the return type because `warn` cannot fail.

See also:

- Values
- @import
- Errors
- Root Source File

# Comments §

comments.zig

    const assert = @import("std").debug.assert;

    test "comments" {
        // Comments in Zig start with "//" and end at the next LF byte (end of line).
        // The below line is a comment, and won't be executed.

        //assert(false);

        const x = true;  // another comment
        assert(x);
    }

    $ zig test comments.zig
    1/1 test "comments"...OK
    All 1 tests passed.

There are no multiline comments in Zig (e.g. like `/* */` comments in C). This helps allow Zig to have the property that each line of code can be tokenized out of context.

## Doc comments §

A doc comment is one that begins with exactly three slashes (i.e. `///` but not `////`); multiple doc comments in a row are merged together to form a multiline doc comment. The doc comment documents whatever immediately follows it.

    /// A structure for storing a timestamp, with nanosecond precision (this is a
    /// multiline doc comment).
    const Timestamp = struct {
        /// The number of seconds since the epoch (this is also a doc comment).
        seconds: i64,  // signed so we can represent pre-1970 (not a doc comment)
        /// The number of nanoseconds past the second (doc comment again).
        nanos: u32,

        /// Returns a `Timestamp` struct representing the Unix epoch; that is, the
        /// moment of 1970 Jan 1 00:00:00 UTC (this is a doc comment too).
        pub fn unixEpoch() Timestamp {
            return Timestamp{
                .seconds = 0,
                .nanos = 0,
            };
        }
    };

Doc comments are only allowed in certain places; eventually, it will become a compile error to have a doc comment in an unexpected place, such as in the middle of an expression, or just before a non-doc comment.

# Values §

values.zig

    // Top-level declarations are order-independent:
    const warn = std.debug.warn;
    const std = @import("std");
    const os = std.os;
    const assert = std.debug.assert;

    pub fn main() void {
        // integers
        const one_plus_one: i32 = 1 + 1;
        warn("1 + 1 = {}\n", .{one_plus_one});

        // floats
        const seven_div_three: f32 = 7.0 / 3.0;
        warn("7.0 / 3.0 = {}\n", .{seven_div_three});

        // boolean
        warn("{}\n{}\n{}\n", .{
            true and false,
            true or false,
            !true,
        });

        // optional
        var optional_value: ?[]const u8 = null;
        assert(optional_value == null);

        warn("\noptional 1\ntype: {}\nvalue: {}\n", .{
            @typeName(@TypeOf(optional_value)),
            optional_value,
        });

        optional_value = "hi";
        assert(optional_value != null);

        warn("\noptional 2\ntype: {}\nvalue: {}\n", .{
            @typeName(@TypeOf(optional_value)),
            optional_value,
        });

        // error union
        var number_or_error: anyerror!i32 = error.ArgNotFound;

        warn("\nerror union 1\ntype: {}\nvalue: {}\n", .{
            @typeName(@TypeOf(number_or_error)),
            number_or_error,
        });

        number_or_error = 1234;

        warn("\nerror union 2\ntype: {}\nvalue: {}\n", .{
            @typeName(@TypeOf(number_or_error)),
            number_or_error,
        });
    }

    $ zig build-exe values.zig
    $ ./values
    1 + 1 = 2
    7.0 / 3.0 = 2.33333325e+00
    false
    true
    false

    optional 1
    type: ?[]const u8
    value: null

    optional 2
    type: ?[]const u8
    value: hi

    error union 1
    type: anyerror!i32
    value: error.ArgNotFound

    error union 2
    type: anyerror!i32
    value: 1234

## Primitive Types §

Name | C Equivalent | Description
--- | --- | ---
`i8` | `int8_t` | signed 8-bit integer
`u8` | `uint8_t` | unsigned 8-bit integer
`i16` | `int16_t` | signed 16-bit integer
`u16` | `uint16_t` | unsigned 16-bit integer
`i32` | `int32_t` | signed 32-bit integer
`u32` | `uint32_t` | unsigned 32-bit integer
`i64` | `int64_t` | signed 64-bit integer
`u64` | `uint64_t` | unsigned 64-bit integer
`i128` | `__int128` | signed 128-bit integer
`u128` | `unsigned __int128` | unsigned 128-bit integer
`isize` | `intptr_t` | signed pointer sized integer
`usize` | `uintptr_t` | unsigned pointer sized integer
`c_short` | `short` | for ABI compatibility with C
`c_ushort` | `unsigned short` | for ABI compatibility with C
`c_int` | `int` | for ABI compatibility with C
`c_uint` | `unsigned int` | for ABI compatibility with C
`c_long` | `long` | for ABI compatibility with C
`c_ulong` | `unsigned long` | for ABI compatibility with C
`c_longlong` | `long long` | for ABI compatibility with C
`c_ulonglong` | `unsigned long long` | for ABI compatibility with C
`c_longdouble` | `long double` | for ABI compatibility with C
`c_void` | `void` | for ABI compatibility with C
`f16` | `_Float16` | 16-bit floating point (10-bit mantissa) IEEE-754-2008 binary16
`f32` | `float` | 32-bit floating point (23-bit mantissa) IEEE-754-2008 binary32
`f64` | `double` | 64-bit floating point (52-bit mantissa) IEEE-754-2008 binary64
`f128` | `_Float128` | 128-bit floating point (112-bit mantissa) IEEE-754-2008 binary128
`bool` | `bool` | `true` or `false`
`void` | (none) | 0 bit type
`noreturn` | (none) | the type of `break`, `continue`, `return`, `unreachable`, and `while (true) {}`
`type` | (none) | the type of types
`anyerror` | (none) | an error code
`comptime_int` | (none) | Only allowed for comptime-known values. The type of integer literals.
`comptime_float` | (none) | Only allowed for comptime-known values. The type of float literals.

In addition to the integer types above, arbitrary bit-width integers can be referenced by using an identifier of `i` or `u` followed by digits. For example, the identifier `i7` refers to a signed 7-bit integer. The maximum allowed bit-width of an integer type is `65535`.

See also:

- Integers
- Floats
- void
- Errors
- @Type

## Primitive Values §

Name | Description
--- | ---
`true` and `false` | `bool` values
`null` | used to set an optional type to `null`
`undefined` | used to leave a value unspecified

See also:

- Optionals
- undefined

## String Literals and Character Literals §

String literals are single-item constant Pointers to null-terminated UTF-8 encoded byte arrays. The type of string literals encodes both the length, and the fact that they are null-terminated, and thus they can be coerced to both Slices and Null-Terminated Pointers. Dereferencing string literals converts them to Arrays.

Character literals have type `comptime_int`, the same as Integer Literals. All Escape Sequences are valid in both string literals and character literals.

test.zig

    const assert = @import("std").debug.assert;
    const mem = @import("std").mem;

    test "string literals" {
        const bytes = "hello";
        assert(@TypeOf(bytes) == *const [5:0]u8);
        assert(bytes.len == 5);
        assert(bytes[1] == 'e');
        assert(bytes[5] == 0);
        assert('e' == '\x65');
        assert('\u{1f4a9}' == 128169);
        assert('💯' == 128175);
        assert(mem.eql(u8, "hello", "h\x65llo"));
    }

    $ zig test test.zig
    1/1 test "string literals"...OK
    All 1 tests passed.

See also:

- Arrays
- Zig Test
- Source Encoding

### Escape Sequences §

Escape Sequence | Name
--- | ---
`\n` | Newline
`\r` | Carriage Return
`\t` | Tab
`\\` | Backslash
`\'` | Single Quote
`\"` | Double Quote
`\xNN` | hexadecimal 8-bit character code (2 digits)
`\u{NNNNNN}` | hexadecimal Unicode character code UTF-8 encoded (1 or more digits)

Note that the maximum valid Unicode point is `0x10ffff`.

### Multiline String Literals §

Multiline string literals have no escapes and can span across multiple lines. To start a multiline string literal, use the `\\` token. Just like a comment, the string literal goes until the end of the line. The end of the line is not included in the string literal. However, if the next line begins with `\\` then a newline is appended and the string literal continues.

    const hello_world_in_c =
        \\#include <stdio.h>
        \\
        \\int main(int argc, char **argv) {
        \\    printf("hello world\n");
        \\    return 0;
        \\}
    ;

See also:

- @embedFile

## Assignment §

Use the `const` keyword to assign a value to an identifier:

test.zig

    const x = 1234;

    fn foo() void {
        // It works at global scope as well as inside functions.
        const y = 5678;

        // Once assigned, an identifier cannot be changed.
        y += 1;
    }

    test "assignment" {
        foo();
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:8:7: error: cannot assign to constant
        y += 1;
          ^

`const` applies to all of the bytes that the identifier immediately addresses. Pointers have their own const-ness.

If you need a variable that you can modify, use the `var` keyword:

test.zig

    const assert = @import("std").debug.assert;

    test "var" {
        var y: i32 = 5678;

        y += 1;

        assert(y == 5679);
    }

    $ zig test test.zig
    1/1 test "var"...OK
    All 1 tests passed.

Variables must be initialized:

test.zig

    test "initialization" {
        var x: i32;

        x = 1;
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:2:5: error: variables must be initialized
        var x: i32;
        ^

### undefined §

Use `undefined` to leave variables uninitialized:

test.zig

    const assert = @import("std").debug.assert;

    test "init with undefined" {
        var x: i32 = undefined;
        x = 1;
        assert(x == 1);
    }

    $ zig test test.zig
    1/1 test "init with undefined"...OK
    All 1 tests passed.

`undefined` can be coerced to any type. Once this happens, it is no longer possible to detect that the value is `undefined`. `undefined` means the value could be anything, even something that is nonsense according to the type. Translated into English, `undefined` means "Not a meaningful value. Using this value would be a bug. The value will be unused, or overwritten before being used."

In Debug mode, Zig writes `0xaa` bytes to undefined memory. This is to catch bugs early, and to help detect use of undefined memory in a debugger.

# Variables §

A variable is a unit of Memory storage.

Variables are never allowed to shadow identifiers from an outer scope.

It is generally preferable to use `const` rather than `var` when declaring a variable. This causes less work for both humans and computers to do when reading code, and creates more optimization opportunities.

## Global Variables §

Global variables are considered to be a top level declaration, which means that they are order-independent and lazily analyzed. The initialization value of global variables is implicitly comptime. If a global variable is `const` then its value is `comptime`-known, otherwise it is runtime-known.

global_variables.zig

    var y: i32 = add(10, x);
    const x: i32 = add(12, 34);

    test "global variables" {
        assert(x == 46);
        assert(y == 56);
    }

    fn add(a: i32, b: i32) i32 {
        return a + b;
    }

    const std = @import("std");
    const assert = std.debug.assert;

    $ zig test global_variables.zig
    1/1 test "global variables"...OK
    All 1 tests passed.

Global variables may be declared inside a struct, union, or enum:

namespaced_global.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "namespaced global variable" {
        assert(foo() == 1235);
        assert(foo() == 1236);
    }

    fn foo() i32 {
        const S = struct {
            var x: i32 = 1234;
        };
        S.x += 1;
        return S.x;
    }

    $ zig test namespaced_global.zig
    1/1 test "namespaced global variable"...OK
    All 1 tests passed.

The `extern` keyword can be used to link against a variable that is exported from another object. The `export` keyword or @export builtin function can be used to make a variable available to other objects at link time. In both cases, the type of the variable must be C ABI compatible.

See also:

- Exporting a C Library

## Thread Local Variables §

A variable may be specified to be a thread-local variable using the `threadlocal` keyword:

tls.zig

    const std = @import("std");
    const assert = std.debug.assert;

    threadlocal var x: i32 = 1234;

    test "thread local storage" {
        const thread1 = try std.Thread.spawn({}, testTls);
        const thread2 = try std.Thread.spawn({}, testTls);
        testTls({});
        thread1.wait();
        thread2.wait();
    }

    fn testTls(context: void) void {
        assert(x == 1234);
        x += 1;
        assert(x == 1235);
    }

    $ zig test tls.zig
    1/1 test "thread local storage"...OK
    All 1 tests passed.

For Single Threaded Builds, all thread local variables are treated as Global Variables.

Thread local variables may not be `const`.

## Local Variables §

Local variables occur inside Functions, comptime blocks, and @cImport blocks.

When a local variable is `const`, it means that after initialization, the variable's value will not change. If the initialization value of a `const` variable is comptime-known, then the variable is also `comptime`-known.

A local variable may be qualified with the `comptime` keyword. This causes the variable's value to be `comptime`-known, and all loads and stores of the variable to happen during semantic analysis of the program, rather than at runtime. All variables declared in a `comptime` expression are implicitly `comptime` variables.

comptime_vars.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "comptime vars" {
        var x: i32 = 1;
        comptime var y: i32 = 1;

        x += 1;
        y += 1;

        assert(x == 2);
        assert(y == 2);

        if (y != 2) {
            // This compile error never triggers because y is a comptime variable,
            // and so `y != 2` is a comptime value, and this if is statically evaluated.
            @compileError("wrong y value");
        }
    }

    $ zig test comptime_vars.zig
    1/1 test "comptime vars"...OK
    All 1 tests passed.

# Integers §

## Integer Literals §

    const decimal_int = 98222;
    const hex_int = 0xff;
    const another_hex_int = 0xFF;
    const octal_int = 0o755;
    const binary_int = 0b11110000;

    // underscores may be placed between two digits as a visual separator
    const one_billion = 1_000_000_000;
    const binary_mask = 0b1_1111_1111;
    const permissions = 0o7_5_5;
    const big_address = 0xFF80_0000_0000_0000;

## Runtime Integer Values §

Integer literals have no size limitation, and if any undefined behavior occurs, the compiler catches it.

However, once an integer value is no longer known at compile-time, it must have a known size, and is vulnerable to undefined behavior.

    fn divide(a: i32, b: i32) i32 {
        return a / b;
    }

In this function, values `a` and `b` are known only at runtime, and thus this division operation is vulnerable to both Integer Overflow and Division by Zero.

Operators such as `+` and `-` cause undefined behavior on integer overflow. Also available are operations such as `+%` and `-%` which are defined to have wrapping arithmetic on all targets.

Zig supports arbitrary bit-width integers, referenced by using an identifier of `i` or `u` followed by digits. For example, the identifier `i7` refers to a signed 7-bit integer. The maximum allowed bit-width of an integer type is `65535`.

See also:

- Wrapping Operations

# Floats §

Zig has the following floating point types:

- `f16` - IEEE-754-2008 binary16
- `f32` - IEEE-754-2008 binary32
- `f64` - IEEE-754-2008 binary64
- `f128` - IEEE-754-2008 binary128
- `c_longdouble` - matches `long double` for the target C ABI

## Float Literals §

Float literals have type `comptime_float` which is guaranteed to have the same precision and operations of the largest other floating point type, which is `f128`.

Float literals coerce to any floating point type, and to any integer type when there is no fractional component.

    const floating_point = 123.0E+77;
    const another_float = 123.0;
    const yet_another = 123.0e+77;

    const hex_floating_point = 0x103.70p-5;
    const another_hex_float = 0x103.70;
    const yet_another_hex_float = 0x103.70P-5;

    // underscores may be placed between two digits as a visual separator
    const lightspeed = 299_792_458.000_000;
    const nanosecond = 0.000_000_001;
    const more_hex = 0x1234_5678.9ABC_CDEFp-10;

There is no syntax for NaN, infinity, or negative infinity. For these special values, one must use the standard library:

    const std = @import("std");

    const inf = std.math.inf(f32);
    const negative_inf = -std.math.inf(f64);
    const nan = std.math.nan(f128);

## Floating Point Operations §

By default floating point operations use `Strict` mode, but you can switch to `Optimized` mode on a per-block basis:

foo.zig

    const std = @import("std");
    const builtin = std.builtin;
    const big = @as(f64, 1 << 40);

    export fn foo_strict(x: f64) f64 {
        return x + big - big;
    }

    export fn foo_optimized(x: f64) f64 {
        @setFloatMode(.Optimized);
        return x + big - big;
    }

    $ zig build-obj foo.zig --release-fast

For this test we have to separate code into two object files - otherwise the optimizer figures out all the values at compile-time, which operates in strict mode.

float_mode.zig

    const warn = @import("std").debug.warn;

    extern fn foo_strict(x: f64) f64;
    extern fn foo_optimized(x: f64) f64;

    pub fn main() void {
        const x = 0.001;
        warn("optimized = {}\n", .{foo_optimized(x)});
        warn("strict = {}\n", .{foo_strict(x)});
    }

    $ zig build-exe float_mode.zig --object foo.o
    $ ./float_mode
    optimized = 1.0e-03
    strict = 9.765625e-04

See also:

- @setFloatMode
- Division by Zero

# Operators §

There is no operator overloading. When you see an operator in Zig, you know that it is doing something from this table, and nothing else.

## Table of Operators §

Syntax | Relevant Types | Description | Example
--- | --- | --- | ---
    a + b
    a += b
| Integers Floats | Addition. Can cause overflow for integers. Invokes Peer Type Resolution for the operands. See also @addWithOverflow. |
    2 + 5 == 7
    a +% b
    a +%= b
| Integers | Wrapping Addition. Guaranteed to have twos-complement wrapping behavior. Invokes Peer Type Resolution for the operands. See also @addWithOverflow. |
    @as(u32, std.math.maxInt(u32)) +% 1 == 0
    a - b
    a -= b
| Integers Floats | Subtraction. Can cause overflow for integers. Invokes Peer Type Resolution for the operands. See also @subWithOverflow. |
    2 - 5 == -3
    a -% b
    a -%= b
| Integers | Wrapping Subtraction. Guaranteed to have twos-complement wrapping behavior. Invokes Peer Type Resolution for the operands. See also @subWithOverflow. |
    @as(u32, 0) -% 1 == std.math.maxInt(u32)
    -a
| Integers Floats | Negation. Can cause overflow for integers. |
    -1 == 0 - 1
    -%a
| Integers | Wrapping Negation. Guaranteed to have twos-complement wrapping behavior. |
    -%@as(i32, std.math.minInt(i32)) == std.math.minInt(i32)
    a * b
    a *= b
| Integers Floats | Multiplication. Can cause overflow for integers. Invokes Peer Type Resolution for the operands. See also @mulWithOverflow. |
    2 * 5 == 10
    a *% b
    a *%= b
| Integers | Wrapping Multiplication. Guaranteed to have twos-complement wrapping behavior. Invokes Peer Type Resolution for the operands. See also @mulWithOverflow. |
    @as(u8, 200) *% 2 == 144
    a / b
    a /= b
| Integers Floats | Division. Can cause overflow for integers. Can cause Division by Zero for integers. Can cause Division by Zero for floats in FloatMode.Optimized Mode. For non-compile-time-known signed integers, must use @divTrunc, @divFloor, or @divExact instead of `/`. Invokes Peer Type Resolution for the operands. |
    10 / 5 == 2
    a % b
    a %= b
| Integers Floats | Remainder Division. Can cause Division by Zero for integers. Can cause Division by Zero for floats in FloatMode.Optimized Mode. For non-compile-time-known signed integers, must use @rem or @mod instead of `%`. Invokes Peer Type Resolution for the operands. |
    10 % 3 == 1
    a << b
    a <<= b
| Integers | Bit Shift Left. `b` must be comptime-known or have a type with log2 number of bits as `a`. See also @shlExact. See also @shlWithOverflow. |
    1 << 8 == 256
    a >> b
    a >>= b
| Integers | Bit Shift Right. `b` must be comptime-known or have a type with log2 number of bits as `a`. See also @shrExact. |
    10 >> 1 == 5
    a & b
    a &= b
| Integers | Bitwise AND. Invokes Peer Type Resolution for the operands. |
    0b011 & 0b101 == 0b001
    a | b
    a |= b
| Integers | Bitwise OR. Invokes Peer Type Resolution for the operands. |
    0b010 | 0b100 == 0b110
    a ^ b
    a ^= b
| Integers | Bitwise XOR. Invokes Peer Type Resolution for the operands. |
    0b011 ^ 0b101 == 0b110
    ~a
| Integers | Bitwise NOT. |
    ~@as(u8, 0b10101111) == 0b01010000
    a orelse b
| Optionals | If `a` is `null`, returns `b` ("default value"), otherwise returns the unwrapped value of `a`. Note that `b` may be a value of type noreturn. |
    const value: ?u32 = null;
    const unwrapped = value orelse 1234;
    unwrapped == 1234
    a.?
| Optionals | Equivalent to:
    a orelse unreachable
    const value: ?u32 = 5678;
    value.? == 5678
    a catch b
    a catch |err| b
| Error Unions | If `a` is an `error`, returns `b` ("default value"), otherwise returns the unwrapped value of `a`. Note that `b` may be a value of type noreturn. `err` is the `error` and is in scope of the expression `b`. |
    const value: anyerror!u32 = error.Broken;
    const unwrapped = value catch 1234;
    unwrapped == 1234
    a and b
| bool | If `a` is `false`, returns `false` without evaluating `b`. Otherwise, returns `b`. |
    (false and true) == false
    a or b
| bool | If `a` is `true`, returns `true` without evaluating `b`. Otherwise, returns `b`. |
    false or true == true
    !a
| bool | Boolean NOT. |
    !false == true
    a == b
| Integers Floats bool type | Returns `true` if a and b are equal, otherwise returns `false`. Invokes Peer Type Resolution for the operands. |
    (1 == 1) == true
    a == null
| Optionals | Returns `true` if a is `null`, otherwise returns `false`. |
    const value: ?u32 = null;
    value == null
    a != b
| Integers Floats bool type | Returns `false` if a and b are equal, otherwise returns `true`. Invokes Peer Type Resolution for the operands. |
    (1 != 1) == false
    a > b
| Integers Floats | Returns `true` if a is greater than b, otherwise returns `false`. Invokes Peer Type Resolution for the operands. |
    (2 > 1) == true
    a >= b
| Integers Floats | Returns `true` if a is greater than or equal to b, otherwise returns `false`. Invokes Peer Type Resolution for the operands. |
    (2 >= 1) == true
    a < b
| Integers Floats | Returns `true` if a is less than b, otherwise returns `false`. Invokes Peer Type Resolution for the operands. |
    (1 < 2) == true>
    a <= b
| Integers Floats | Returns `true` if a is less than or equal to b, otherwise returns `false`. Invokes Peer Type Resolution for the operands. |
    (1 <= 2) == true
    a ++ b
| Arrays | Array concatenation. Only available when `a` and `b` are compile-time known. |
    const mem = @import("std").mem;
    const array1 = [_]u32{1,2};
    const array2 = [_]u32{3,4};
    const together = array1 ++ array2;
    mem.eql(u32, together, &[_]u32{1,2,3,4})
    a ** b
| Arrays | Array multiplication. Only available when `a` and `b` are compile-time known. |
    const mem = @import("std").mem;
    const pattern = "ab" ** 3;
    mem.eql(u8, pattern, "ababab")
    a.*
| Pointers | Pointer dereference. |
    const x: u32 = 1234;
    const ptr = &x;
    ptr.* == 1234
    &a
| All types | Address of. |
    const x: u32 = 1234;
    const ptr = &x;
    ptr.* == 1234
    a || b
| Error Set Type | Merging Error Sets |
    const A = error{One};
    const B = error{Two};
    (A || B) == error{One, Two}

## Precedence §

    x() x[] x.y
    a!b
    !x -x -%x ~x &x ?x
    x{} x.* x.?
    ! * / % ** *% ||
    + - ++ +% -%
    << >>
    & ^ |
    == != < > <= >=
    and
    or
    orelse catch
    = *= /= %= += -= <<= >>= &= ^= |=

# Arrays §

arrays.zig

    const assert = @import("std").debug.assert;
    const mem = @import("std").mem;

    // array literal
    const message = [_]u8{ 'h', 'e', 'l', 'l', 'o' };

    // get the size of an array
    comptime {
        assert(message.len == 5);
    }

    // A string literal is a pointer to an array literal.
    const same_message = "hello";

    comptime {
        assert(mem.eql(u8, &message, same_message));
    }

    test "iterate over an array" {
        var sum: usize = 0;
        for (message) |byte| {
            sum += byte;
        }
        assert(sum == 'h' + 'e' + 'l' * 2 + 'o');
    }

    // modifiable array
    var some_integers: [100]i32 = undefined;

    test "modify an array" {
        for (some_integers) |*item, i| {
            item.* = @intCast(i32, i);
        }
        assert(some_integers[10] == 10);
        assert(some_integers[99] == 99);
    }

    // array concatenation works if the values are known
    // at compile time
    const part_one = [_]i32{ 1, 2, 3, 4 };
    const part_two = [_]i32{ 5, 6, 7, 8 };
    const all_of_it = part_one ++ part_two;
    comptime {
        assert(mem.eql(i32, &all_of_it, &[_]i32{ 1, 2, 3, 4, 5, 6, 7, 8 }));
    }

    // remember that string literals are arrays
    const hello = "hello";
    const world = "world";
    const hello_world = hello ++ " " ++ world;
    comptime {
        assert(mem.eql(u8, hello_world, "hello world"));
    }

    // ** does repeating patterns
    const pattern = "ab" ** 3;
    comptime {
        assert(mem.eql(u8, pattern, "ababab"));
    }

    // initialize an array to zero
    const all_zero = [_]u16{0} ** 10;

    comptime {
        assert(all_zero.len == 10);
        assert(all_zero[5] == 0);
    }

    // use compile-time code to initialize an array
    var fancy_array = init: {
        var initial_value: [10]Point = undefined;
        for (initial_value) |*pt, i| {
            pt.* = Point{
                .x = @intCast(i32, i),
                .y = @intCast(i32, i) * 2,
            };
        }
        break :init initial_value;
    };
    const Point = struct {
        x: i32,
        y: i32,
    };

    test "compile-time array initalization" {
        assert(fancy_array[4].x == 4);
        assert(fancy_array[4].y == 8);
    }

    // call a function to initialize an array
    var more_points = [_]Point{makePoint(3)} ** 10;
    fn makePoint(x: i32) Point {
        return Point{
            .x = x,
            .y = x * 2,
        };
    }
    test "array initialization with function calls" {
        assert(more_points[4].x == 3);
        assert(more_points[4].y == 6);
        assert(more_points.len == 10);
    }

    $ zig test arrays.zig
    1/4 test "iterate over an array"...OK
    2/4 test "modify an array"...OK
    3/4 test "compile-time array initalization"...OK
    4/4 test "array initialization with function calls"...OK
    All 4 tests passed.

See also:

- for
- Slices

## Anonymous List Literals §

Similar to Enum Literals and Anonymous Struct Literals the type can be omitted from array literals:

anon_list.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "anonymous list literal syntax" {
        var array: [4]u8 = .{11, 22, 33, 44};
        assert(array[0] == 11);
        assert(array[1] == 22);
        assert(array[2] == 33);
        assert(array[3] == 44);
    }

    $ zig test anon_list.zig
    1/1 test "anonymous list literal syntax"...OK
    All 1 tests passed.

If there is no type in the result location then an anonymous list literal actually turns into a struct with numbered field names:

infer_list_literal.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "fully anonymous list literal" {
        dump(.{ @as(u32, 1234), @as(f64, 12.34), true, "hi"});
    }

    fn dump(args: var) void {
        assert(args.@"0" == 1234);
        assert(args.@"1" == 12.34);
        assert(args.@"2");
        assert(args.@"3"[0] == 'h');
        assert(args.@"3"[1] == 'i');
    }

    $ zig test infer_list_literal.zig
    1/1 test "fully anonymous list literal"...OK
    All 1 tests passed.

## Multidimensional Arrays §

Mutlidimensional arrays can be created by nesting arrays:

multidimensional.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const mat4x4 = [4][4]f32{
        [_]f32{ 1.0, 0.0, 0.0, 0.0 },
        [_]f32{ 0.0, 1.0, 0.0, 1.0 },
        [_]f32{ 0.0, 0.0, 1.0, 0.0 },
        [_]f32{ 0.0, 0.0, 0.0, 1.0 },
    };
    test "multidimensional arrays" {
        // Access the 2D array by indexing the outer array, and then the inner array.
        assert(mat4x4[1][1] == 1.0);

        // Here we iterate with for loops.
        for (mat4x4) |row, row_index| {
            for (row) |cell, column_index| {
                if (row_index == column_index) {
                    assert(cell == 1.0);
                }
            }
        }
    }

    $ zig test multidimensional.zig
    1/1 test "multidimensional arrays"...OK
    All 1 tests passed.

## Sentinel-Terminated Arrays §

The syntax `[N:x]T` describes an array which has a sentinel element of value `x` at the index corresponding to `len`.

null_terminated_array.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "null terminated array" {
        const array = [_:0]u8 {1, 2, 3, 4};

        assert(@TypeOf(array) == [4:0]u8);
        assert(array.len == 4);
        assert(array[4] == 0);
    }

    $ zig test null_terminated_array.zig
    1/1 test "null terminated array"...OK
    All 1 tests passed.

See also:

- Sentinel-Terminated Pointers
- Sentinel-Terminated Slices

# Vectors §

A vector is a group of Integers, Floats, or Pointers which are operated on in parallel using a single instruction (SIMD). Vector types are created with the builtin function @Vector.

TODO talk about C ABI interop

## SIMD §

TODO Zig's SIMD abilities are just beginning to be fleshed out. Here are some talking points to update the docs with: * What kind of operations can you do? All the operations on integers and floats? What about mixing scalar and vector? * How to convert to/from vectors/arrays * How to access individual elements from vectors, how to loop over the elements * "shuffle" * Advice on writing high perf software, how to abstract the best way

# Pointers §

Zig has two kinds of pointers:

- `*T` - pointer to exactly one item.
  - Supports deref syntax: `ptr.*`
- `[*]T` - pointer to unknown number of items.
  - Supports index syntax: `ptr[i]`
  - Supports slice syntax: `ptr[start..end]`
  - Supports pointer arithmetic: `ptr + x`, `ptr - x`
  - `T` must have a known size, which means that it cannot be `c_void` or any other @OpaqueType.

These types are closely related to Arrays and Slices:

- `*[N]T` - pointer to N items, same as single-item pointer to array.
  - Supports index syntax: `array_ptr[i]`
  - Supports slice syntax: `array_ptr[start..end]`
  - Supports len property: `array_ptr.len`

- `[]T` - pointer to runtime-known number of items.
  - Supports index syntax: `slice[i]`
  - Supports slice syntax: `slice[start..end]`
  - Supports len property: `slice.len`

Use `&x` to obtain a single-item pointer:

test.zig

    const assert = @import("std").debug.assert;

    test "address of syntax" {
        // Get the address of a variable:
        const x: i32 = 1234;
        const x_ptr = &x;

        // Dereference a pointer:
        assert(x_ptr.* == 1234);

        // When you get the address of a const variable, you get a const pointer to a single item.
        assert(@TypeOf(x_ptr) == *const i32);

        // If you want to mutate the value, you'd need an address of a mutable variable:
        var y: i32 = 5678;
        const y_ptr = &y;
        assert(@TypeOf(y_ptr) == *i32);
        y_ptr.* += 1;
        assert(y_ptr.* == 5679);
    }

    test "pointer array access" {
        // Taking an address of an individual element gives a
        // pointer to a single item. This kind of pointer
        // does not support pointer arithmetic.
        var array = [_]u8{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
        const ptr = &array[2];
        assert(@TypeOf(ptr) == *u8);

        assert(array[2] == 3);
        ptr.* += 1;
        assert(array[2] == 4);
    }

    $ zig test test.zig
    1/2 test "address of syntax"...OK
    2/2 test "pointer array access"...OK
    All 2 tests passed.

In Zig, we generally prefer Slices rather than Sentinel-Terminated Pointers. You can turn an array or pointer into a slice using slice syntax.

Slices have bounds checking and are therefore protected against this kind of undefined behavior. This is one reason we prefer slices to pointers.

test.zig

    const assert = @import("std").debug.assert;

    test "pointer slicing" {
        var array = [_]u8{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
        const slice = array[2..4];
        assert(slice.len == 2);

        assert(array[3] == 4);
        slice[1] += 1;
        assert(array[3] == 5);
    }

    $ zig test test.zig
    1/1 test "pointer slicing"...OK
    All 1 tests passed.

Pointers work at compile-time too, as long as the code does not depend on an undefined memory layout:

test.zig

    const assert = @import("std").debug.assert;

    test "comptime pointers" {
        comptime {
            var x: i32 = 1;
            const ptr = &x;
            ptr.* += 1;
            x += 1;
            assert(ptr.* == 3);
        }
    }

    $ zig test test.zig
    1/1 test "comptime pointers"...OK
    All 1 tests passed.

To convert an integer address into a pointer, use `@intToPtr`. To convert a pointer to an integer, use `@ptrToInt`:

test.zig

    const assert = @import("std").debug.assert;

    test "@ptrToInt and @intToPtr" {
        const ptr = @intToPtr(*i32, 0xdeadbee0);
        const addr = @ptrToInt(ptr);
        assert(@TypeOf(addr) == usize);
        assert(addr == 0xdeadbee0);
    }

    $ zig test test.zig
    1/1 test "@ptrToInt and @intToPtr"...OK
    All 1 tests passed.

Zig is able to preserve memory addresses in comptime code, as long as the pointer is never dereferenced:

test.zig

    const assert = @import("std").debug.assert;

    test "comptime @intToPtr" {
        comptime {
            // Zig is able to do this at compile-time, as long as
            // ptr is never dereferenced.
            const ptr = @intToPtr(*i32, 0xdeadbee0);
            const addr = @ptrToInt(ptr);
            assert(@TypeOf(addr) == usize);
            assert(addr == 0xdeadbee0);
        }
    }

    $ zig test test.zig
    1/1 test "comptime @intToPtr"...OK
    All 1 tests passed.

See also:

- Optional Pointers
- @intToPtr
- @ptrToInt
- C Pointers
- Pointers to Zero Bit Types

## volatile §

Loads and stores are assumed to not have side effects. If a given load or store should have side effects, such as Memory Mapped Input/Output (MMIO), use `volatile`. In the following code, loads and stores with `mmio_ptr` are guaranteed to all happen and in the same order as in source code:

test.zig

    const assert = @import("std").debug.assert;

    test "volatile" {
        const mmio_ptr = @intToPtr(*volatile u8, 0x12345678);
        assert(@TypeOf(mmio_ptr) == *volatile u8);
    }

    $ zig test test.zig
    1/1 test "volatile"...OK
    All 1 tests passed.

Note that `volatile` is unrelated to concurrency and Atomics. If you see code that is using `volatile` for something other than Memory Mapped Input/Output, it is probably a bug.

To convert one pointer type to another, use @ptrCast. This is an unsafe operation that Zig cannot protect you against. Use `@ptrCast` only when other conversions are not possible.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "pointer casting" {
        const bytes align(@alignOf(u32)) = [_]u8{ 0x12, 0x12, 0x12, 0x12 };
        const u32_ptr = @ptrCast(*const u32, &bytes);
        assert(u32_ptr.* == 0x12121212);

        // Even this example is contrived - there are better ways to do the above than
        // pointer casting. For example, using a slice narrowing cast:
        const u32_value = std.mem.bytesAsSlice(u32, bytes[0..])[0];
        assert(u32_value == 0x12121212);

        // And even another way, the most straightforward way to do it:
        assert(@bitCast(u32, bytes) == 0x12121212);
    }

    test "pointer child type" {
        // pointer types have a `child` field which tells you the type they point to.
        assert((*u32).Child == u32);
    }

    $ zig test test.zig
    1/2 test "pointer casting"...OK
    2/2 test "pointer child type"...OK
    All 2 tests passed.

## Alignment §

Each type has an alignment - a number of bytes such that, when a value of the type is loaded from or stored to memory, the memory address must be evenly divisible by this number. You can use @alignOf to find out this value for any type.

Alignment depends on the CPU architecture, but is always a power of two, and less than `1 << 29`.

In Zig, a pointer type has an alignment value. If the value is equal to the alignment of the underlying type, it can be omitted from the type:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "variable alignment" {
        var x: i32 = 1234;
        const align_of_i32 = @alignOf(@TypeOf(x));
        assert(@TypeOf(&x) == *i32);
        assert(*i32 == *align(align_of_i32) i32);
        if (std.Target.current.cpu.arch == .x86_64) {
            assert((*i32).alignment == 4);
        }
    }

    $ zig test test.zig
    1/1 test "variable alignment"...OK
    All 1 tests passed.

In the same way that a `*i32` can be coerced to a `*const i32`, a pointer with a larger alignment can be implicitly cast to a pointer with a smaller alignment, but not vice versa.

You can specify alignment on variables and functions. If you do this, then pointers to them get the specified alignment:

test.zig

    const assert = @import("std").debug.assert;

    var foo: u8 align(4) = 100;

    test "global variable alignment" {
        assert(@TypeOf(&foo).alignment == 4);
        assert(@TypeOf(&foo) == *align(4) u8);
        const as_pointer_to_array: *[1]u8 = &foo;
        const as_slice: []u8 = as_pointer_to_array;
        assert(@TypeOf(as_slice) == []align(4) u8);
    }

    fn derp() align(@sizeOf(usize) * 2) i32 { return 1234; }
    fn noop1() align(1) void {}
    fn noop4() align(4) void {}

    test "function alignment" {
        assert(derp() == 1234);
        assert(@TypeOf(noop1) == fn() align(1) void);
        assert(@TypeOf(noop4) == fn() align(4) void);
        noop1();
        noop4();
    }

    $ zig test test.zig
    1/2 test "global variable alignment"...OK
    2/2 test "function alignment"...OK
    All 2 tests passed.

If you have a pointer or a slice that has a small alignment, but you know that it actually has a bigger alignment, use @alignCast to change the pointer into a more aligned pointer. This is a no-op at runtime, but inserts a safety check:

test.zig

    const std = @import("std");

    test "pointer alignment safety" {
        var array align(4) = [_]u32{ 0x11111111, 0x11111111 };
        const bytes = std.mem.sliceAsBytes(array[0..]);
        std.debug.assert(foo(bytes) == 0x11111111);
    }
    fn foo(bytes: []u8) u32 {
        const slice4 = bytes[1..5];
        const int_slice = std.mem.bytesAsSlice(u32, @alignCast(4, slice4));
        return int_slice[0];
    }

    $ zig test test.zig
    1/1 test "pointer alignment safety"...incorrect alignment
    /home/andy/dev/zig/docgen_tmp/test.zig:10:63: 0x20544b in foo (test)
        const int_slice = std.mem.bytesAsSlice(u32, @alignCast(4, slice4));
                                                                  ^
    /home/andy/dev/zig/docgen_tmp/test.zig:6:25: 0x204bea in test "pointer alignment safety" (test)
        std.debug.assert(foo(bytes) == 0x11111111);
                            ^
    /home/andy/dev/zig/lib/std/special/test_runner.zig:47:28: 0x22bd0e in std.special.main (test)
            } else test_fn.func();
                               ^
    /home/andy/dev/zig/lib/std/start.zig:253:37: 0x2057ad in std.start.posixCallMainAndExit (test)
                const result = root.main() catch |err| {
                                        ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x2054ef in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^

    Tests failed. Use the following command to reproduce the failure:
    /home/andy/dev/zig/docgen_tmp/test

## allowzero §

This pointer attribute allows a pointer to have address zero. This is only ever needed on the freestanding OS target, where the address zero is mappable. If you want to represent null pointers, use Optional Pointers instead. Optional Pointers with `allowzero` are not the same size as pointers. In this code example, if the pointer did not have the `allowzero` attribute, this would be a Pointer Cast Invalid Null panic:

allowzero.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "allowzero" {
        var zero: usize = 0;
        var ptr = @intToPtr(*allowzero i32, zero);
        assert(@ptrToInt(ptr) == 0);
    }

    $ zig test allowzero.zig
    1/1 test "allowzero"...OK
    All 1 tests passed.

## Sentinel-Terminated Pointers §

The syntax `[*:x]T` describes a pointer that has a length determined by a sentinel value. This provides protection against buffer overflow and overreads.

test.zig

    const std = @import("std");

    // This is also available as `std.c.printf`.
    pub extern "c" fn printf(format: [*:0]const u8, ...) c_int;

    pub fn main() anyerror!void {
        _ = printf("Hello, world!\n"); // OK

        const msg = "Hello, world!\n";
        const non_null_terminated_msg: [msg.len]u8 = msg.*;
        _ = printf(&non_null_terminated_msg);
    }

    $ zig build-exe test.zig -lc
    ./docgen_tmp/test.zig:11:17: error: expected type '[*:0]const u8', found '*const [14]u8'
        _ = printf(&non_null_terminated_msg);
                    ^
    ./docgen_tmp/test.zig:11:17: note: destination pointer requires a terminating '0' sentinel
        _ = printf(&non_null_terminated_msg);
                    ^

See also:

- Sentinel-Terminated Slices
- Sentinel-Terminated Arrays

# Slices §

test.zig

    const assert = @import("std").debug.assert;

    test "basic slices" {
        var array = [_]i32{ 1, 2, 3, 4 };
        // A slice is a pointer and a length. The difference between an array and
        // a slice is that the array's length is part of the type and known at
        // compile-time, whereas the slice's length is known at runtime.
        // Both can be accessed with the `len` field.
        var known_at_runtime_zero: usize = 0;
        const slice = array[known_at_runtime_zero..array.len];
        assert(&slice[0] == &array[0]);
        assert(slice.len == array.len);

        // Using the address-of operator on a slice gives a pointer to a single
        // item, while using the `ptr` field gives an unknown length pointer.
        assert(@TypeOf(slice.ptr) == [*]i32);
        assert(@TypeOf(&slice[0]) == *i32);
        assert(@ptrToInt(slice.ptr) == @ptrToInt(&slice[0]));

        // Slices have array bounds checking. If you try to access something out
        // of bounds, you'll get a safety check failure:
        slice[10] += 1;

        // Note that `slice.ptr` does not invoke safety checking, while `&slice[0]`
        // asserts that the slice has len >= 1.
    }

    $ zig test test.zig
    1/1 test "basic slices"...index out of bounds
    /home/andy/dev/zig/docgen_tmp/test.zig:22:10: 0x204cc6 in test "basic slices" (test)
        slice[10] += 1;
             ^
    /home/andy/dev/zig/lib/std/special/test_runner.zig:47:28: 0x22bc1e in std.special.main (test)
            } else test_fn.func();
                               ^
    /home/andy/dev/zig/lib/std/start.zig:253:37: 0x20574d in std.start.posixCallMainAndExit (test)
                const result = root.main() catch |err| {
                                        ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20548f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^

    Tests failed. Use the following command to reproduce the failure:
    /home/andy/dev/zig/docgen_tmp/test

This is one reason we prefer slices to pointers.

slices.zig

    const std = @import("std");
    const assert = std.debug.assert;
    const mem = std.mem;
    const fmt = std.fmt;

    test "using slices for strings" {
        // Zig has no concept of strings. String literals are const pointers to
        // arrays of u8, and by convention parameters that are "strings" are
        // expected to be UTF-8 encoded slices of u8.
        // Here we coerce [5]u8 to []const u8
        const hello: []const u8 = "hello";
        const world: []const u8 = "世界";

        var all_together: [100]u8 = undefined;
        // You can use slice syntax on an array to convert an array into a slice.
        const all_together_slice = all_together[0..];
        // String concatenation example.
        const hello_world = try fmt.bufPrint(all_together_slice, "{} {}", .{ hello, world });

        // Generally, you can use UTF-8 and not worry about whether something is a
        // string. If you don't need to deal with individual characters, no need
        // to decode.
        assert(mem.eql(u8, hello_world, "hello 世界"));
    }

    test "slice pointer" {
        var array: [10]u8 = undefined;
        const ptr = &array;

        // You can use slicing syntax to convert a pointer into a slice:
        const slice = ptr[0..5];
        slice[2] = 3;
        assert(slice[2] == 3);
        // The slice is mutable because we sliced a mutable pointer.
        // Furthermore, it is actually a pointer to an array, since the start
        // and end indexes were both comptime-known.
        assert(@TypeOf(slice) == *[5]u8);

        // You can also slice a slice:
        const slice2 = slice[2..3];
        assert(slice2.len == 1);
        assert(slice2[0] == 3);
    }

    $ zig test slices.zig
    1/2 test "using slices for strings"...OK
    2/2 test "slice pointer"...OK
    All 2 tests passed.

See also:

- Pointers
- for
- Arrays

## Sentinel-Terminated Slices §

The syntax `[:x]T` is a slice which has a runtime known length and also guarantees a sentinel value at the element indexed by the length. The type does not guarantee that there are no sentinel elements before that. Sentinel-terminated slices allow element access to the `len` index.

null_terminated_slice.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "null terminated slice" {
        const slice: [:0]const u8 = "hello";

        assert(slice.len == 5);
        assert(slice[5] == 0);
    }

    $ zig test null_terminated_slice.zig
    1/1 test "null terminated slice"...OK
    All 1 tests passed.

See also:

- Sentinel-Terminated Pointers
- Sentinel-Terminated Arrays

# struct §

structs.zig

    // Declare a struct.
    // Zig gives no guarantees about the order of fields and the size of
    // the struct but the fields are guaranteed to be ABI-aligned.
    const Point = struct {
        x: f32,
        y: f32,
    };

    // Maybe we want to pass it to OpenGL so we want to be particular about
    // how the bytes are arranged.
    const Point2 = packed struct {
        x: f32,
        y: f32,
    };

    // Declare an instance of a struct.
    const p = Point {
        .x = 0.12,
        .y = 0.34,
    };

    // Maybe we're not ready to fill out some of the fields.
    var p2 = Point {
        .x = 0.12,
        .y = undefined,
    };

    // Structs can have methods
    // Struct methods are not special, they are only namespaced
    // functions that you can call with dot syntax.
    const Vec3 = struct {
        x: f32,
        y: f32,
        z: f32,

        pub fn init(x: f32, y: f32, z: f32) Vec3 {
            return Vec3 {
                .x = x,
                .y = y,
                .z = z,
            };
        }

        pub fn dot(self: Vec3, other: Vec3) f32 {
            return self.x * other.x + self.y * other.y + self.z * other.z;
        }
    };

    const assert = @import("std").debug.assert;
    test "dot product" {
        const v1 = Vec3.init(1.0, 0.0, 0.0);
        const v2 = Vec3.init(0.0, 1.0, 0.0);
        assert(v1.dot(v2) == 0.0);

        // Other than being available to call with dot syntax, struct methods are
        // not special. You can reference them as any other declaration inside
        // the struct:
        assert(Vec3.dot(v1, v2) == 0.0);
    }

    // Structs can have global declarations.
    // Structs can have 0 fields.
    const Empty = struct {
        pub const PI = 3.14;
    };
    test "struct namespaced variable" {
        assert(Empty.PI == 3.14);
        assert(@sizeOf(Empty) == 0);

        // you can still instantiate an empty struct
        const does_nothing = Empty {};
    }

    // struct field order is determined by the compiler for optimal performance.
    // however, you can still calculate a struct base pointer given a field pointer:
    fn setYBasedOnX(x: *f32, y: f32) void {
        const point = @fieldParentPtr(Point, "x", x);
        point.y = y;
    }
    test "field parent pointer" {
        var point = Point {
            .x = 0.1234,
            .y = 0.5678,
        };
        setYBasedOnX(&point.x, 0.9);
        assert(point.y == 0.9);
    }

    // You can return a struct from a function. This is how we do generics
    // in Zig:
    fn LinkedList(comptime T: type) type {
        return struct {
            pub const Node = struct {
                prev: ?*Node,
                next: ?*Node,
                data: T,
            };

            first: ?*Node,
            last:  ?*Node,
            len:   usize,
        };
    }

    test "linked list" {
        // Functions called at compile-time are memoized. This means you can
        // do this:
        assert(LinkedList(i32) == LinkedList(i32));

        var list = LinkedList(i32) {
            .first = null,
            .last = null,
            .len = 0,
        };
        assert(list.len == 0);

        // Since types are first class values you can instantiate the type
        // by assigning it to a variable:
        const ListOfInts = LinkedList(i32);
        assert(ListOfInts == LinkedList(i32));

        var node = ListOfInts.Node {
            .prev = null,
            .next = null,
            .data = 1234,
        };
        var list2 = LinkedList(i32) {
            .first = &node,
            .last = &node,
            .len = 1,
        };
        assert(list2.first.?.data == 1234);
    }

    $ zig test structs.zig
    1/4 test "dot product"...OK
    2/4 test "struct namespaced variable"...OK
    3/4 test "field parent pointer"...OK
    4/4 test "linked list"...OK
    All 4 tests passed.

## Default Field Values §

Each struct field may have an expression indicating the default field value. Such expressions are executed at comptime, and allow the field to be omitted in a struct literal expression:

test.zig

    const Foo = struct {
        a: i32 = 1234,
        b: i32,
    };

    test "default struct initialization fields" {
        const x = Foo{
            .b = 5,
        };
        if (x.a + x.b != 1239) {
            @compileError("it's even comptime known!");
        }
    }

    $ zig test test.zig
    1/1 test "default struct initialization fields"...OK
    All 1 tests passed.

## extern struct §

An `extern struct` has in-memory layout guaranteed to match the C ABI for the target.

This kind of struct should only be used for compatibility with the C ABI. Every other use case should be solved with packed struct or normal struct.

See also:

- extern union
- extern enum

## packed struct §

Unlike normal structs, `packed` structs have guaranteed in-memory layout:

- Fields remain in the order declared.
- There is no padding between fields.
- Zig supports arbitrary width Integers and although normally, integers with fewer than 8 bits will still use 1 byte of memory, in packed structs, they use exactly their bit width.
- `bool` fields use exactly 1 bit.
- A packed enum field uses exactly the bit width of its integer tag type.
- A packed union field uses exactly the bit width of the union field with the largest bit width.
- Non-ABI-aligned fields are packed into the smallest possible ABI-aligned integers in accordance with the target endianness.

This means that a `packed struct` can participate in a @bitCast or a @ptrCast to reinterpret memory. This even works at comptime:

test.zig

    const std = @import("std");
    const builtin = std.builtin;
    const assert = std.debug.assert;

    const Full = packed struct {
        number: u16,
    };
    const Divided = packed struct {
        half1: u8,
        quarter3: u4,
        quarter4: u4,
    };

    test "@bitCast between packed structs" {
        doTheTest();
        comptime doTheTest();
    }

    fn doTheTest() void {
        assert(@sizeOf(Full) == 2);
        assert(@sizeOf(Divided) == 2);
        var full = Full{ .number = 0x1234 };
        var divided = @bitCast(Divided, full);
        switch (builtin.endian) {
            .Big => {
                assert(divided.half1 == 0x12);
                assert(divided.quarter3 == 0x3);
                assert(divided.quarter4 == 0x4);
            },
            .Little => {
                assert(divided.half1 == 0x34);
                assert(divided.quarter3 == 0x2);
                assert(divided.quarter4 == 0x1);
            },
        }
    }

    $ zig test test.zig
    1/1 test "@bitCast between packed structs"...OK
    All 1 tests passed.

Zig allows the address to be taken of a non-byte-aligned field:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const BitField = packed struct {
        a: u3,
        b: u3,
        c: u2,
    };

    var foo = BitField{
        .a = 1,
        .b = 2,
        .c = 3,
    };

    test "pointer to non-byte-aligned field" {
        const ptr = &foo.b;
        assert(ptr.* == 2);
    }

    $ zig test test.zig
    1/1 test "pointer to non-byte-aligned field"...OK
    All 1 tests passed.

However, the pointer to a non-byte-aligned field has special properties and cannot be passed when a normal pointer is expected:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const BitField = packed struct {
        a: u3,
        b: u3,
        c: u2,
    };

    var bit_field = BitField{
        .a = 1,
        .b = 2,
        .c = 3,
    };

    test "pointer to non-bit-aligned field" {
        assert(bar(&bit_field.b) == 2);
    }

    fn bar(x: *const u3) u3 {
        return x.*;
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:17:26: error: expected type '*const u3', found '*align(:3:1) u3'
        assert(bar(&bit_field.b) == 2);
                             ^

In this case, the function `bar` cannot be called becuse the pointer to the non-ABI-aligned field mentions the bit offset, but the function expects an ABI-aligned pointer.

Pointers to non-ABI-aligned fields share the same address as the other fields within their host integer:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const BitField = packed struct {
        a: u3,
        b: u3,
        c: u2,
    };

    var bit_field = BitField{
        .a = 1,
        .b = 2,
        .c = 3,
    };

    test "pointer to non-bit-aligned field" {
        assert(@ptrToInt(&bit_field.a) == @ptrToInt(&bit_field.b));
        assert(@ptrToInt(&bit_field.a) == @ptrToInt(&bit_field.c));
    }

    $ zig test test.zig
    1/1 test "pointer to non-bit-aligned field"...OK
    All 1 tests passed.

This can be observed with @bitOffsetOf and byteOffsetOf:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const BitField = packed struct {
        a: u3,
        b: u3,
        c: u2,
    };

    test "pointer to non-bit-aligned field" {
        comptime {
            assert(@bitOffsetOf(BitField, "a") == 0);
            assert(@bitOffsetOf(BitField, "b") == 3);
            assert(@bitOffsetOf(BitField, "c") == 6);

            assert(@byteOffsetOf(BitField, "a") == 0);
            assert(@byteOffsetOf(BitField, "b") == 0);
            assert(@byteOffsetOf(BitField, "c") == 0);
        }
    }

    $ zig test test.zig
    1/1 test "pointer to non-bit-aligned field"...OK
    All 1 tests passed.

Packed structs have 1-byte alignment. However if you have an overaligned pointer to a packed struct, Zig should correctly understand the alignment of fields. However there is a bug:

test.zig

    const S = packed struct {
        a: u32,
        b: u32,
    };
    test "overaligned pointer to packed struct" {
        var foo: S align(4) = undefined;
        const ptr: *align(4) S = &foo;
        const ptr_to_b: *u32 = &ptr.b;
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:8:32: error: expected type '*u32', found '*align(1) u32'
        const ptr_to_b: *u32 = &ptr.b;
                                   ^

When this bug is fixed, the above test in the documentation will unexpectedly pass, which will cause the test suite to fail, notifying the bug fixer to update these docs.

It's also planned to be able to set alignment of struct fields.

Using packed structs with volatile is problematic, and may be a compile error in the future. For details on this subscribe to this issue. TODO update these docs with a recommendation on how to use packed structs with MMIO (the use case for volatile packed structs) once this issue is resolved. Don't worry, there will be a good solution for this use case in zig.

## Struct Naming §

Since all structs are anonymous, Zig infers the type name based on a few rules.

- If the struct is in the initialization expression of a variable, it gets named after that variable.
- If the struct is in the `return` expression, it gets named after the function it is returning from, with the parameter values serialized.
- Otherwise, the struct gets a name such as `(anonymous struct at file.zig:7:38)`.

struct_name.zig

    const std = @import("std");

    pub fn main() void {
        const Foo = struct {};
        std.debug.warn("variable: {}\n", .{@typeName(Foo)});
        std.debug.warn("anonymous: {}\n", .{@typeName(struct {})});
        std.debug.warn("function: {}\n", .{@typeName(List(i32))});
    }

    fn List(comptime T: type) type {
        return struct {
            x: T,
        };
    }

    $ zig build-exe struct_name.zig
    $ ./struct_name
    variable: Foo
    anonymous: struct:6:51
    function: List(i32)

## Anonymous Struct Literals §

Zig allows omitting the struct type of a literal. When the result is coerced, the struct literal will directly instantiate the result location, with no copy:

struct_result.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const Point = struct {x: i32, y: i32};

    test "anonymous struct literal" {
        var pt: Point = .{
            .x = 13,
            .y = 67,
        };
        assert(pt.x == 13);
        assert(pt.y == 67);
    }

    $ zig test struct_result.zig
    1/1 test "anonymous struct literal"...OK
    All 1 tests passed.

The struct type can be inferred. Here the result location does not include a type, and so Zig infers the type:

struct_anon.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "fully anonymous struct" {
        dump(.{
            .int = @as(u32, 1234),
            .float = @as(f64, 12.34),
            .b = true,
            .s = "hi",
        });
    }

    fn dump(args: var) void {
        assert(args.int == 1234);
        assert(args.float == 12.34);
        assert(args.b);
        assert(args.s[0] == 'h');
        assert(args.s[1] == 'i');
    }

    $ zig test struct_anon.zig
    1/1 test "fully anonymous struct"...OK
    All 1 tests passed.

See also:

- comptime
- @fieldParentPtr

# enum §

enums.zig

    const assert = @import("std").debug.assert;
    const mem = @import("std").mem;

    // Declare an enum.
    const Type = enum {
        Ok,
        NotOk,
    };

    // Declare a specific instance of the enum variant.
    const c = Type.Ok;

    // If you want access to the ordinal value of an enum, you
    // can specify the tag type.
    const Value = enum(u2) {
        Zero,
        One,
        Two,
    };

    // Now you can cast between u2 and Value.
    // The ordinal value starts from 0, counting up for each member.
    test "enum ordinal value" {
        assert(@enumToInt(Value.Zero) == 0);
        assert(@enumToInt(Value.One) == 1);
        assert(@enumToInt(Value.Two) == 2);
    }

    // You can override the ordinal value for an enum.
    const Value2 = enum(u32) {
        Hundred = 100,
        Thousand = 1000,
        Million = 1000000,
    };
    test "set enum ordinal value" {
        assert(@enumToInt(Value2.Hundred) == 100);
        assert(@enumToInt(Value2.Thousand) == 1000);
        assert(@enumToInt(Value2.Million) == 1000000);
    }

    // Enums can have methods, the same as structs and unions.
    // Enum methods are not special, they are only namespaced
    // functions that you can call with dot syntax.
    const Suit = enum {
        Clubs,
        Spades,
        Diamonds,
        Hearts,

        pub fn isClubs(self: Suit) bool {
            return self == Suit.Clubs;
        }
    };
    test "enum method" {
        const p = Suit.Spades;
        assert(!p.isClubs());
    }

    // An enum variant of different types can be switched upon.
    const Foo = enum {
        String,
        Number,
        None,
    };
    test "enum variant switch" {
        const p = Foo.Number;
        const what_is_it = switch (p) {
            Foo.String => "this is a string",
            Foo.Number => "this is a number",
            Foo.None => "this is a none",
        };
        assert(mem.eql(u8, what_is_it, "this is a number"));
    }

    // @TagType can be used to access the integer tag type of an enum.
    const Small = enum {
        One,
        Two,
        Three,
        Four,
    };
    test "@TagType" {
        assert(@TagType(Small) == u2);
    }

    // @typeInfo tells us the field count and the fields names:
    test "@typeInfo" {
        assert(@typeInfo(Small).Enum.fields.len == 4);
        assert(mem.eql(u8, @typeInfo(Small).Enum.fields[1].name, "Two"));
    }

    // @tagName gives a []const u8 representation of an enum value:
    test "@tagName" {
        assert(mem.eql(u8, @tagName(Small.Three), "Three"));
    }

    $ zig test enums.zig
    1/7 test "enum ordinal value"...OK
    2/7 test "set enum ordinal value"...OK
    3/7 test "enum method"...OK
    4/7 test "enum variant switch"...OK
    5/7 test "@TagType"...OK
    6/7 test "@typeInfo"...OK
    7/7 test "@tagName"...OK
    All 7 tests passed.

See also:

- @typeInfo
- @tagName
- @sizeOf

## extern enum §

By default, enums are not guaranteed to be compatible with the C ABI:

test.zig

    const Foo = enum { A, B, C };
    export fn entry(foo: Foo) void { }

    $ zig build-obj test.zig
    ./docgen_tmp/test.zig:2:22: error: parameter of type 'Foo' not allowed in function with calling convention 'C'
    export fn entry(foo: Foo) void {
                         ^

For a C-ABI-compatible enum, use `extern enum`:

test.zig

    const Foo = extern enum { A, B, C };
    export fn entry(foo: Foo) void { }

    $ zig build-obj test.zig

## packed enum §

By default, the size of enums is not guaranteed.

`packed enum` causes the size of the enum to be the same as the size of the integer tag type of the enum:

test.zig

    const std = @import("std");

    test "packed enum" {
        const Number = packed enum(u8) {
            One,
            Two,
            Three,
        };
        std.debug.assert(@sizeOf(Number) == @sizeOf(u8));
    }

    $ zig test test.zig
    1/1 test "packed enum"...OK
    All 1 tests passed.

This makes the enum eligible to be in a packed struct.

## Enum Literals §

Enum literals allow specifying the name of an enum field without specifying the enum type:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const Color = enum {
        Auto,
        Off,
        On,
    };

    test "enum literals" {
        const color1: Color = .Auto;
        const color2 = Color.Auto;
        assert(color1 == color2);
    }

    test "switch using enum literals" {
        const color = Color.On;
        const result = switch (color) {
            .Auto => false,
            .On => true,
            .Off => false,
        };
        assert(result);
    }

    $ zig test test.zig
    1/2 test "enum literals"...OK
    2/2 test "switch using enum literals"...OK
    All 2 tests passed.

## Non-exhaustive enum §

A Non-exhaustive enum can be created by adding a trailing '_' field. It must specify a tag type and cannot consume every enumeration value.

@intToEnum on a non-exhaustive enum cannot fail.

A switch on a non-exhaustive enum can include a '_' prong as an alternative to an `else` prong with the difference being that it makes it a compile error if all the known tag names are not handled by the switch.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const Number = enum(u8) {
        One,
        Two,
        Three,
        _,
    };

    test "switch on non-exhaustive enum" {
        const number = Number.One;
        const result = switch (number) {
            .One => true,
            .Two,
            .Three => false,
            _ => false,
        };
        assert(result);
        const is_one = switch (number) {
            .One => true,
            else => false,
        };
        assert(is_one);
    }

    $ zig test test.zig
    1/1 test "switch on non-exhaustive enum"...OK
    All 1 tests passed.

# union §

A bare `union` defines a set of possible types that a value can be as a list of fields. Only one field can be active at a time. The in-memory representation of bare unions is not guaranteed. Bare unions cannot be used to reinterpret memory. For that, use @ptrCast, or use an extern union or a packed union which have guaranteed in-memory layout. Accessing the non-active field is safety-checked Undefined Behavior:

test.zig

    const Payload = union {
        Int: i64,
        Float: f64,
        Bool: bool,
    };
    test "simple union" {
        var payload = Payload{ .Int = 1234 };
        payload.Float = 12.34;
    }

    $ zig test test.zig
    1/1 test "simple union"...access of inactive union field
    /home/andy/dev/zig/docgen_tmp/test.zig:8:12: 0x204c1b in test "simple union" (test)
        payload.Float = 12.34;
               ^
    /home/andy/dev/zig/lib/std/special/test_runner.zig:47:28: 0x22bb1e in std.special.main (test)
            } else test_fn.func();
                               ^
    /home/andy/dev/zig/lib/std/start.zig:253:37: 0x20564d in std.start.posixCallMainAndExit (test)
                const result = root.main() catch |err| {
                                        ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20538f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^

    Tests failed. Use the following command to reproduce the failure:
    /home/andy/dev/zig/docgen_tmp/test

You can activate another field by assigning the entire union:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const Payload = union {
        Int: i64,
        Float: f64,
        Bool: bool,
    };
    test "simple union" {
        var payload = Payload{ .Int = 1234 };
        assert(payload.Int == 1234);
        payload = Payload{ .Float = 12.34 };
        assert(payload.Float == 12.34);
    }

    $ zig test test.zig
    1/1 test "simple union"...OK
    All 1 tests passed.

In order to use switch with a union, it must be a Tagged union.

To initialize a union when the tag is a comptime-known name, see @unionInit.

## Tagged union §

Unions can be declared with an enum tag type. This turns the union into a tagged union, which makes it eligible to use with switch expressions. One can use @TagType to obtain the enum type from the union type. Tagged unions coerce to their tag type: Type Coercion: unions and enums.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const ComplexTypeTag = enum {
        Ok,
        NotOk,
    };
    const ComplexType = union(ComplexTypeTag) {
        Ok: u8,
        NotOk: void,
    };

    test "switch on tagged union" {
        const c = ComplexType{ .Ok = 42 };
        assert(@as(ComplexTypeTag, c) == ComplexTypeTag.Ok);

        switch (c) {
            ComplexTypeTag.Ok => |value| assert(value == 42),
            ComplexTypeTag.NotOk => unreachable,
        }
    }

    test "@TagType" {
        assert(@TagType(ComplexType) == ComplexTypeTag);
    }

    test "coerce to enum" {
        const c1 = ComplexType{ .Ok = 42 };
        const c2 = ComplexType.NotOk;

        assert(c1 == .Ok);
        assert(c2 == .NotOk);
    }

    $ zig test test.zig
    1/3 test "switch on tagged union"...OK
    2/3 test "@TagType"...OK
    3/3 test "coerce to enum"...OK
    All 3 tests passed.

In order to modify the payload of a tagged union in a switch expression, place a `*` before the variable name to make it a pointer:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const ComplexTypeTag = enum {
        Ok,
        NotOk,
    };
    const ComplexType = union(ComplexTypeTag) {
        Ok: u8,
        NotOk: void,
    };

    test "modify tagged union in switch" {
        var c = ComplexType{ .Ok = 42 };
        assert(@as(ComplexTypeTag, c) == ComplexTypeTag.Ok);

        switch (c) {
            ComplexTypeTag.Ok => |*value| value.* += 1,
            ComplexTypeTag.NotOk => unreachable,
        }

        assert(c.Ok == 43);
    }

    $ zig test test.zig
    1/1 test "modify tagged union in switch"...OK
    All 1 tests passed.

Unions can be made to infer the enum tag type. Further, unions can have methods just like structs and enums.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const Variant = union(enum) {
        Int: i32,
        Bool: bool,

        // void can be omitted when inferring enum tag type.
        None,

        fn truthy(self: Variant) bool {
            return switch (self) {
                Variant.Int => |x_int| x_int != 0,
                Variant.Bool => |x_bool| x_bool,
                Variant.None => false,
            };
        }
    };

    test "union method" {
        var v1 = Variant{ .Int = 1 };
        var v2 = Variant{ .Bool = false };

        assert(v1.truthy());
        assert(!v2.truthy());
    }

    $ zig test test.zig
    1/1 test "union method"...OK
    All 1 tests passed.

@tagName can be used to return a comptime `[]const u8` value representing the field name:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const Small2 = union(enum) {
        A: i32,
        B: bool,
        C: u8,
    };
    test "@tagName" {
        assert(std.mem.eql(u8, @tagName(Small2.C), "C"));
    }

    $ zig test test.zig
    1/1 test "@tagName"...OK
    All 1 tests passed.

## extern union §

An `extern union` has memory layout guaranteed to be compatible with the target C ABI.

See also:

- extern struct

## packed union §

A `packed union` has well-defined in-memory layout and is eligible to be in a packed struct.

## Anonymous Union Literals §

Anonymous Struct Literals syntax can be used to initialize unions without specifying the type:

anon_union.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const Number = union {
        int: i32,
        float: f64,
    };

    test "anonymous union literal syntax" {
        var i: Number = .{.int = 42};
        var f = makeNumber();
        assert(i.int == 42);
        assert(f.float == 12.34);
    }

    fn makeNumber() Number {
        return .{.float = 12.34};
    }

    $ zig test anon_union.zig
    1/1 test "anonymous union literal syntax"...OK
    All 1 tests passed.

# blocks §

Blocks are used to limit the scope of variable declarations:

test.zig

    test "access variable after block scope" {
        {
            var x: i32 = 1;
        }
        x += 1;
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:5:5: error: use of undeclared identifier 'x'
        x += 1;
        ^

Blocks are expressions. When labeled, `break` can be used to return a value from the block:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "labeled break from labeled block expression" {
        var y: i32 = 123;

        const x = blk: {
            y += 1;
            break :blk y;
        };
        assert(x == 124);
        assert(y == 124);
    }

    $ zig test test.zig
    1/1 test "labeled break from labeled block expression"...OK
    All 1 tests passed.

Here, `blk` can be any name.

See also:

- Labeled while
- Labeled for

## Shadowing §

It is never allowed for an identifier to "hide" another one by using the same name:

test.zig

    const pi = 3.14;

    test "inside test block" {
        // Let's even go inside another block
        {
            var pi: i32 = 1234;
        }
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:6:9: error: redefinition of 'pi'
            var pi: i32 = 1234;
            ^
    ./docgen_tmp/test.zig:1:1: note: previous definition is here
    const pi = 3.14;
    ^

Because of this, when you read Zig code you can rely on an identifier always meaning the same thing, within the scope it is defined. Note that you can, however use the same name if the scopes are separate:

test.zig

    test "separate scopes" {
        {
            const pi = 3.14;
        }
        {
            var pi: bool = true;
        }
    }

    $ zig test test.zig
    1/1 test "separate scopes"...OK
    All 1 tests passed.

# switch §

switch.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "switch simple" {
        const a: u64 = 10;
        const zz: u64 = 103;

        // All branches of a switch expression must be able to be coerced to a
        // common type.
        //
        // Branches cannot fallthrough. If fallthrough behavior is desired, combine
        // the cases and use an if.
        const b = switch (a) {
            // Multiple cases can be combined via a ','
            1, 2, 3 => 0,

            // Ranges can be specified using the ... syntax. These are inclusive
            // both ends.
            5...100 => 1,

            // Branches can be arbitrarily complex.
            101 => blk: {
                const c: u64 = 5;
                break :blk c * 2 + 1;
            },

            // Switching on arbitrary expressions is allowed as long as the
            // expression is known at compile-time.
            zz => zz,
            comptime blk: {
                const d: u32 = 5;
                const e: u32 = 100;
                break :blk d + e;
            } => 107,

            // The else branch catches everything not already captured.
            // Else branches are mandatory unless the entire range of values
            // is handled.
            else => 9,
        };

        assert(b == 1);
    }

    // Switch expressions can be used outside a function:
    const os_msg = switch (std.Target.current.os.tag) {
        .linux => "we found a linux user",
        else => "not a linux user",
    };

    // Inside a function, switch statements implicitly are compile-time
    // evaluated if the target expression is compile-time known.
    test "switch inside function" {
        switch (std.Target.current.os.tag) {
            .fuchsia => {
                // On an OS other than fuchsia, block is not even analyzed,
                // so this compile error is not triggered.
                // On fuchsia this compile error would be triggered.
                @compileError("fuchsia not supported");
            },
            else => {},
        }
    }

    $ zig test switch.zig
    1/2 test "switch simple"...OK
    2/2 test "switch inside function"...OK
    All 2 tests passed.

`switch` can be used to capture the field values of a Tagged union. Modifications to the field values can be done by placing a `*` before the capture variable name, turning it into a pointer.

test.zig

    const assert = @import("std").debug.assert;

    test "switch on tagged union" {
        const Point = struct {
            x: u8,
            y: u8,
        };
        const Item = union(enum) {
            A: u32,
            C: Point,
            D,
            E: u32,
        };

        var a = Item{ .C = Point{ .x = 1, .y = 2 } };

        // Switching on more complex enums is allowed.
        const b = switch (a) {
            // A capture group is allowed on a match, and will return the enum
            // value matched. If the payload types of both cases are the same
            // they can be put into the same switch prong.
            Item.A, Item.E => |item| item,

            // A reference to the matched value can be obtained using `*` syntax.
            Item.C => |*item| blk: {
                item.*.x += 1;
                break :blk 6;
            },

            // No else is required if the types cases was exhaustively handled
            Item.D => 8,
        };

        assert(b == 6);
        assert(a.C.x == 2);
    }

    $ zig test test.zig
    1/1 test "switch on tagged union"...OK
    All 1 tests passed.

See also:

- comptime
- enum
- @compileError
- Compile Variables

## Exhaustive Switching §

When a `switch` expression does not have an `else` clause, it must exhaustively list all the possible values. Failure to do so is a compile error:

test.zig

    const Color = enum {
        Auto,
        Off,
        On,
    };

    test "exhaustive switching" {
        const color = Color.Off;
        switch (color) {
            Color.Auto => {},
            Color.On => {},
        }
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:9:5: error: enumeration value 'Color.Off' not handled in switch
        switch (color) {
        ^
    ./docgen_tmp/test.zig:7:29: note: referenced here
    test "exhaustive switching" {
                                ^

## Switching with Enum Literals §

Enum Literals can be useful to use with `switch` to avoid repetitively specifying enum or union types:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const Color = enum {
        Auto,
        Off,
        On,
    };

    test "enum literals with switch" {
        const color = Color.Off;
        const result = switch (color) {
            .Auto => false,
            .On => false,
            .Off => true,
        };
        assert(result);
    }

    $ zig test test.zig
    1/1 test "enum literals with switch"...OK
    All 1 tests passed.

# while §

A while loop is used to repeatedly execute an expression until some condition is no longer true.

while.zig

    const assert = @import("std").debug.assert;

    test "while basic" {
        var i: usize = 0;
        while (i < 10) {
            i += 1;
        }
        assert(i == 10);
    }

    $ zig test while.zig
    1/1 test "while basic"...OK
    All 1 tests passed.

Use `break` to exit a while loop early.

while.zig

    const assert = @import("std").debug.assert;

    test "while break" {
        var i: usize = 0;
        while (true) {
            if (i == 10)
                break;
            i += 1;
        }
        assert(i == 10);
    }

    $ zig test while.zig
    1/1 test "while break"...OK
    All 1 tests passed.

Use `continue` to jump back to the beginning of the loop.

while.zig

    const assert = @import("std").debug.assert;

    test "while continue" {
        var i: usize = 0;
        while (true) {
            i += 1;
            if (i < 10)
                continue;
            break;
        }
        assert(i == 10);
    }

    $ zig test while.zig
    1/1 test "while continue"...OK
    All 1 tests passed.

While loops support a continue expression which is executed when the loop is continued. The `continue` keyword respects this expression.

while.zig

    const assert = @import("std").debug.assert;

    test "while loop continue expression" {
        var i: usize = 0;
        while (i < 10) : (i += 1) {}
        assert(i == 10);
    }

    test "while loop continue expression, more complicated" {
        var i: usize = 1;
        var j: usize = 1;
        while (i * j < 2000) : ({ i *= 2; j *= 3; }) {
            const my_ij = i * j;
            assert(my_ij < 2000);
        }
    }

    $ zig test while.zig
    1/2 test "while loop continue expression"...OK
    2/2 test "while loop continue expression, more complicated"...OK
    All 2 tests passed.

While loops are expressions. The result of the expression is the result of the `else` clause of a while loop, which is executed when the condition of the while loop is tested as false.

`break`, like `return`, accepts a value parameter. This is the result of the `while` expression. When you `break` from a while loop, the `else` branch is not evaluated.

while.zig

    const assert = @import("std").debug.assert;

    test "while else" {
        assert(rangeHasNumber(0, 10, 5));
        assert(!rangeHasNumber(0, 10, 15));
    }

    fn rangeHasNumber(begin: usize, end: usize, number: usize) bool {
        var i = begin;
        return while (i < end) : (i += 1) {
            if (i == number) {
                break true;
            }
        } else false;
    }

    $ zig test while.zig
    1/1 test "while else"...OK
    All 1 tests passed.

## Labeled while §

When a `while` loop is labeled, it can be referenced from a `break` or `continue` from within a nested loop:

test.zig

    test "nested break" {
        outer: while (true) {
            while (true) {
                break :outer;
            }
        }
    }

    test "nested continue" {
        var i: usize = 0;
        outer: while (i < 10) : (i += 1) {
            while (true) {
                continue :outer;
            }
        }
    }

    $ zig test test.zig
    1/2 test "nested break"...OK
    2/2 test "nested continue"...OK
    All 2 tests passed.

## while with Optionals §

Just like if expressions, while loops can take an optional as the condition and capture the payload. When null is encountered the loop exits.

When the `|x|` syntax is present on a `while` expression, the while condition must have an Optional Type.

The `else` branch is allowed on optional iteration. In this case, it will be executed on the first null value encountered.

while.zig

    const assert = @import("std").debug.assert;

    test "while null capture" {
        var sum1: u32 = 0;
        numbers_left = 3;
        while (eventuallyNullSequence()) |value| {
            sum1 += value;
        }
        assert(sum1 == 3);

        var sum2: u32 = 0;
        numbers_left = 3;
        while (eventuallyNullSequence()) |value| {
            sum2 += value;
        } else {
            assert(sum2 == 3);
        }
    }

    var numbers_left: u32 = undefined;
    fn eventuallyNullSequence() ?u32 {
        return if (numbers_left == 0) null else blk: {
            numbers_left -= 1;
            break :blk numbers_left;
        };
    }

    $ zig test while.zig
    1/1 test "while null capture"...OK
    All 1 tests passed.

## while with Error Unions §

Just like if expressions, while loops can take an error union as the condition and capture the payload or the error code. When the condition results in an error code the else branch is evaluated and the loop is finished.

When the `else |x|` syntax is present on a `while` expression, the while condition must have an Error Union Type.

while.zig

    const assert = @import("std").debug.assert;

    test "while error union capture" {
        var sum1: u32 = 0;
        numbers_left = 3;
        while (eventuallyErrorSequence()) |value| {
            sum1 += value;
        } else |err| {
            assert(err == error.ReachedZero);
        }
    }

    var numbers_left: u32 = undefined;

    fn eventuallyErrorSequence() anyerror!u32 {
        return if (numbers_left == 0) error.ReachedZero else blk: {
            numbers_left -= 1;
            break :blk numbers_left;
        };
    }

    $ zig test while.zig
    1/1 test "while error union capture"...OK
    All 1 tests passed.

## inline while §

While loops can be inlined. This causes the loop to be unrolled, which allows the code to do some things which only work at compile time, such as use types as first class values.

test.zig

    const assert = @import("std").debug.assert;

    test "inline while loop" {
        comptime var i = 0;
        var sum: usize = 0;
        inline while (i < 3) : (i += 1) {
            const T = switch (i) {
                0 => f32,
                1 => i8,
                2 => bool,
                else => unreachable,
            };
            sum += typeNameLength(T);
        }
        assert(sum == 9);
    }

    fn typeNameLength(comptime T: type) usize {
        return @typeName(T).len;
    }

    $ zig test test.zig
    1/1 test "inline while loop"...OK
    All 1 tests passed.

It is recommended to use `inline` loops only for one of these reasons:

- You need the loop to execute at comptime for the semantics to work.
- You have a benchmark to prove that forcibly unrolling the loop in this way is measurably faster.

See also:

- if
- Optionals
- Errors
- comptime
- unreachable

# for §

for.zig

    const assert = @import("std").debug.assert;

    test "for basics" {
        const items = [_]i32 { 4, 5, 3, 4, 0 };
        var sum: i32 = 0;

        // For loops iterate over slices and arrays.
        for (items) |value| {
            // Break and continue are supported.
            if (value == 0) {
                continue;
            }
            sum += value;
        }
        assert(sum == 16);

        // To iterate over a portion of a slice, reslice.
        for (items[0..1]) |value| {
            sum += value;
        }
        assert(sum == 20);

        // To access the index of iteration, specify a second capture value.
        // This is zero-indexed.
        var sum2: i32 = 0;
        for (items) |value, i| {
            assert(@TypeOf(i) == usize);
            sum2 += @intCast(i32, i);
        }
        assert(sum2 == 10);
    }

    test "for reference" {
        var items = [_]i32 { 3, 4, 2 };

        // Iterate over the slice by reference by
        // specifying that the capture value is a pointer.
        for (items) |*value| {
            value.* += 1;
        }

        assert(items[0] == 4);
        assert(items[1] == 5);
        assert(items[2] == 3);
    }

    test "for else" {
        // For allows an else attached to it, the same as a while loop.
        var items = [_]?i32 { 3, 4, null, 5 };

        // For loops can also be used as expressions.
        // Similar to while loops, when you break from a for loop, the else branch is not evaluated.
        var sum: i32 = 0;
        const result = for (items) |value| {
            if (value != null) {
                sum += value.?;
            }
        } else blk: {
            assert(sum == 12);
            break :blk sum;
        };
        assert(result == 12);
    }

    $ zig test for.zig
    1/3 test "for basics"...OK
    2/3 test "for reference"...OK
    3/3 test "for else"...OK
    All 3 tests passed.

## Labeled for §

When a `for` loop is labeled, it can be referenced from a `break` or `continue` from within a nested loop:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "nested break" {
        var count: usize = 0;
        outer: for ([_]i32{ 1, 2, 3, 4, 5 }) |_| {
            for ([_]i32{ 1, 2, 3, 4, 5 }) |_| {
                count += 1;
                break :outer;
            }
        }
        assert(count == 1);
    }

    test "nested continue" {
        var count: usize = 0;
        outer: for ([_]i32{ 1, 2, 3, 4, 5, 6, 7, 8 }) |_| {
            for ([_]i32{ 1, 2, 3, 4, 5 }) |_| {
                count += 1;
                continue :outer;
            }
        }

        assert(count == 8);
    }

    $ zig test test.zig
    1/2 test "nested break"...OK
    2/2 test "nested continue"...OK
    All 2 tests passed.

## inline for §

For loops can be inlined. This causes the loop to be unrolled, which allows the code to do some things which only work at compile time, such as use types as first class values. The capture value and iterator value of inlined for loops are compile-time known.

test.zig

    const assert = @import("std").debug.assert;

    test "inline for loop" {
        const nums = [_]i32{2, 4, 6};
        var sum: usize = 0;
        inline for (nums) |i| {
            const T = switch (i) {
                2 => f32,
                4 => i8,
                6 => bool,
                else => unreachable,
            };
            sum += typeNameLength(T);
        }
        assert(sum == 9);
    }

    fn typeNameLength(comptime T: type) usize {
        return @typeName(T).len;
    }

    $ zig test test.zig
    1/1 test "inline for loop"...OK
    All 1 tests passed.

It is recommended to use `inline` loops only for one of these reasons:

- You need the loop to execute at comptime for the semantics to work.
- You have a benchmark to prove that forcibly unrolling the loop in this way is measurably faster.

See also:

- while
- comptime
- Arrays
- Slices

# if §

if.zig

    // If expressions have three uses, corresponding to the three types:
    // * bool
    // * ?T
    // * anyerror!T

    const assert = @import("std").debug.assert;

    test "if expression" {
        // If expressions are used instead of a ternary expression.
        const a: u32 = 5;
        const b: u32 = 4;
        const result = if (a != b) 47 else 3089;
        assert(result == 47);
    }

    test "if boolean" {
        // If expressions test boolean conditions.
        const a: u32 = 5;
        const b: u32 = 4;
        if (a != b) {
            assert(true);
        } else if (a == 9) {
            unreachable;
        } else {
            unreachable;
        }
    }

    test "if optional" {
        // If expressions test for null.

        const a: ?u32 = 0;
        if (a) |value| {
            assert(value == 0);
        } else {
            unreachable;
        }

        const b: ?u32 = null;
        if (b) |value| {
            unreachable;
        } else {
            assert(true);
        }

        // The else is not required.
        if (a) |value| {
            assert(value == 0);
        }

        // To test against null only, use the binary equality operator.
        if (b == null) {
            assert(true);
        }

        // Access the value by reference using a pointer capture.
        var c: ?u32 = 3;
        if (c) |*value| {
            value.* = 2;
        }

        if (c) |value| {
            assert(value == 2);
        } else {
            unreachable;
        }
    }

    test "if error union" {
        // If expressions test for errors.
        // Note the |err| capture on the else.

        const a: anyerror!u32 = 0;
        if (a) |value| {
            assert(value == 0);
        } else |err| {
            unreachable;
        }

        const b: anyerror!u32 = error.BadValue;
        if (b) |value| {
            unreachable;
        } else |err| {
            assert(err == error.BadValue);
        }

        // The else and |err| capture is strictly required.
        if (a) |value| {
            assert(value == 0);
        } else |_| {}

        // To check only the error value, use an empty block expression.
        if (b) |_| {} else |err| {
            assert(err == error.BadValue);
        }

        // Access the value by reference using a pointer capture.
        var c: anyerror!u32 = 3;
        if (c) |*value| {
            value.* = 9;
        } else |err| {
            unreachable;
        }

        if (c) |value| {
            assert(value == 9);
        } else |err| {
            unreachable;
        }
    }

    $ zig test if.zig
    1/4 test "if expression"...OK
    2/4 test "if boolean"...OK
    3/4 test "if optional"...OK
    4/4 test "if error union"...OK
    All 4 tests passed.

See also:

- Optionals
- Errors

# defer §

defer.zig

    const std = @import("std");
    const assert = std.debug.assert;
    const warn = std.debug.warn;

    // defer will execute an expression at the end of the current scope.
    fn deferExample() usize {
        var a: usize = 1;

        {
            defer a = 2;
            a = 1;
        }
        assert(a == 2);

        a = 5;
        return a;
    }

    test "defer basics" {
        assert(deferExample() == 5);
    }

    // If multiple defer statements are specified, they will be executed in
    // the reverse order they were run.
    fn deferUnwindExample() void {
        warn("\n", .{});

        defer {
            warn("1 ", .{});
        }
        defer {
            warn("2 ", .{});
        }
        if (false) {
            // defers are not run if they are never executed.
            defer {
                warn("3 ", .{});
            }
        }
    }

    test "defer unwinding" {
        deferUnwindExample();
    }

    // The errdefer keyword is similar to defer, but will only execute if the
    // scope returns with an error.
    //
    // This is especially useful in allowing a function to clean up properly
    // on error, and replaces goto error handling tactics as seen in c.
    fn deferErrorExample(is_error: bool) !void {
        warn("\nstart of function\n", .{});

        // This will always be executed on exit
        defer {
            warn("end of function\n", .{});
        }

        errdefer {
            warn("encountered an error!\n", .{});
        }

        if (is_error) {
            return error.DeferError;
        }
    }

    test "errdefer unwinding" {
        deferErrorExample(false) catch {};
        deferErrorExample(true) catch {};
    }

    $ zig test defer.zig
    1/3 test "defer basics"...OK
    2/3 test "defer unwinding"...
    2 1 OK
    3/3 test "errdefer unwinding"...
    start of function
    end of function

    start of function
    encountered an error!
    end of function
    OK
    All 3 tests passed.

See also:

- Errors

# unreachable §

In `Debug` and `ReleaseSafe` mode, and when using `zig test`, `unreachable` emits a call to `panic` with the message `reached unreachable code`.

In `ReleaseFast` mode, the optimizer uses the assumption that `unreachable` code will never be hit to perform optimizations. However, `zig test` even in `ReleaseFast` mode still emits `unreachable` as calls to `panic`.

## Basics §

test.zig

    // unreachable is used to assert that control flow will never happen upon a
    // particular location:
    test "basic math" {
        const x = 1;
        const y = 2;
        if (x + y != 3) {
            unreachable;
        }
    }

    $ zig test test.zig
    1/1 test "basic math"...OK
    All 1 tests passed.

In fact, this is how assert is implemented:

test.zig

    fn assert(ok: bool) void {
        if (!ok) unreachable; // assertion failure
    }

    // This test will fail because we hit unreachable.
    test "this will fail" {
        assert(false);
    }

    $ zig test test.zig
    1/1 test "this will fail"...reached unreachable code
    /home/andy/dev/zig/docgen_tmp/test.zig:2:14: 0x2052cb in assert (test)
        if (!ok) unreachable; // assertion failure
                 ^
    /home/andy/dev/zig/docgen_tmp/test.zig:7:11: 0x204b8e in test "this will fail" (test)
        assert(false);
              ^
    /home/andy/dev/zig/lib/std/special/test_runner.zig:47:28: 0x22bade in std.special.main (test)
            } else test_fn.func();
                               ^
    /home/andy/dev/zig/lib/std/start.zig:253:37: 0x20560d in std.start.posixCallMainAndExit (test)
                const result = root.main() catch |err| {
                                        ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20534f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^

    Tests failed. Use the following command to reproduce the failure:
    /home/andy/dev/zig/docgen_tmp/test

## At Compile-Time §

test.zig

    const assert = @import("std").debug.assert;

    test "type of unreachable" {
        comptime {
            // The type of unreachable is noreturn.

            // However this assertion will still fail because
            // evaluating unreachable at compile-time is a compile error.

            assert(@TypeOf(unreachable) == noreturn);
        }
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:10:16: error: unreachable code
            assert(@TypeOf(unreachable) == noreturn);
                   ^
    ./docgen_tmp/test.zig:3:28: note: referenced here
    test "type of unreachable" {
                               ^

See also:

- Zig Test
- Build Mode
- comptime

# noreturn §

`noreturn` is the type of:

- `break`
- `continue`
- `return`
- `unreachable`
- `while (true) {}`

When resolving types together, such as `if` clauses or `switch` prongs, the `noreturn` type is compatible with every other type. Consider:

test.zig

    fn foo(condition: bool, b: u32) void {
        const a = if (condition) b else return;
        @panic("do something with a");
    }
    test "noreturn" {
        foo(false, 1);
    }

    $ zig test test.zig
    1/1 test "noreturn"...OK
    All 1 tests passed.

Another use case for `noreturn` is the `exit` function:

test.zig

    pub extern "kernel32" fn ExitProcess(exit_code: c_uint) callconv(.Stdcall) noreturn;

    test "foo" {
        const value = bar() catch ExitProcess(1);
        assert(value == 1234);
    }

    fn bar() anyerror!u32 {
        return 1234;
    }

    const assert = @import("std").debug.assert;

    $ zig test test.zig -target x86_64-windows
    Created /home/andy/dev/zig/zig-cache/o/zMCfPPh__UsVTYa6CBcgFV8MDgBX1h1MPB4_1EwqLAnnVB1cdQ4Wymr7GK1McWwM/test.exe but skipping execution because it is non-native.

# Functions §

functions.zig

    const assert = @import("std").debug.assert;

    // Functions are declared like this
    fn add(a: i8, b: i8) i8 {
        if (a == 0) {
            return b;
        }

        return a + b;
    }

    // The export specifier makes a function externally visible in the generated
    // object file, and makes it use the C ABI.
    export fn sub(a: i8, b: i8) i8 { return a - b; }

    // The extern specifier is used to declare a function that will be resolved
    // at link time, when linking statically, or at runtime, when linking
    // dynamically.
    // The callconv specifier changes the calling convention of the function.
    extern "kernel32" fn ExitProcess(exit_code: u32) callconv(.Stdcall) noreturn;
    extern "c" fn atan2(a: f64, b: f64) f64;

    // The @setCold builtin tells the optimizer that a function is rarely called.
    fn abort() noreturn {
        @setCold(true);
        while (true) {}
    }

    // The naked calling convention makes a function not have any function prologue or epilogue.
    // This can be useful when integrating with assembly.
    fn _start() callconv(.Naked) noreturn {
        abort();
    }

    // The inline specifier forces a function to be inlined at all call sites.
    // If the function cannot be inlined, it is a compile-time error.
    inline fn shiftLeftOne(a: u32) u32 {
        return a << 1;
    }

    // The pub specifier allows the function to be visible when importing.
    // Another file can use @import and call sub2
    pub fn sub2(a: i8, b: i8) i8 { return a - b; }

    // Functions can be used as values and are equivalent to pointers.
    const call2_op = fn (a: i8, b: i8) i8;
    fn do_op(fn_call: call2_op, op1: i8, op2: i8) i8 {
        return fn_call(op1, op2);
    }

    test "function" {
        assert(do_op(add, 5, 6) == 11);
        assert(do_op(sub2, 5, 6) == -1);
    }

    $ zig test functions.zig
    1/1 test "function"...OK
    All 1 tests passed.

Function values are like pointers:

test.zig

    const assert = @import("std").debug.assert;

    comptime {
        assert(@TypeOf(foo) == fn()void);
        assert(@sizeOf(fn()void) == @sizeOf(?fn()void));
    }

    fn foo() void { }

    $ zig build-obj test.zig

## Pass-by-value Parameters §

Primitive types such as Integers and Floats passed as parameters are copied, and then the copy is available in the function body. This is called "passing by value". Copying a primitive type is essentially free and typically involves nothing more than setting a register.

Structs, unions, and arrays can sometimes be more efficiently passed as a reference, since a copy could be arbitrarily expensive depending on the size. When these types are passed as parameters, Zig may choose to copy and pass by value, or pass by reference, whichever way Zig decides will be faster. This is made possible, in part, by the fact that parameters are immutable.

test.zig

    const Point = struct {
        x: i32,
        y: i32,
    };

    fn foo(point: Point) i32 {
        // Here, `point` could be a reference, or a copy. The function body
        // can ignore the difference and treat it as a value. Be very careful
        // taking the address of the parameter - it should be treated as if
        // the address will become invalid when the function returns.
        return point.x + point.y;
    }

    const assert = @import("std").debug.assert;

    test "pass struct to function" {
        assert(foo(Point{ .x = 1, .y = 2 }) == 3);
    }

    $ zig test test.zig
    1/1 test "pass struct to function"...OK
    All 1 tests passed.

For extern functions, Zig follows the C ABI for passing structs and unions by value.

## Function Parameter Type Inference §

Function parameters can be declared with `var` in place of the type. In this case the parameter types will be inferred when the function is called. Use @TypeOf and @typeInfo to get information about the inferred type.

test.zig

    const assert = @import("std").debug.assert;

    fn addFortyTwo(x: var) @TypeOf(x) {
        return x + 42;
    }

    test "fn type inference" {
        assert(addFortyTwo(1) == 43);
        assert(@TypeOf(addFortyTwo(1)) == comptime_int);
        var y: i64 = 2;
        assert(addFortyTwo(y) == 44);
        assert(@TypeOf(addFortyTwo(y)) == i64);
    }

    $ zig test test.zig
    1/1 test "fn type inference"...OK
    All 1 tests passed.

## Function Reflection §

test.zig

    const assert = @import("std").debug.assert;

    test "fn reflection" {
        assert(@TypeOf(assert).ReturnType == void);
        assert(@TypeOf(assert).is_var_args == false);
    }

    $ zig test test.zig
    1/1 test "fn reflection"...OK
    All 1 tests passed.

# Errors §

## Error Set Type §

An error set is like an enum. However, each error name across the entire compilation gets assigned an unsigned integer greater than 0. You are allowed to declare the same error name more than once, and if you do, it gets assigned the same integer value.

The number of unique error values across the entire compilation should determine the size of the error set type. However right now it is hard coded to be a `u16`. See #768.

You can coerce an error from a subset to a superset:

test.zig

    const std = @import("std");

    const FileOpenError = error {
        AccessDenied,
        OutOfMemory,
        FileNotFound,
    };

    const AllocationError = error {
        OutOfMemory,
    };

    test "coerce subset to superset" {
        const err = foo(AllocationError.OutOfMemory);
        std.debug.assert(err == FileOpenError.OutOfMemory);
    }

    fn foo(err: AllocationError) FileOpenError {
        return err;
    }

    $ zig test test.zig
    1/1 test "coerce subset to superset"...OK
    All 1 tests passed.

But you cannot coerce an error from a superset to a subset:

test.zig

    const FileOpenError = error {
        AccessDenied,
        OutOfMemory,
        FileNotFound,
    };

    const AllocationError = error {
        OutOfMemory,
    };

    test "coerce superset to subset" {
        foo(FileOpenError.OutOfMemory) catch {};
    }

    fn foo(err: FileOpenError) AllocationError {
        return err;
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:16:12: error: expected type 'AllocationError', found 'FileOpenError'
        return err;
               ^
    ./docgen_tmp/test.zig:2:5: note: 'error.AccessDenied' not a member of destination error set
        AccessDenied,
        ^
    ./docgen_tmp/test.zig:4:5: note: 'error.FileNotFound' not a member of destination error set
        FileNotFound,
        ^

There is a shortcut for declaring an error set with only 1 value, and then getting that value:

    const err = error.FileNotFound;

This is equivalent to:

    const err = (error {FileNotFound}).FileNotFound;

This becomes useful when using Inferred Error Sets.

### The Global Error Set §

`anyerror` refers to the global error set. This is the error set that contains all errors in the entire compilation unit. It is a superset of all other error sets and a subset of none of them.

You can coerce any error set to the global one, and you can explicitly cast an error of the global error set to a non-global one. This inserts a language-level assert to make sure the error value is in fact in the destination error set.

The global error set should generally be avoided because it prevents the compiler from knowing what errors are possible at compile-time. Knowing the error set at compile-time is better for generated documentation and helpful error messages, such as forgetting a possible error value in a switch.

## Error Union Type §

An error set type and normal type can be combined with the `!` binary operator to form an error union type. You are likely to use an error union type more often than an error set type by itself.

Here is a function to parse a string into a 64-bit integer:

test.zig

    const std = @import("std");
    const maxInt = std.math.maxInt;

    pub fn parseU64(buf: []const u8, radix: u8) !u64 {
        var x: u64 = 0;

        for (buf) |c| {
            const digit = charToDigit(c);

            if (digit >= radix) {
                return error.InvalidChar;
            }

            // x *= radix
            if (@mulWithOverflow(u64, x, radix, &x)) {
                return error.Overflow;
            }

            // x += digit
            if (@addWithOverflow(u64, x, digit, &x)) {
                return error.Overflow;
            }
        }

        return x;
    }

    fn charToDigit(c: u8) u8 {
        return switch (c) {
            '0' ... '9' => c - '0',
            'A' ... 'Z' => c - 'A' + 10,
            'a' ... 'z' => c - 'a' + 10,
            else => maxInt(u8),
        };
    }

    test "parse u64" {
        const result = try parseU64("1234", 10);
        std.debug.assert(result == 1234);
    }

    $ zig test test.zig
    1/1 test "parse u64"...OK
    All 1 tests passed.

Notice the return type is `!u64`. This means that the function either returns an unsigned 64 bit integer, or an error. We left off the error set to the left of the `!`, so the error set is inferred.

Within the function definition, you can see some return statements that return an error, and at the bottom a return statement that returns a `u64`. Both types coerce to `anyerror!u64`.

What it looks like to use this function varies depending on what you're trying to do. One of the following:

- You want to provide a default value if it returned an error.
- If it returned an error then you want to return the same error.
- You know with complete certainty it will not return an error, so want to unconditionally unwrap it.
- You want to take a different action for each possible error.

### catch §

If you want to provide a default value, you can use the `catch` binary operator:

    fn doAThing(str: []u8) void {
        const number = parseU64(str, 10) catch 13;
        // ...
    }

In this code, `number` will be equal to the successfully parsed string, or a default value of 13. The type of the right hand side of the binary `catch` operator must match the unwrapped error union type, or be of type `noreturn`.

### try §

Let's say you wanted to return the error if you got one, otherwise continue with the function logic:

    fn doAThing(str: []u8) !void {
        const number = parseU64(str, 10) catch |err| return err;
        // ...
    }

There is a shortcut for this. The `try` expression:

    fn doAThing(str: []u8) !void {
        const number = try parseU64(str, 10);
        // ...
    }

`try` evaluates an error union expression. If it is an error, it returns from the current function with the same error. Otherwise, the expression results in the unwrapped value.

Maybe you know with complete certainty that an expression will never be an error. In this case you can do this:

    const number = parseU64("1234", 10) catch unreachable;

Here we know for sure that "1234" will parse successfully. So we put the `unreachable` value on the right hand side. `unreachable` generates a panic in Debug and ReleaseSafe modes and undefined behavior in ReleaseFast mode. So, while we're debugging the application, if there was a surprise error here, the application would crash appropriately.

Finally, you may want to take a different action for every situation. For that, we combine the if and switch expression:

    fn doAThing(str: []u8) void {
        if (parseU64(str, 10)) |number| {
            doSomethingWithNumber(number);
        } else |err| switch (err) {
            error.Overflow => {
                // handle overflow...
            },
            // we promise that InvalidChar won't happen (or crash in debug mode if it does)
            error.InvalidChar => unreachable,
        }
    }

### errdefer §

The other component to error handling is defer statements. In addition to an unconditional defer, Zig has `errdefer`, which evaluates the deferred expression on block exit path if and only if the function returned with an error from the block.

Example:

    fn createFoo(param: i32) !Foo {
        const foo = try tryToAllocateFoo();
        // now we have allocated foo. we need to free it if the function fails.
        // but we want to return it if the function succeeds.
        errdefer deallocateFoo(foo);

        const tmp_buf = allocateTmpBuffer() orelse return error.OutOfMemory;
        // tmp_buf is truly a temporary resource, and we for sure want to clean it up
        // before this block leaves scope
        defer deallocateTmpBuffer(tmp_buf);

        if (param > 1337) return error.InvalidParam;

        // here the errdefer will not run since we're returning success from the function.
        // but the defer will run!
        return foo;
    }

The neat thing about this is that you get robust error handling without the verbosity and cognitive overhead of trying to make sure every exit path is covered. The deallocation code is always directly following the allocation code.

A couple of other tidbits about error handling:

- These primitives give enough expressiveness that it's completely practical to have failing to check for an error be a compile error. If you really want to ignore the error, you can add `catch unreachable` and get the added benefit of crashing in Debug and ReleaseSafe modes if your assumption was wrong.
- Since Zig understands error types, it can pre-weight branches in favor of errors not occurring. Just a small optimization benefit that is not available in other languages.

See also:

- defer
- if
- switch

An error union is created with the `!` binary operator. You can use compile-time reflection to access the child type of an error union:

test.zig

    const assert = @import("std").debug.assert;

    test "error union" {
        var foo: anyerror!i32 = undefined;

        // Coerce from child type of an error union:
        foo = 1234;

        // Coerce from an error set:
        foo = error.SomeError;

        // Use compile-time reflection to access the payload type of an error union:
        comptime assert(@TypeOf(foo).Payload == i32);

        // Use compile-time reflection to access the error set type of an error union:
        comptime assert(@TypeOf(foo).ErrorSet == anyerror);
    }

    $ zig test test.zig
    1/1 test "error union"...OK
    All 1 tests passed.

### Merging Error Sets §

Use the `||` operator to merge two error sets together. The resulting error set contains the errors of both error sets. Doc comments from the left-hand side override doc comments from the right-hand side. In this example, the doc comments for `C.PathNotFound` is `A doc comment`.

This is especially useful for functions which return different error sets depending on comptime branches. For example, the Zig standard library uses `LinuxFileOpenError || WindowsFileOpenError` for the error set of opening files.

test.zig

    const A = error{
        NotDir,

        /// A doc comment
        PathNotFound,
    };
    const B = error{
        OutOfMemory,

        /// B doc comment
        PathNotFound,
    };

    const C = A || B;

    fn foo() C!void {
        return error.NotDir;
    }

    test "merge error sets" {
        if (foo()) {
            @panic("unexpected");
        } else |err| switch (err) {
            error.OutOfMemory => @panic("unexpected"),
            error.PathNotFound => @panic("unexpected"),
            error.NotDir => {},
        }
    }

    $ zig test test.zig
    1/1 test "merge error sets"...OK
    All 1 tests passed.

### Inferred Error Sets §

Because many functions in Zig return a possible error, Zig supports inferring the error set. To infer the error set for a function, use this syntax:

test.zig

    // With an inferred error set
    pub fn add_inferred(comptime T: type, a: T, b: T) !T {
        var answer: T = undefined;
        return if (@addWithOverflow(T, a, b, &answer)) error.Overflow else answer;
    }

    // With an explicit error set
    pub fn add_explicit(comptime T: type, a: T, b: T) Error!T {
        var answer: T = undefined;
        return if (@addWithOverflow(T, a, b, &answer)) error.Overflow else answer;
    }

    const Error = error {
        Overflow,
    };

    const std = @import("std");

    test "inferred error set" {
        if (add_inferred(u8, 255, 1)) |_| unreachable else |err| switch (err) {
            error.Overflow => {}, // ok
        }
    }

    $ zig test test.zig
    1/1 test "inferred error set"...OK
    All 1 tests passed.

When a function has an inferred error set, that function becomes generic and thus it becomes trickier to do certain things with it, such as obtain a function pointer, or have an error set that is consistent across different build targets. Additionally, inferred error sets are incompatible with recursion.

In these situations, it is recommended to use an explicit error set. You can generally start with an empty error set and let compile errors guide you toward completing the set.

These limitations may be overcome in a future version of Zig.

## Error Return Traces §

Error Return Traces show all the points in the code that an error was returned to the calling function. This makes it practical to use try everywhere and then still be able to know what happened if an error ends up bubbling all the way out of your application.

test.zig

    pub fn main() !void {
        try foo(12);
    }

    fn foo(x: i32) !void {
        if (x >= 5) {
            try bar();
        } else {
            try bang2();
        }
    }

    fn bar() !void {
        if (baz()) {
            try quux();
        } else |err| switch (err) {
            error.FileNotFound => try hello(),
            else => try another(),
        }
    }

    fn baz() !void {
        try bang1();
    }

    fn quux() !void {
        try bang2();
    }

    fn hello() !void {
        try bang2();
    }

    fn another() !void {
        try bang1();
    }

    fn bang1() !void {
        return error.FileNotFound;
    }

    fn bang2() !void {
        return error.PermissionDenied;
    }

    $ zig build-exe test.zig
    $ ./test
    error: PermissionDenied
    /home/andy/dev/zig/docgen_tmp/test.zig:39:5: 0x22ef02 in bang1 (test)
        return error.FileNotFound;
        ^
    /home/andy/dev/zig/docgen_tmp/test.zig:23:5: 0x22eddf in baz (test)
        try bang1();
        ^
    /home/andy/dev/zig/docgen_tmp/test.zig:43:5: 0x22eda2 in bang2 (test)
        return error.PermissionDenied;
        ^
    /home/andy/dev/zig/docgen_tmp/test.zig:31:5: 0x22eecf in hello (test)
        try bang2();
        ^
    /home/andy/dev/zig/docgen_tmp/test.zig:17:31: 0x22ed6e in bar (test)
            error.FileNotFound => try hello(),
                                  ^
    /home/andy/dev/zig/docgen_tmp/test.zig:7:9: 0x22ec5c in foo (test)
            try bar();
            ^
    /home/andy/dev/zig/docgen_tmp/test.zig:2:5: 0x22a9e4 in main (test)
        try foo(12);
        ^

Look closely at this example. This is no stack trace.

You can see that the final error bubbled up was `PermissionDenied`, but the original error that started this whole thing was `FileNotFound`. In the `bar` function, the code handles the original error code, and then returns another one, from the switch statement. Error Return Traces make this clear, whereas a stack trace would look like this:

test.zig

    pub fn main() void {
        foo(12);
    }

    fn foo(x: i32) void {
        if (x >= 5) {
            bar();
        } else {
            bang2();
        }
    }

    fn bar() void {
        if (baz()) {
            quux();
        } else {
            hello();
        }
    }

    fn baz() bool {
        return bang1();
    }

    fn quux() void {
        bang2();
    }

    fn hello() void {
        bang2();
    }

    fn bang1() bool {
        return false;
    }

    fn bang2() void {
        @panic("PermissionDenied");
    }

    $ zig build-exe test.zig
    $ ./test
    PermissionDenied
    /home/andy/dev/zig/docgen_tmp/test.zig:38:5: 0x2302d6 in bang2 (test)
        @panic("PermissionDenied");
        ^
    /home/andy/dev/zig/docgen_tmp/test.zig:30:10: 0x230a38 in hello (test)
        bang2();
             ^
    /home/andy/dev/zig/docgen_tmp/test.zig:17:14: 0x2302ba in bar (test)
            hello();
                 ^
    /home/andy/dev/zig/docgen_tmp/test.zig:7:12: 0x22e955 in foo (test)
            bar();
               ^
    /home/andy/dev/zig/docgen_tmp/test.zig:2:8: 0x22a7ed in main (test)
        foo(12);
           ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x2046ef in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x2044cf in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

Here, the stack trace does not explain how the control flow in `bar` got to the `hello()` call. One would have to open a debugger or further instrument the application in order to find out. The error return trace, on the other hand, shows exactly how the error bubbled up.

This debugging feature makes it easier to iterate quickly on code that robustly handles all error conditions. This means that Zig developers will naturally find themselves writing correct, robust code in order to increase their development pace.

Error Return Traces are enabled by default in Debug and ReleaseSafe builds and disabled by default in ReleaseFast and ReleaseSmall builds.

There are a few ways to activate this error return tracing feature:

- Return an error from main
- An error makes its way to `catch unreachable` and you have not overridden the default panic handler
- Use errorReturnTrace to access the current return trace. You can use `std.debug.dumpStackTrace` to print it. This function returns comptime-known null when building without error return tracing support.

### Implementation Details §

To analyze performance cost, there are two cases:

- when no errors are returned
- when returning errors

For the case when no errors are returned, the cost is a single memory write operation, only in the first non-failable function in the call graph that calls a failable function, i.e. when a function returning `void` calls a function returning `error`. This is to initialize this struct in the stack memory:

    pub const StackTrace = struct {
        index: usize,
        instruction_addresses: [N]usize,
    };

Here, N is the maximum function call depth as determined by call graph analysis. Recursion is ignored and counts for 2.

A pointer to `StackTrace` is passed as a secret parameter to every function that can return an error, but it's always the first parameter, so it can likely sit in a register and stay there.

That's it for the path when no errors occur. It's practically free in terms of performance.

When generating the code for a function that returns an error, just before the `return` statement (only for the `return` statements that return errors), Zig generates a call to this function:

    // marked as "no-inline" in LLVM IR
    fn __zig_return_error(stack_trace: *StackTrace) void {
        stack_trace.instruction_addresses[stack_trace.index] = @returnAddress();
        stack_trace.index = (stack_trace.index + 1) % N;
    }

The cost is 2 math operations plus some memory reads and writes. The memory accessed is constrained and should remain cached for the duration of the error return bubbling.

As for code size cost, 1 function call before a return statement is no big deal. Even so, I have a plan to make the call to `__zig_return_error` a tail call, which brings the code size cost down to actually zero. What is a return statement in code without error return tracing can become a jump instruction in code with error return tracing.

# Optionals §

One area that Zig provides safety without compromising efficiency or readability is with the optional type.

The question mark symbolizes the optional type. You can convert a type to an optional type by putting a question mark in front of it, like this:

    // normal integer
    const normal_int: i32 = 1234;

    // optional integer
    const optional_int: ?i32 = 5678;

Now the variable `optional_int` could be an `i32`, or `null`.

Instead of integers, let's talk about pointers. Null references are the source of many runtime exceptions, and even stand accused of being the worst mistake of computer science.

Zig does not have them.

Instead, you can use an optional pointer. This secretly compiles down to a normal pointer, since we know we can use 0 as the null value for the optional type. But the compiler can check your work and make sure you don't assign null to something that can't be null.

Typically the downside of not having null is that it makes the code more verbose to write. But, let's compare some equivalent C code and Zig code.

Task: call malloc, if the result is null, return null.

C code

    // malloc prototype included for reference
    void *malloc(size_t size);

    struct Foo *do_a_thing(void) {
        char *ptr = malloc(1234);
        if (!ptr) return NULL;
        // ...
    }

Zig code

    // malloc prototype included for reference
    extern fn malloc(size: size_t) ?*u8;

    fn doAThing() ?*Foo {
        const ptr = malloc(1234) orelse return null;
        // ...
    }

Here, Zig is at least as convenient, if not more, than C. And, the type of "ptr" is `*u8` not `?*u8`. The `orelse` keyword unwrapped the optional type and therefore `ptr` is guaranteed to be non-null everywhere it is used in the function.

The other form of checking against NULL you might see looks like this:

    void do_a_thing(struct Foo *foo) {
        // do some stuff

        if (foo) {
            do_something_with_foo(foo);
        }

        // do some stuff
    }

In Zig you can accomplish the same thing:

    fn doAThing(optional_foo: ?*Foo) void {
        // do some stuff

        if (optional_foo) |foo| {
          doSomethingWithFoo(foo);
        }

        // do some stuff
    }

Once again, the notable thing here is that inside the if block, `foo` is no longer an optional pointer, it is a pointer, which cannot be null.

One benefit to this is that functions which take pointers as arguments can be annotated with the "nonnull" attribute - `__attribute__((nonnull))` in GCC. The optimizer can sometimes make better decisions knowing that pointer arguments cannot be null.

## Optional Type §

An optional is created by putting `?` in front of a type. You can use compile-time reflection to access the child type of an optional:

test.zig

    const assert = @import("std").debug.assert;

    test "optional type" {
        // Declare an optional and coerce from null:
        var foo: ?i32 = null;

        // Coerce from child type of an optional
        foo = 1234;

        // Use compile-time reflection to access the child type of the optional:
        comptime assert(@TypeOf(foo).Child == i32);
    }

    $ zig test test.zig
    1/1 test "optional type"...OK
    All 1 tests passed.

## null §

Just like undefined, `null` has its own type, and the only way to use it is to cast it to a different type:

    const optional_value: ?i32 = null;

## Optional Pointers §

An optional pointer is guaranteed to be the same size as a pointer. The `null` of the optional is guaranteed to be address 0.

test.zig

    const assert = @import("std").debug.assert;

    test "optional pointers" {
        // Pointers cannot be null. If you want a null pointer, use the optional
        // prefix `?` to make the pointer type optional.
        var ptr: ?*i32 = null;

        var x: i32 = 1;
        ptr = &x;

        assert(ptr.?.* == 1);

        // Optional pointers are the same size as normal pointers, because pointer
        // value 0 is used as the null value.
        assert(@sizeOf(?*i32) == @sizeOf(*i32));
    }

    $ zig test test.zig
    1/1 test "optional pointers"...OK
    All 1 tests passed.

# Casting §

A type cast converts a value of one type to another. Zig has Type Coercion for conversions that are known to be completely safe and unambiguous, and Explicit Casts for conversions that one would not want to happen on accident. There is also a third kind of type conversion called Peer Type Resolution for the case when a result type must be decided given multiple operand types.

## Type Coercion §

Type coercion occurs when one type is expected, but different type is provided:

test.zig

    test "type coercion - variable declaration" {
        var a: u8 = 1;
        var b: u16 = a;
    }

    test "type coercion - function call" {
        var a: u8 = 1;
        foo(a);
    }

    fn foo(b: u16) void {}

    test "type coercion - @as builtin" {
        var a: u8 = 1;
        var b = @as(u16, a);
    }

    $ zig test test.zig
    1/3 test "type coercion - variable declaration"...OK
    2/3 test "type coercion - function call"...OK
    3/3 test "type coercion - @as builtin"...OK
    All 3 tests passed.

Type coercions are only allowed when it is completely unambiguous how to get from one type to another, and the transformation is guaranteed to be safe. There is one exception, which is C Pointers.

### Type Coercion: Stricter Qualification §

Values which have the same representation at runtime can be cast to increase the strictness of the qualifiers, no matter how nested the qualifiers are:

- `const` - non-const to const is allowed
- `volatile` - non-volatile to volatile is allowed
- `align` - bigger to smaller alignment is allowed
- error sets to supersets is allowed

These casts are no-ops at runtime since the value representation does not change.

test.zig

    test "type coercion - const qualification" {
        var a: i32 = 1;
        var b: *i32 = &a;
        foo(b);
    }

    fn foo(a: *const i32) void {}

    $ zig test test.zig
    1/1 test "type coercion - const qualification"...OK
    All 1 tests passed.

In addition, pointers coerce to const optional pointers:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;
    const mem = std.mem;

    test "cast *[1][*]const u8 to [*]const ?[*]const u8" {
        const window_name = [1][*]const u8{"window name"};
        const x: [*]const ?[*]const u8 = &window_name;
        assert(mem.eql(u8, std.mem.spanZ(@ptrCast([*:0]const u8, x[0].?)), "window name"));
    }

    $ zig test test.zig
    1/1 test "cast *[1][*]const u8 to [*]const ?[*]const u8"...OK
    All 1 tests passed.

### Type Coercion: Integer and Float Widening §

Integers coerce to integer types which can represent every value of the old type, and likewise Floats coerce to float types which can represent every value of the old type.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;
    const mem = std.mem;

    test "integer widening" {
        var a: u8 = 250;
        var b: u16 = a;
        var c: u32 = b;
        var d: u64 = c;
        var e: u64 = d;
        var f: u128 = e;
        assert(f == a);
    }

    test "implicit unsigned integer to signed integer" {
        var a: u8 = 250;
        var b: i16 = a;
        assert(b == 250);
    }

    test "float widening" {
        // Note: there is an open issue preventing this from working on aarch64:
        // https://github.com/ziglang/zig/issues/3282
        if (std.Target.current.cpu.arch == .aarch64) return error.SkipZigTest;

        var a: f16 = 12.34;
        var b: f32 = a;
        var c: f64 = b;
        var d: f128 = c;
        assert(d == a);
    }

    $ zig test test.zig
    1/3 test "integer widening"...OK
    2/3 test "implicit unsigned integer to signed integer"...OK
    3/3 test "float widening"...OK
    All 3 tests passed.

### Type Coercion: Arrays and Pointers §

coerce_arrays_and_ptrs.zig

    const std = @import("std");
    const assert = std.debug.assert;

    // This cast exists primarily so that string literals can be
    // passed to functions that accept const slices. However
    // it is probably going to be removed from the language when
    // https://github.com/ziglang/zig/issues/265 is implemented.
    test "[N]T to []const T" {
        var x1: []const u8 = "hello";
        var x2: []const u8 = &[5]u8{ 'h', 'e', 'l', 'l', 111 };
        assert(std.mem.eql(u8, x1, x2));

        var y: []const f32 = &[2]f32{ 1.2, 3.4 };
        assert(y[0] == 1.2);
    }

    // Likewise, it works when the destination type is an error union.
    test "[N]T to E![]const T" {
        var x1: anyerror![]const u8 = "hello";
        var x2: anyerror![]const u8 = &[5]u8{ 'h', 'e', 'l', 'l', 111 };
        assert(std.mem.eql(u8, try x1, try x2));

        var y: anyerror![]const f32 = &[2]f32{ 1.2, 3.4 };
        assert((try y)[0] == 1.2);
    }

    // Likewise, it works when the destination type is an optional.
    test "[N]T to ?[]const T" {
        var x1: ?[]const u8 = "hello";
        var x2: ?[]const u8 = &[5]u8{ 'h', 'e', 'l', 'l', 111 };
        assert(std.mem.eql(u8, x1.?, x2.?));

        var y: ?[]const f32 = &[2]f32{ 1.2, 3.4 };
        assert(y.?[0] == 1.2);
    }

    // In this cast, the array length becomes the slice length.
    test "*[N]T to []T" {
        var buf: [5]u8 = "hello".*;
        const x: []u8 = &buf;
        assert(std.mem.eql(u8, x, "hello"));

        const buf2 = [2]f32{ 1.2, 3.4 };
        const x2: []const f32 = &buf2;
        assert(std.mem.eql(f32, x2, &[2]f32{ 1.2, 3.4 }));
    }

    // Single-item pointers to arrays can be coerced to
    // unknown length pointers.
    test "*[N]T to [*]T" {
        var buf: [5]u8 = "hello".*;
        const x: [*]u8 = &buf;
        assert(x[4] == 'o');
        // x[5] would be an uncaught out of bounds pointer dereference!
    }

    // Likewise, it works when the destination type is an optional.
    test "*[N]T to ?[*]T" {
        var buf: [5]u8 = "hello".*;
        const x: ?[*]u8 = &buf;
        assert(x.?[4] == 'o');
    }

    // Single-item pointers can be cast to len-1 single-item arrays.
    test "*T to *[1]T" {
        var x: i32 = 1234;
        const y: *[1]i32 = &x;
        const z: [*]i32 = y;
        assert(z[0] == 1234);
    }

    $ zig test coerce_arrays_and_ptrs.zig
    1/7 test "[N]T to []const T"...OK
    2/7 test "[N]T to E![]const T"...OK
    3/7 test "[N]T to ?[]const T"...OK
    4/7 test "*[N]T to []T"...OK
    5/7 test "*[N]T to [*]T"...OK
    6/7 test "*[N]T to ?[*]T"...OK
    7/7 test "*T to *[1]T"...OK
    All 7 tests passed.

See also:

- C Pointers

### Type Coercion: Optionals §

The payload type of Optionals, as well as null, coerce to the optional type.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "coerce to optionals" {
        const x: ?i32 = 1234;
        const y: ?i32 = null;

        assert(x.? == 1234);
        assert(y == null);
    }

    $ zig test test.zig
    1/1 test "coerce to optionals"...OK
    All 1 tests passed.

It works nested inside the Error Union Type, too:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "coerce to optionals wrapped in error union" {
        const x: anyerror!?i32 = 1234;
        const y: anyerror!?i32 = null;

        assert((try x).? == 1234);
        assert((try y) == null);
    }

    $ zig test test.zig
    1/1 test "coerce to optionals wrapped in error union"...OK
    All 1 tests passed.

### Type Coercion: Error Unions §

The payload type of an Error Union Type as well as the Error Set Type coerce to the error union type:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "coercion to error unions" {
        const x: anyerror!i32 = 1234;
        const y: anyerror!i32 = error.Failure;

        assert((try x) == 1234);
        std.testing.expectError(error.Failure, y);
    }

    $ zig test test.zig
    1/1 test "coercion to error unions"...OK
    All 1 tests passed.

### Type Coercion: Compile-Time Known Numbers §

When a number is comptime-known to be representable in the destination type, it may be coerced:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "coercing large integer type to smaller one when value is comptime known to fit" {
        const x: u64 = 255;
        const y: u8 = x;
        assert(y == 255);
    }

    $ zig test test.zig
    1/1 test "coercing large integer type to smaller one when value is comptime known to fit"...OK
    All 1 tests passed.

### Type Coercion: unions and enums §

Tagged unions can be coerced to enums, and enums can be coerced to tagged unions when they are comptime-known to be a field of the union that has only one possible value, such as void:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const E = enum {
        One,
        Two,
        Three,
    };

    const U = union(E) {
        One: i32,
        Two: f32,
        Three,
    };

    test "coercion between unions and enums" {
        var u = U{ .Two = 12.34 };
        var e: E = u;
        assert(e == E.Two);

        const three = E.Three;
        var another_u: U = three;
        assert(another_u == E.Three);
    }

    $ zig test test.zig
    1/1 test "coercion between unions and enums"...OK
    All 1 tests passed.

See also:

- union
- enum

### Type Coercion: Zero Bit Types §

Zero Bit Types may be coerced to single-item Pointers, regardless of const.

TODO document the reasoning for this

TODO document whether vice versa should work and why

test.zig

    test "coercion of zero bit types" {
        var x: void = {};
        var y: *void = x;
        //var z: void = y; // TODO
    }

    $ zig test test.zig
    1/1 test "coercion of zero bit types"...OK
    All 1 tests passed.

### Type Coercion: undefined §

undefined can be cast to any type.

## Explicit Casts §

Explicit casts are performed via Builtin Functions. Some explicit casts are safe; some are not. Some explicit casts perform language-level assertions; some do not. Some explicit casts are no-ops at runtime; some are not.

- @bitCast - change type but maintain bit representation
- @alignCast - make a pointer have more alignment
- @boolToInt - convert true to 1 and false to 0
- @enumToInt - obtain the integer tag value of an enum or tagged union
- @errSetCast - convert to a smaller error set
- @errorToInt - obtain the integer value of an error code
- @floatCast - convert a larger float to a smaller float
- @floatToInt - obtain the integer part of a float value
- @intCast - convert between integer types
- @intToEnum - obtain an enum value based on its integer tag value
- @intToError - obtain an error code based on its integer value
- @intToFloat - convert an integer to a float value
- @intToPtr - convert an address to a pointer
- @ptrCast - convert between pointer types
- @ptrToInt - obtain the address of a pointer
- @truncate - convert between integer types, chopping off bits

## Peer Type Resolution §

Peer Type Resolution occurs in these places:

- switch expressions
- if expressions
- while expressions
- for expressions
- Multiple break statements in a block
- Some binary operations

This kind of type resolution chooses a type that all peer types can coerce into. Here are some examples:

peer_type_resolution.zig

    const std = @import("std");
    const assert = std.debug.assert;
    const mem = std.mem;

    test "peer resolve int widening" {
        var a: i8 = 12;
        var b: i16 = 34;
        var c = a + b;
        assert(c == 46);
        assert(@TypeOf(c) == i16);
    }

    test "peer resolve arrays of different size to const slice" {
        assert(mem.eql(u8, boolToStr(true), "true"));
        assert(mem.eql(u8, boolToStr(false), "false"));
        comptime assert(mem.eql(u8, boolToStr(true), "true"));
        comptime assert(mem.eql(u8, boolToStr(false), "false"));
    }
    fn boolToStr(b: bool) []const u8 {
        return if (b) "true" else "false";
    }

    test "peer resolve array and const slice" {
        testPeerResolveArrayConstSlice(true);
        comptime testPeerResolveArrayConstSlice(true);
    }
    fn testPeerResolveArrayConstSlice(b: bool) void {
        const value1 = if (b) "aoeu" else @as([]const u8, "zz");
        const value2 = if (b) @as([]const u8, "zz") else "aoeu";
        assert(mem.eql(u8, value1, "aoeu"));
        assert(mem.eql(u8, value2, "zz"));
    }

    test "peer type resolution: ?T and T" {
        assert(peerTypeTAndOptionalT(true, false).? == 0);
        assert(peerTypeTAndOptionalT(false, false).? == 3);
        comptime {
            assert(peerTypeTAndOptionalT(true, false).? == 0);
            assert(peerTypeTAndOptionalT(false, false).? == 3);
        }
    }
    fn peerTypeTAndOptionalT(c: bool, b: bool) ?usize {
        if (c) {
            return if (b) null else @as(usize, 0);
        }

        return @as(usize, 3);
    }

    test "peer type resolution: *[0]u8 and []const u8" {
        assert(peerTypeEmptyArrayAndSlice(true, "hi").len == 0);
        assert(peerTypeEmptyArrayAndSlice(false, "hi").len == 1);
        comptime {
            assert(peerTypeEmptyArrayAndSlice(true, "hi").len == 0);
            assert(peerTypeEmptyArrayAndSlice(false, "hi").len == 1);
        }
    }
    fn peerTypeEmptyArrayAndSlice(a: bool, slice: []const u8) []const u8 {
        if (a) {
            return &[_]u8{};
        }

        return slice[0..1];
    }
    test "peer type resolution: *[0]u8, []const u8, and anyerror![]u8" {
        {
            var data = "hi".*;
            const slice = data[0..];
            assert((try peerTypeEmptyArrayAndSliceAndError(true, slice)).len == 0);
            assert((try peerTypeEmptyArrayAndSliceAndError(false, slice)).len == 1);
        }
        comptime {
            var data = "hi".*;
            const slice = data[0..];
            assert((try peerTypeEmptyArrayAndSliceAndError(true, slice)).len == 0);
            assert((try peerTypeEmptyArrayAndSliceAndError(false, slice)).len == 1);
        }
    }
    fn peerTypeEmptyArrayAndSliceAndError(a: bool, slice: []u8) anyerror![]u8 {
        if (a) {
            return &[_]u8{};
        }

        return slice[0..1];
    }

    test "peer type resolution: *const T and ?*T" {
        const a = @intToPtr(*const usize, 0x123456780);
        const b = @intToPtr(?*usize, 0x123456780);
        assert(a == b);
        assert(b == a);
    }

    $ zig test peer_type_resolution.zig
    1/7 test "peer resolve int widening"...OK
    2/7 test "peer resolve arrays of different size to const slice"...OK
    3/7 test "peer resolve array and const slice"...OK
    4/7 test "peer type resolution: ?T and T"...OK
    5/7 test "peer type resolution: *[0]u8 and []const u8"...OK
    6/7 test "peer type resolution: *[0]u8, []const u8, and anyerror![]u8"...OK
    7/7 test "peer type resolution: *const T and ?*T"...OK
    All 7 tests passed.

# Zero Bit Types §

For some types, @sizeOf is 0:

- void
- The Integers `u0` and `i0`.
- Arrays and Vectors with len 0, or with an element type that is a zero bit type.
- An enum with only 1 tag.
- A struct with all fields being zero bit types.
- A union with only 1 field which is a zero bit type.
- Pointers to Zero Bit Types are themselves zero bit types.

These types can only ever have one possible value, and thus require 0 bits to represent. Code that makes use of these types is not included in the final generated code:

    export fn entry() void {
        var x: void = {};
        var y: void = {};
        x = y;
    }

When this turns into machine code, there is no code generated in the body of `entry`, even in Debug mode. For example, on x86_64:

    0000000000000010 <entry>:
      10:	55                   	push   %rbp
      11:	48 89 e5             	mov    %rsp,%rbp
      14:	5d                   	pop    %rbp
      15:	c3                   	retq

These assembly instructions do not have any code associated with the void values - they only perform the function call prologue and epilog.

## void §

`void` can be useful for instantiating generic types. For example, given a `Map(Key, Value)`, one can pass `void` for the `Value` type to make it into a `Set`:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "turn HashMap into a set with void" {
        var map = std.HashMap(i32, void, hash_i32, eql_i32).init(std.testing.allocator);
        defer map.deinit();

        _ = try map.put(1, {});
        _ = try map.put(2, {});

        assert(map.contains(2));
        assert(!map.contains(3));

        _ = map.remove(2);
        assert(!map.contains(2));
    }

    fn hash_i32(x: i32) u32 {
        return @bitCast(u32, x);
    }

    fn eql_i32(a: i32, b: i32) bool {
        return a == b;
    }

    $ zig test test.zig
    1/1 test "turn HashMap into a set with void"...OK
    All 1 tests passed.

Note that this is different from using a dummy value for the hash map value. By using `void` as the type of the value, the hash map entry type has no value field, and thus the hash map takes up less space. Further, all the code that deals with storing and loading the value is deleted, as seen above.

`void` is distinct from `c_void`, which is defined like this: `pub const c_void = @OpaqueType();`. `void` has a known size of 0 bytes, and `c_void` has an unknown, but non-zero, size.

Expressions of type `void` are the only ones whose value can be ignored. For example:

test.zig

    test "ignoring expression value" {
        foo();
    }

    fn foo() i32 {
        return 1234;
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:2:8: error: expression value is ignored
        foo();
           ^
    ./docgen_tmp/test.zig:1:34: note: referenced here
    test "ignoring expression value" {
                                     ^

However, if the expression has type `void`, there will be no error. Function return values can also be explicitly ignored by assigning them to `_`.

test.zig

    test "void is ignored" {
        returnsVoid();
    }

    test "explicitly ignoring expression value" {
        _ = foo();
    }

    fn returnsVoid() void {}

    fn foo() i32 {
        return 1234;
    }

    $ zig test test.zig
    1/2 test "void is ignored"...OK
    2/2 test "explicitly ignoring expression value"...OK
    All 2 tests passed.

## Pointers to Zero Bit Types §

Pointers to zero bit types also have zero bits. They always compare equal to each other:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "pointer to empty struct" {
        const Empty = struct {};
        var a = Empty{};
        var b = Empty{};
        var ptr_a = &a;
        var ptr_b = &b;
        comptime assert(ptr_a == ptr_b);
    }

    $ zig test test.zig
    1/1 test "pointer to empty struct"...OK
    All 1 tests passed.

The type being pointed to can only ever be one value; therefore loads and stores are never generated. ptrToInt and intToPtr are not allowed:

test.zig

    const Empty = struct {};

    test "@ptrToInt for pointer to zero bit type" {
        var a = Empty{};
        _ = @ptrToInt(&a);
    }

    test "@intToPtr for pointer to zero bit type" {
        _ = @intToPtr(*Empty, 0x1);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:4:5: error: pointer to size 0 type has no address
        var a = Empty{};
        ^
    ./docgen_tmp/test.zig:5:9: note: referenced here
        _ = @ptrToInt(&a);
            ^
    ./docgen_tmp/test.zig:9:19: error: type '*Empty' has 0 bits and cannot store information
        _ = @intToPtr(*Empty, 0x1);
                      ^
    ./docgen_tmp/test.zig:9:9: note: referenced here
        _ = @intToPtr(*Empty, 0x1);
            ^

# Result Location Semantics §

TODO add documentation for this

# usingnamespace §

`usingnamespace` is a top level declaration that imports all the public declarations of the operand, which must be a struct, union, or enum, into the current scope:

usingnamespace.zig

    usingnamespace @import("std");

    test "using std namespace" {
        debug.assert(true);
    }

    $ zig test usingnamespace.zig
    1/1 test "using std namespace"...OK
    All 1 tests passed.

Instead of the above pattern, it is generally recommended to explicitly alias individual declarations. However, `usingnamespace` has an important use case when organizing the public API of a file or package. For example, one might have `c.zig` with all of the C imports:

    pub usingnamespace @cImport({
        @cInclude("epoxy/gl.h");
        @cInclude("GLFW/glfw3.h");
        @cDefine("STBI_ONLY_PNG", "");
        @cDefine("STBI_NO_STDIO", "");
        @cInclude("stb_image.h");
    });

The above example demonstrates using `pub` to qualify the `usingnamespace` additionally makes the imported declarations `pub`. This can be used to forward declarations, giving precise control over what declarations a given file exposes.

# comptime §

Zig places importance on the concept of whether an expression is known at compile-time. There are a few different places this concept is used, and these building blocks are used to keep the language small, readable, and powerful.

## Introducing the Compile-Time Concept §

### Compile-Time Parameters §

Compile-time parameters is how Zig implements generics. It is compile-time duck typing.

    fn max(comptime T: type, a: T, b: T) T {
        return if (a > b) a else b;
    }
    fn gimmeTheBiggerFloat(a: f32, b: f32) f32 {
        return max(f32, a, b);
    }
    fn gimmeTheBiggerInteger(a: u64, b: u64) u64 {
        return max(u64, a, b);
    }

In Zig, types are first-class citizens. They can be assigned to variables, passed as parameters to functions, and returned from functions. However, they can only be used in expressions which are known at compile-time, which is why the parameter `T` in the above snippet must be marked with `comptime`.

A `comptime` parameter means that:

- At the callsite, the value must be known at compile-time, or it is a compile error.
- In the function definition, the value is known at compile-time.

For example, if we were to introduce another function to the above snippet:

test.zig

    fn max(comptime T: type, a: T, b: T) T {
        return if (a > b) a else b;
    }
    test "try to pass a runtime type" {
        foo(false);
    }
    fn foo(condition: bool) void {
        const result = max(
            if (condition) f32 else u64,
            1234,
            5678);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:9:9: error: values of type 'type' must be comptime known
            if (condition) f32 else u64,
            ^

This is an error because the programmer attempted to pass a value only known at run-time to a function which expects a value known at compile-time.

Another way to get an error is if we pass a type that violates the type checker when the function is analyzed. This is what it means to have compile-time duck typing.

For example:

test.zig

    fn max(comptime T: type, a: T, b: T) T {
        return if (a > b) a else b;
    }
    test "try to compare bools" {
        _ = max(bool, true, false);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:2:18: error: operator not allowed for type 'bool'
        return if (a > b) a else b;
                     ^
    ./docgen_tmp/test.zig:5:12: note: called from here
        _ = max(bool, true, false);
               ^
    ./docgen_tmp/test.zig:4:29: note: called from here
    test "try to compare bools" {
                                ^

On the flip side, inside the function definition with the `comptime` parameter, the value is known at compile-time. This means that we actually could make this work for the bool type if we wanted to:

test.zig

    fn max(comptime T: type, a: T, b: T) T {
        if (T == bool) {
            return a or b;
        } else if (a > b) {
            return a;
        } else {
            return b;
        }
    }
    test "try to compare bools" {
        @import("std").debug.assert(max(bool, false, true) == true);
    }

    $ zig test test.zig
    1/1 test "try to compare bools"...OK
    All 1 tests passed.

This works because Zig implicitly inlines `if` expressions when the condition is known at compile-time, and the compiler guarantees that it will skip analysis of the branch not taken.

This means that the actual function generated for `max` in this situation looks like this:

    fn max(a: bool, b: bool) bool {
        return a or b;
    }

All the code that dealt with compile-time known values is eliminated and we are left with only the necessary run-time code to accomplish the task.

This works the same way for `switch` expressions - they are implicitly inlined when the target expression is compile-time known.

### Compile-Time Variables §

In Zig, the programmer can label variables as `comptime`. This guarantees to the compiler that every load and store of the variable is performed at compile-time. Any violation of this results in a compile error.

This combined with the fact that we can `inline` loops allows us to write a function which is partially evaluated at compile-time and partially at run-time.

For example:

comptime_vars.zig

    const assert = @import("std").debug.assert;

    const CmdFn = struct {
        name: []const u8,
        func: fn(i32) i32,
    };

    const cmd_fns = [_]CmdFn{
        CmdFn {.name = "one", .func = one},
        CmdFn {.name = "two", .func = two},
        CmdFn {.name = "three", .func = three},
    };
    fn one(value: i32) i32 { return value + 1; }
    fn two(value: i32) i32 { return value + 2; }
    fn three(value: i32) i32 { return value + 3; }

    fn performFn(comptime prefix_char: u8, start_value: i32) i32 {
        var result: i32 = start_value;
        comptime var i = 0;
        inline while (i < cmd_fns.len) : (i += 1) {
            if (cmd_fns[i].name[0] == prefix_char) {
                result = cmd_fns[i].func(result);
            }
        }
        return result;
    }

    test "perform fn" {
        assert(performFn('t', 1) == 6);
        assert(performFn('o', 0) == 1);
        assert(performFn('w', 99) == 99);
    }

    $ zig test comptime_vars.zig
    1/1 test "perform fn"...OK
    All 1 tests passed.

This example is a bit contrived, because the compile-time evaluation component is unnecessary; this code would work fine if it was all done at run-time. But it does end up generating different code. In this example, the function `performFn` is generated three different times, for the different values of `prefix_char` provided:

    // From the line:
    // assert(performFn('t', 1) == 6);
    fn performFn(start_value: i32) i32 {
        var result: i32 = start_value;
        result = two(result);
        result = three(result);
        return result;
    }

    // From the line:
    // assert(performFn('o', 0) == 1);
    fn performFn(start_value: i32) i32 {
        var result: i32 = start_value;
        result = one(result);
        return result;
    }

    // From the line:
    // assert(performFn('w', 99) == 99);
    fn performFn(start_value: i32) i32 {
        var result: i32 = start_value;
        return result;
    }

Note that this happens even in a debug build; in a release build these generated functions still pass through rigorous LLVM optimizations. The important thing to note, however, is not that this is a way to write more optimized code, but that it is a way to make sure that what should happen at compile-time, does happen at compile-time. This catches more errors and as demonstrated later in this article, allows expressiveness that in other languages requires using macros, generated code, or a preprocessor to accomplish.

### Compile-Time Expressions §

In Zig, it matters whether a given expression is known at compile-time or run-time. A programmer can use a `comptime` expression to guarantee that the expression will be evaluated at compile-time. If this cannot be accomplished, the compiler will emit an error. For example:

test.zig

    extern fn exit() noreturn;

    test "foo" {
        comptime {
            exit();
        }
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:5:9: error: unable to evaluate constant expression
            exit();
            ^
    ./docgen_tmp/test.zig:5:13: note: referenced here
            exit();
                ^

It doesn't make sense that a program could call `exit()` (or any other external function) at compile-time, so this is a compile error. However, a `comptime` expression does much more than sometimes cause a compile error.

Within a `comptime` expression:

- All variables are `comptime` variables.
- All `if`, `while`, `for`, and `switch` expressions are evaluated at compile-time, or emit a compile error if this is not possible.
- All function calls cause the compiler to interpret the function at compile-time, emitting a compile error if the function tries to do something that has global run-time side effects.

This means that a programmer can create a function which is called both at compile-time and run-time, with no modification to the function required.

Let's look at an example:

test.zig

    const assert = @import("std").debug.assert;

    fn fibonacci(index: u32) u32 {
        if (index < 2) return index;
        return fibonacci(index - 1) + fibonacci(index - 2);
    }

    test "fibonacci" {
        // test fibonacci at run-time
        assert(fibonacci(7) == 13);

        // test fibonacci at compile-time
        comptime {
            assert(fibonacci(7) == 13);
        }
    }

    $ zig test test.zig
    1/1 test "fibonacci"...OK
    All 1 tests passed.

Imagine if we had forgotten the base case of the recursive function and tried to run the tests:

test.zig

    const assert = @import("std").debug.assert;

    fn fibonacci(index: u32) u32 {
        //if (index < 2) return index;
        return fibonacci(index - 1) + fibonacci(index - 2);
    }

    test "fibonacci" {
        comptime {
            assert(fibonacci(7) == 13);
        }
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:5:28: error: operation caused overflow
        return fibonacci(index - 1) + fibonacci(index - 2);
                               ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:10:25: note: called from here
            assert(fibonacci(7) == 13);
                            ^
    ./docgen_tmp/test.zig:8:18: note: called from here
    test "fibonacci" {
                     ^
    ./docgen_tmp/test.zig:5:21: note: referenced here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:10:25: note: referenced here
            assert(fibonacci(7) == 13);
                            ^

The compiler produces an error which is a stack trace from trying to evaluate the function at compile-time.

Luckily, we used an unsigned integer, and so when we tried to subtract 1 from 0, it triggered undefined behavior, which is always a compile error if the compiler knows it happened. But what would have happened if we used a signed integer?

test.zig

    const assert = @import("std").debug.assert;

    fn fibonacci(index: i32) i32 {
        //if (index < 2) return index;
        return fibonacci(index - 1) + fibonacci(index - 2);
    }

    test "fibonacci" {
        comptime {
            assert(fibonacci(7) == 13);
        }
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:5:21: error: evaluation exceeded 1000 backwards branches
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:5:21: note: called from here
        return fibonacci(index - 1) + fibonacci(index - 2);
                        ^
    ./docgen_tmp/test.zig:10:25: note: referenced here
            assert(fibonacci(7) == 13);
                            ^

The compiler noticed that evaluating this function at compile-time took a long time, and thus emitted a compile error and gave up. If the programmer wants to increase the budget for compile-time computation, they can use a built-in function called @setEvalBranchQuota to change the default number 1000 to something else.

What if we fix the base case, but put the wrong value in the `assert` line?

test.zig

    const assert = @import("std").debug.assert;

    fn fibonacci(index: i32) i32 {
        if (index < 2) return index;
        return fibonacci(index - 1) + fibonacci(index - 2);
    }

    test "fibonacci" {
        comptime {
            assert(fibonacci(7) == 99999);
        }
    }

    $ zig test test.zig
    ./lib/std/debug.zig:228:14: error: unable to evaluate constant expression
        if (!ok) unreachable; // assertion failure
                 ^
    ./docgen_tmp/test.zig:10:15: note: called from here
            assert(fibonacci(7) == 99999);
                  ^
    ./docgen_tmp/test.zig:8:18: note: called from here
    test "fibonacci" {
                     ^
    ./docgen_tmp/test.zig:10:15: note: referenced here
            assert(fibonacci(7) == 99999);
                  ^

What happened is Zig started interpreting the `assert` function with the parameter `ok` set to `false`. When the interpreter hit `unreachable` it emitted a compile error, because reaching unreachable code is undefined behavior, and undefined behavior causes a compile error if it is detected at compile-time.

In the global scope (outside of any function), all expressions are implicitly `comptime` expressions. This means that we can use functions to initialize complex static data. For example:

test.zig

    const first_25_primes = firstNPrimes(25);
    const sum_of_first_25_primes = sum(&first_25_primes);

    fn firstNPrimes(comptime n: usize) [n]i32 {
        var prime_list: [n]i32 = undefined;
        var next_index: usize = 0;
        var test_number: i32 = 2;
        while (next_index < prime_list.len) : (test_number += 1) {
            var test_prime_index: usize = 0;
            var is_prime = true;
            while (test_prime_index < next_index) : (test_prime_index += 1) {
                if (test_number % prime_list[test_prime_index] == 0) {
                    is_prime = false;
                    break;
                }
            }
            if (is_prime) {
                prime_list[next_index] = test_number;
                next_index += 1;
            }
        }
        return prime_list;
    }

    fn sum(numbers: []const i32) i32 {
        var result: i32 = 0;
        for (numbers) |x| {
            result += x;
        }
        return result;
    }

    test "variable values" {
        @import("std").debug.assert(sum_of_first_25_primes == 1060);
    }

    $ zig test test.zig
    1/1 test "variable values"...OK
    All 1 tests passed.

When we compile this program, Zig generates the constants with the answer pre-computed. Here are the lines from the generated LLVM IR:

    @0 = internal unnamed_addr constant [25 x i32] [i32 2, i32 3, i32 5, i32 7, i32 11, i32 13, i32 17, i32 19, i32 23, i32 29, i32 31, i32 37, i32 41, i32 43, i32 47, i32 53, i32 59, i32 61, i32 67, i32 71, i32 73, i32 79, i32 83, i32 89, i32 97]
    @1 = internal unnamed_addr constant i32 1060

Note that we did not have to do anything special with the syntax of these functions. For example, we could call the `sum` function as is with a slice of numbers whose length and values were only known at run-time.

## Generic Data Structures §

Zig uses these capabilities to implement generic data structures without introducing any special-case syntax. If you followed along so far, you may already know how to create a generic data structure.

Here is an example of a generic `List` data structure, that we will instantiate with the type `i32`. In Zig we refer to the type as `List(i32)`.

    fn List(comptime T: type) type {
        return struct {
            items: []T,
            len: usize,
        };
    }

That's it. It's a function that returns an anonymous `struct`. For the purposes of error messages and debugging, Zig infers the name `"List(i32)"` from the function name and parameters invoked when creating the anonymous struct.

To keep the language small and uniform, all aggregate types in Zig are anonymous. To give a type a name, we assign it to a constant:

    const Node = struct {
        next: *Node,
        name: []u8,
    };

This works because all top level declarations are order-independent, and as long as there isn't an actual infinite regression, values can refer to themselves, directly or indirectly. In this case, `Node` refers to itself as a pointer, which is not actually an infinite regression, so it works fine.

## Case Study: printf in Zig §

Putting all of this together, let's see how `printf` works in Zig.

printf.zig

    const warn = @import("std").debug.warn;

    const a_number: i32 = 1234;
    const a_string = "foobar";

    pub fn main() void {
        warn("here is a string: '{}' here is a number: {}\n", .{a_string, a_number});
    }

    $ zig build-exe printf.zig
    $ ./printf
    here is a string: 'foobar' here is a number: 1234

Let's crack open the implementation of this and see how it works:

    /// Calls print and then flushes the buffer.
    pub fn printf(self: *OutStream, comptime format: []const u8, args: var) anyerror!void {
        const State = enum {
            Start,
            OpenBrace,
            CloseBrace,
        };

        comptime var start_index: usize = 0;
        comptime var state = State.Start;
        comptime var next_arg: usize = 0;

        inline for (format) |c, i| {
            switch (state) {
                State.Start => switch (c) {
                    '{' => {
                        if (start_index < i) try self.write(format[start_index..i]);
                        state = State.OpenBrace;
                    },
                    '}' => {
                        if (start_index < i) try self.write(format[start_index..i]);
                        state = State.CloseBrace;
                    },
                    else => {},
                },
                State.OpenBrace => switch (c) {
                    '{' => {
                        state = State.Start;
                        start_index = i;
                    },
                    '}' => {
                        try self.printValue(args[next_arg]);
                        next_arg += 1;
                        state = State.Start;
                        start_index = i + 1;
                    },
                    else => @compileError("Unknown format character: " ++ c),
                },
                State.CloseBrace => switch (c) {
                    '}' => {
                        state = State.Start;
                        start_index = i;
                    },
                    else => @compileError("Single '}' encountered in format string"),
                },
            }
        }
        comptime {
            if (args.len != next_arg) {
                @compileError("Unused arguments");
            }
            if (state != State.Start) {
                @compileError("Incomplete format string: " ++ format);
            }
        }
        if (start_index < format.len) {
            try self.write(format[start_index..format.len]);
        }
        try self.flush();
    }

This is a proof of concept implementation; the actual function in the standard library has more formatting capabilities.

Note that this is not hard-coded into the Zig compiler; this is userland code in the standard library.

When this function is analyzed from our example code above, Zig partially evaluates the function and emits a function that actually looks like this:

    pub fn printf(self: *OutStream, arg0: i32, arg1: []const u8) !void {
        try self.write("here is a string: '");
        try self.printValue(arg0);
        try self.write("' here is a number: ");
        try self.printValue(arg1);
        try self.write("\n");
        try self.flush();
    }

`printValue` is a function that takes a parameter of any type, and does different things depending on the type:

    pub fn printValue(self: *OutStream, value: var) !void {
        switch (@typeInfo(@TypeOf(value))) {
            .Int => {
                return self.printInt(T, value);
            },
            .Float => {
                return self.printFloat(T, value);
            },
            else => {
                @compileError("Unable to print type '" ++ @typeName(T) ++ "'");
            },
        }
    }

And now, what happens if we give too many arguments to `printf`?

test.zig

    const warn = @import("std").debug.warn;

    const a_number: i32 = 1234;
    const a_string = "foobar";

    test "printf too many arguments" {
        warn("here is a string: '{}' here is a number: {}\n", .{
            a_string,
            a_number,
            a_number,
        });
    }

    $ zig test test.zig
    ./lib/std/fmt.zig:302:13: error: Unused arguments
                @compileError("Unused arguments");
                ^
    ./lib/std/io/out_stream.zig:28:34: note: called from here
                return std.fmt.format(self, format, args);
                                     ^
    ./lib/std/debug.zig:65:25: note: called from here
        noasync stderr.print(fmt, args) catch return;
                            ^
    ./docgen_tmp/test.zig:7:9: note: called from here
        warn("here is a string: '{}' here is a number: {}\n", .{
            ^
    ./docgen_tmp/test.zig:6:34: note: called from here
    test "printf too many arguments" {
                                     ^
    ./lib/std/io/out_stream.zig:28:34: error: expected type 'std.os.WriteError!void', found '@TypeOf(std.fmt.format).ReturnType.ErrorSet!void'
                return std.fmt.format(self, format, args);
                                     ^
    ./lib/std/debug.zig:65:25: note: called from here
        noasync stderr.print(fmt, args) catch return;
                            ^
    ./docgen_tmp/test.zig:7:9: note: called from here
        warn("here is a string: '{}' here is a number: {}\n", .{
            ^
    ./docgen_tmp/test.zig:6:34: note: called from here
    test "printf too many arguments" {
                                     ^
    ./lib/std/io/out_stream.zig:28:34: note: error set '@TypeOf(std.fmt.format).ReturnType.ErrorSet' cannot cast into error set 'std.os.WriteError'
                return std.fmt.format(self, format, args);
                                     ^

Zig gives programmers the tools needed to protect themselves against their own mistakes.

Zig doesn't care whether the format argument is a string literal, only that it is a compile-time known value that can be coerced to a `[]const u8`:

printf.zig

    const warn = @import("std").debug.warn;

    const a_number: i32 = 1234;
    const a_string = "foobar";
    const fmt = "here is a string: '{}' here is a number: {}\n";

    pub fn main() void {
        warn(fmt, .{a_string, a_number});
    }

    $ zig build-exe printf.zig
    $ ./printf
    here is a string: 'foobar' here is a number: 1234

This works fine.

Zig does not special case string formatting in the compiler and instead exposes enough power to accomplish this task in userland. It does so without introducing another language on top of Zig, such as a macro language or a preprocessor language. It's Zig all the way down.

See also:

- inline while
- inline for

# Assembly §

For some use cases, it may be necessary to directly control the machine code generated by Zig programs, rather than relying on Zig's code generation. For these cases, one can use inline assembly. Here is an example of implementing Hello, World on x86_64 Linux using inline assembly:

test.zig

    pub fn main() noreturn {
        const msg = "hello world\n";
        _ = syscall3(SYS_write, STDOUT_FILENO, @ptrToInt(msg), msg.len);
        _ = syscall1(SYS_exit, 0);
        unreachable;
    }

    pub const SYS_write = 1;
    pub const SYS_exit = 60;

    pub const STDOUT_FILENO = 1;

    pub fn syscall1(number: usize, arg1: usize) usize {
        return asm volatile ("syscall"
            : [ret] "={rax}" (-> usize)
            : [number] "{rax}" (number),
              [arg1] "{rdi}" (arg1)
            : "rcx", "r11"
        );
    }

    pub fn syscall3(number: usize, arg1: usize, arg2: usize, arg3: usize) usize {
        return asm volatile ("syscall"
            : [ret] "={rax}" (-> usize)
            : [number] "{rax}" (number),
              [arg1] "{rdi}" (arg1),
              [arg2] "{rsi}" (arg2),
              [arg3] "{rdx}" (arg3)
            : "rcx", "r11"
        );
    }

    $ zig build-exe test.zig -target x86_64-linux
    $ ./test
    hello world

Dissecting the syntax:

    // Inline assembly is an expression which returns a value.
    // the `asm` keyword begins the expression.
    _ = asm
    // `volatile` is an optional modifier that tells Zig this
    // inline assembly expression has side-effects. Without
    // `volatile`, Zig is allowed to delete the inline assembly
    // code if the result is unused.
    volatile (
    // Next is a comptime string which is the assembly code.
    // Inside this string one may use `%[ret]`, `%[number]`,
    // or `%[arg1]` where a register is expected, to specify
    // the register that Zig uses for the argument or return value,
    // if the register constraint strings are used. However in
    // the below code, this is not used. A literal `%` can be
    // obtained by escaping it with a double percent: `%%`.
    // Often multiline string syntax comes in handy here.
        \\syscall
    // Next is the output. It is possible in the future Zig will
    // support multiple outputs, depending on how
    // https://github.com/ziglang/zig/issues/215 is resolved.
    // It is allowed for there to be no outputs, in which case
    // this colon would be directly followed by the colon for the inputs.
        :
    // This specifies the name to be used in `%[ret]` syntax in
    // the above assembly string. This example does not use it,
    // but the syntax is mandatory.
        [ret]
    // Next is the output constraint string. This feature is still
    // considered unstable in Zig, and so LLVM/GCC documentation
    // must be used to understand the semantics.
    // http://releases.llvm.org/10.0.0/docs/LangRef.html#inline-asm-constraint-string
    // https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
    // In this example, the constraint string means "the result value of
    // this inline assembly instruction is whatever is in $rax".
        "={rax}"
    // Next is either a value binding, or `->` and then a type. The
    // type is the result type of the inline assembly expression.
    // If it is a value binding, then `%[ret]` syntax would be used
    // to refer to the register bound to the value.
        (-> usize)
    // Next is the list of inputs.
    // The constraint for these inputs means, "when the assembly code is
    // executed, $rax shall have the value of `number` and $rdi shall have
    // the value of `arg1`". Any number of input parameters is allowed,
    // including none.
        : [number] "{rax}" (number),
            [arg1] "{rdi}" (arg1)
    // Next is the list of clobbers. These declare a set of registers whose
    // values will not be preserved by the execution of this assembly code.
    // These do not include output or input registers. The special clobber
    // value of "memory" means that the assembly writes to arbitrary undeclared
    // memory locations - not only the memory pointed to by a declared indirect
    // output. In this example we list $rcx and $r11 because it is known the
    // kernel syscall does not preserve these registers.
        : "rcx", "r11"
    );

For i386 and x86_64 targets, the syntax is AT&T syntax, rather than the more popular Intel syntax. This is due to technical constraints; assembly parsing is provided by LLVM and its support for Intel syntax is buggy and not well tested.

Some day Zig may have its own assembler. This would allow it to integrate more seamlessly into the language, as well as be compatible with the popular NASM syntax. This documentation section will be updated before 1.0.0 is released, with a conclusive statement about the status of AT&T vs Intel/NASM syntax.

## Output Constraints §

Output constraints are still considered to be unstable in Zig, and so LLVM documentation and GCC documentation must be used to understand the semantics.

Note that some breaking changes to output constraints are planned with issue #215.

## Input Constraints §

Input constraints are still considered to be unstable in Zig, and so LLVM documentation and GCC documentation must be used to understand the semantics.

Note that some breaking changes to input constraints are planned with issue #215.

## Clobbers §

Clobbers are the set of registers whose values will not be preserved by the execution of the assembly code. These do not include output or input registers. The special clobber value of `"memory"` means that the assembly causes writes to arbitrary undeclared memory locations - not only the memory pointed to by a declared indirect output.

Failure to declare the full set of clobbers for a given inline assembly expression is unchecked Undefined Behavior.

## Global Assembly §

When an assembly expression occurs in a top level comptime block, this is global assembly.

This kind of assembly has different rules than inline assembly. First, `volatile` is not valid because all global assembly is unconditionally included. Second, there are no inputs, outputs, or clobbers. All global assembly is concatenated verbatim into one long string and assembled together. There are no template substitution rules regarding `%` as there are in inline assembly expressions.

global-asm.zig

    const std = @import("std");
    const assert = std.debug.assert;

    comptime {
        asm (
            \\.global my_func;
            \\.type my_func, @function;
            \\my_func:
            \\  lea (%rdi,%rsi,1),%eax
            \\  retq
        );
    }

    extern fn my_func(a: i32, b: i32) i32;

    test "global assembly" {
        assert(my_func(12, 34) == 46);
    }

    $ zig test global-asm.zig -target x86_64-linux
    1/1 test "global assembly"...OK
    All 1 tests passed.

# Atomics §

TODO: @fence()

TODO: @atomic rmw

TODO: builtin atomic memory ordering enum

# Async Functions §

When a function is called, a frame is pushed to the stack, the function runs until it reaches a return statement, and then the frame is popped from the stack. At the callsite, the following code does not run until the function returns.

An async function is a function whose callsite is split into an `async` initiation, followed by an `await` completion. Its frame is provided explicitly by the caller, and it can be suspended and resumed any number of times.

Zig infers that a function is `async` when it observes that the function contains a suspension point. Async functions can be called the same as normal functions. A function call of an async function is a suspend point.

## Suspend and Resume §

At any point, a function may suspend itself. This causes control flow to return to the callsite (in the case of the first suspension), or resumer (in the case of subsequent suspensions).

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    var x: i32 = 1;

    test "suspend with no resume" {
        var frame = async func();
        assert(x == 2);
    }

    fn func() void {
        x += 1;
        suspend;
        // This line is never reached because the suspend has no matching resume.
        x += 1;
    }

    $ zig test test.zig
    1/1 test "suspend with no resume"...OK
    All 1 tests passed.

In the same way that each allocation should have a corresponding free, Each `suspend` should have a corresponding `resume`. A suspend block allows a function to put a pointer to its own frame somewhere, for example into an event loop, even if that action will perform a `resume` operation on a different thread. @frame provides access to the async function frame pointer.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    var the_frame: anyframe = undefined;
    var result = false;

    test "async function suspend with block" {
        _ = async testSuspendBlock();
        assert(!result);
        resume the_frame;
        assert(result);
    }

    fn testSuspendBlock() void {
        suspend {
            comptime assert(@TypeOf(@frame()) == *@Frame(testSuspendBlock));
            the_frame = @frame();
        }
        result = true;
    }

    $ zig test test.zig
    1/1 test "async function suspend with block"...OK
    All 1 tests passed.

`suspend` causes a function to be `async`.

### Resuming from Suspend Blocks §

Upon entering a `suspend` block, the async function is already considered suspended, and can be resumed. For example, if you started another kernel thread, and had that thread call `resume` on the frame pointer provided by the @frame, the new thread would begin executing after the suspend block, while the old thread continued executing the suspend block.

However, the async function can be directly resumed from the suspend block, in which case it never returns to its resumer and continues executing.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "resume from suspend" {
        var my_result: i32 = 1;
        _ = async testResumeFromSuspend(&my_result);
        std.debug.assert(my_result == 2);
    }
    fn testResumeFromSuspend(my_result: *i32) void {
        suspend {
            resume @frame();
        }
        my_result.* += 1;
        suspend;
        my_result.* += 1;
    }

    $ zig test test.zig
    1/1 test "resume from suspend"...OK
    All 1 tests passed.

This is guaranteed to tail call, and therefore will not cause a new stack frame.

## Async and Await §

In the same way that every `suspend` has a matching `resume`, every `async` has a matching `await`.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "async and await" {
        // Here we have an exception where we do not match an async
        // with an await. The test block is not async and so cannot
        // have a suspend point in it.
        // This is well-defined behavior, and everything is OK here.
        // Note however that there would be no way to collect the
        // return value of amain, if it were something other than void.
        _ = async amain();
    }

    fn amain() void {
        var frame = async func();
        comptime assert(@TypeOf(frame) == @Frame(func));

        const ptr: anyframe->void = &frame;
        const any_ptr: anyframe = ptr;

        resume any_ptr;
        await ptr;
    }

    fn func() void {
        suspend;
    }

    $ zig test test.zig
    1/1 test "async and await"...OK
    All 1 tests passed.

The `await` keyword is used to coordinate with an async function's `return` statement.

`await` is a suspend point, and takes as an operand anything that coerces to `anyframe->T`.

There is a common misconception that `await` resumes the target function. It is the other way around: it suspends until the target function completes. In the event that the target function has already completed, `await` does not suspend; instead it copies the return value directly from the target function's frame.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    var the_frame: anyframe = undefined;
    var final_result: i32 = 0;

    test "async function await" {
        seq('a');
        _ = async amain();
        seq('f');
        resume the_frame;
        seq('i');
        assert(final_result == 1234);
        assert(std.mem.eql(u8, &seq_points, "abcdefghi"));
    }
    fn amain() void {
        seq('b');
        var f = async another();
        seq('e');
        final_result = await f;
        seq('h');
    }
    fn another() i32 {
        seq('c');
        suspend {
            seq('d');
            the_frame = @frame();
        }
        seq('g');
        return 1234;
    }

    var seq_points = [_]u8{0} ** "abcdefghi".len;
    var seq_index: usize = 0;

    fn seq(c: u8) void {
        seq_points[seq_index] = c;
        seq_index += 1;
    }

    $ zig test test.zig
    1/1 test "async function await"...OK
    All 1 tests passed.

In general, `suspend` is lower level than `await`. Most application code will use only `async` and `await`, but event loop implementations will make use of `suspend` internally.

## Async Function Example §

Putting all of this together, here is an example of typical `async`/`await` usage:

async.zig

    const std = @import("std");
    const Allocator = std.mem.Allocator;

    pub fn main() void {
        _ = async amainWrap();

        // Typically we would use an event loop to manage resuming async functions,
        // but in this example we hard code what the event loop would do,
        // to make things deterministic.
        resume global_file_frame;
        resume global_download_frame;
    }

    fn amainWrap() void {
        amain() catch |e| {
            std.debug.warn("{}\n", .{e});
            if (@errorReturnTrace()) |trace| {
                std.debug.dumpStackTrace(trace.*);
            }
            std.process.exit(1);
        };
    }

    fn amain() !void {
        const allocator = std.heap.page_allocator;
        var download_frame = async fetchUrl(allocator, "https://example.com/");
        var awaited_download_frame = false;
        errdefer if (!awaited_download_frame) {
            if (await download_frame) |r| allocator.free(r) else |_| {}
        };

        var file_frame = async readFile(allocator, "something.txt");
        var awaited_file_frame = false;
        errdefer if (!awaited_file_frame) {
            if (await file_frame) |r| allocator.free(r) else |_| {}
        };

        awaited_file_frame = true;
        const file_text = try await file_frame;
        defer allocator.free(file_text);

        awaited_download_frame = true;
        const download_text = try await download_frame;
        defer allocator.free(download_text);

        std.debug.warn("download_text: {}\n", .{download_text});
        std.debug.warn("file_text: {}\n", .{file_text});
    }

    var global_download_frame: anyframe = undefined;
    fn fetchUrl(allocator: *Allocator, url: []const u8) ![]u8 {
        const result = try std.mem.dupe(allocator, u8, "this is the downloaded url contents");
        errdefer allocator.free(result);
        suspend {
            global_download_frame = @frame();
        }
        std.debug.warn("fetchUrl returning\n", .{});
        return result;
    }

    var global_file_frame: anyframe = undefined;
    fn readFile(allocator: *Allocator, filename: []const u8) ![]u8 {
        const result = try std.mem.dupe(allocator, u8, "this is the file contents");
        errdefer allocator.free(result);
        suspend {
            global_file_frame = @frame();
        }
        std.debug.warn("readFile returning\n", .{});
        return result;
    }

    $ zig build-exe async.zig
    $ ./async
    readFile returning
    fetchUrl returning
    download_text: this is the downloaded url contents
    file_text: this is the file contents

Now we remove the `suspend` and `resume` code, and observe the same behavior, with one tiny difference:

blocking.zig

    const std = @import("std");
    const Allocator = std.mem.Allocator;

    pub fn main() void {
        _ = async amainWrap();
    }

    fn amainWrap() void {
        amain() catch |e| {
            std.debug.warn("{}\n", .{e});
            if (@errorReturnTrace()) |trace| {
                std.debug.dumpStackTrace(trace.*);
            }
            std.process.exit(1);
        };
    }

    fn amain() !void {
        const allocator = std.heap.page_allocator;
        var download_frame = async fetchUrl(allocator, "https://example.com/");
        var awaited_download_frame = false;
        errdefer if (!awaited_download_frame) {
            if (await download_frame) |r| allocator.free(r) else |_| {}
        };

        var file_frame = async readFile(allocator, "something.txt");
        var awaited_file_frame = false;
        errdefer if (!awaited_file_frame) {
            if (await file_frame) |r| allocator.free(r) else |_| {}
        };

        awaited_file_frame = true;
        const file_text = try await file_frame;
        defer allocator.free(file_text);

        awaited_download_frame = true;
        const download_text = try await download_frame;
        defer allocator.free(download_text);

        std.debug.warn("download_text: {}\n", .{download_text});
        std.debug.warn("file_text: {}\n", .{file_text});
    }

    fn fetchUrl(allocator: *Allocator, url: []const u8) ![]u8 {
        const result = try std.mem.dupe(allocator, u8, "this is the downloaded url contents");
        errdefer allocator.free(result);
        std.debug.warn("fetchUrl returning\n", .{});
        return result;
    }

    fn readFile(allocator: *Allocator, filename: []const u8) ![]u8 {
        const result = try std.mem.dupe(allocator, u8, "this is the file contents");
        errdefer allocator.free(result);
        std.debug.warn("readFile returning\n", .{});
        return result;
    }

    $ zig build-exe blocking.zig
    $ ./blocking
    fetchUrl returning
    readFile returning
    download_text: this is the downloaded url contents
    file_text: this is the file contents

Previously, the `fetchUrl` and `readFile` functions suspended, and were resumed in an order determined by the `main` function. Now, since there are no suspend points, the order of the printed "... returning" messages is determined by the order of `async` callsites.

# Builtin Functions §

Builtin functions are provided by the compiler and are prefixed with `@`. The `comptime` keyword on a parameter means that the parameter must be known at compile time.

## @addWithOverflow §

    @addWithOverflow(comptime T: type, a: T, b: T, result: *T) bool

Performs `result.* = a + b`. If overflow or underflow occurs, stores the overflowed bits in `result` and returns `true`. If no overflow or underflow occurs, returns `false`.

## @alignCast §

    @alignCast(comptime alignment: u29, ptr: var) var

`ptr` can be `*T`, `fn()`, `?*T`, `?fn()`, or `[]T`. It returns the same type as `ptr` except with the alignment adjusted to the new value.

A pointer alignment safety check is added to the generated code to make sure the pointer is aligned as promised.

## @alignOf §

    @alignOf(comptime T: type) comptime_int

This function returns the number of bytes that this type should be aligned to for the current target to match the C ABI. When the child type of a pointer has this alignment, the alignment can be omitted from the type.

    const assert = @import("std").debug.assert;
    comptime {
        assert(*u32 == *align(@alignOf(u32)) u32);
    }

The result is a target-specific compile time constant. It is guaranteed to be less than or equal to @sizeOf(T).

See also:

- Alignment

## @as §

    @as(comptime T: type, expression) T

Performs Type Coercion. This cast is allowed when the conversion is unambiguous and safe, and is the preferred way to convert between types, whenever possible.

## @asyncCall §

    @asyncCall(frame_buffer: []align(@alignOf(@Frame(anyAsyncFunction))) u8, result_ptr, function_ptr, args: ...) anyframe->T

`@asyncCall` performs an `async` call on a function pointer, which may or may not be an async function.

The provided `frame_buffer` must be large enough to fit the entire function frame. This size can be determined with @frameSize. To provide a too-small buffer invokes safety-checked Undefined Behavior.

`result_ptr` is optional (null may be provided). If provided, the function call will write its result directly to the result pointer, which will be available to read after await completes. Any result location provided to `await` will copy the result from `result_ptr`.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "async fn pointer in a struct field" {
        var data: i32 = 1;
        const Foo = struct {
            bar: async fn (*i32) void,
        };
        var foo = Foo{ .bar = func };
        var bytes: [64]u8 align(@alignOf(@Frame(func))) = undefined;
        const f = @asyncCall(&bytes, {}, foo.bar, &data);
        assert(data == 2);
        resume f;
        assert(data == 4);
    }

    async fn func(y: *i32) void {
        defer y.* += 2;
        y.* += 1;
        suspend;
    }

    $ zig test test.zig
    1/1 test "async fn pointer in a struct field"...OK
    All 1 tests passed.

## @atomicLoad §

    @atomicLoad(comptime T: type, ptr: *const T, comptime ordering: builtin.AtomicOrder) T

This builtin function atomically dereferences a pointer and returns the value.

`T` must be a `bool`, a float, an integer or an enum.

## @atomicRmw §

    @atomicRmw(comptime T: type, ptr: *T, comptime op: builtin.AtomicRmwOp, operand: T, comptime ordering: builtin.AtomicOrder) T

This builtin function atomically modifies memory and then returns the previous value.

`T` must be a `bool`, a float, an integer or an enum.

Supported operations:

- `.Xchg` - stores the operand unmodified. Supports enums, integers and floats.
- `.Add` - for integers, twos complement wraparound addition. Also supports Floats.
- `.Sub` - for integers, twos complement wraparound subtraction. Also supports Floats.
- `.And` - bitwise and
- `.Nand` - bitwise nand
- `.Or` - bitwise or
- `.Xor` - bitwise xor
- `.Max` - stores the operand if it is larger. Supports integers and floats.
- `.Min` - stores the operand if it is smaller. Supports integers and floats.

## @atomicStore §

    @atomicStore(comptime T: type, ptr: *T, value: T, comptime ordering: builtin.AtomicOrder) void

This builtin function atomically stores a value.

`T` must be a `bool`, a float, an integer or an enum.

## @bitCast §

    @bitCast(comptime DestType: type, value: var) DestType

Converts a value of one type to another type.

Asserts that `@sizeOf(@TypeOf(value)) == @sizeOf(DestType)`.

Asserts that `@typeInfo(DestType) != .Pointer`. Use `@ptrCast` or `@intToPtr` if you need this.

Can be used for these things for example:

- Convert `f32` to `u32` bits
- Convert `i32` to `u32` preserving twos complement

Works at compile-time if `value` is known at compile time. It's a compile error to bitcast a struct to a scalar type of the same size since structs have undefined layout. However if the struct is packed then it works.

## @bitOffsetOf §

    @bitOffsetOf(comptime T: type, comptime field_name: []const u8) comptime_int

Returns the bit offset of a field relative to its containing struct.

For non packed structs, this will always be divisible by `8`. For packed structs, non-byte-aligned fields will share a byte offset, but they will have different bit offsets.

See also:

- @byteOffsetOf

## @boolToInt §

    @boolToInt(value: bool) u1

Converts `true` to `u1(1)` and `false` to `u1(0)`.

If the value is known at compile-time, the return type is `comptime_int` instead of `u1`.

## @bitSizeOf §

    @bitSizeOf(comptime T: type) comptime_int

This function returns the number of bits it takes to store `T` in memory. The result is a target-specific compile time constant.

This function measures the size at runtime. For types that are disallowed at runtime, such as `comptime_int` and `type`, the result is `0`.

See also:

- @sizeOf
- @typeInfo

## @breakpoint §

    @breakpoint()

This function inserts a platform-specific debug trap instruction which causes debuggers to break there.

This function is only valid within function scope.

## @mulAdd §

    @mulAdd(comptime T: type, a: T, b: T, c: T) T

Fused multiply add, similar to `(a * b) + c`, except only rounds once, and is thus more accurate.

Supports Floats and Vectors of floats.

## @byteSwap §

    @byteSwap(comptime T: type, operand: T) T

`T` must be an integer type with bit count evenly divisible by 8.

`operand` may be an integer or vector.

Swaps the byte order of the integer. This converts a big endian integer to a little endian integer, and converts a little endian integer to a big endian integer.

Note that for the purposes of memory layout with respect to endianness, the integer type should be related to the number of bytes reported by @sizeOf bytes. This is demonstrated with `u24`. `@sizeOf(u24) == 4`, which means that a `u24` stored in memory takes 4 bytes, and those 4 bytes are what are swapped on a little vs big endian system. On the other hand, if `T` is specified to be `u24`, then only 3 bytes are reversed.

## @bitReverse §

    @bitReverse(comptime T: type, integer: T) T

`T` accepts any integer type.

Reverses the bitpattern of an integer value, including the sign bit if applicable.

For example 0b10110110 (`u8 = 182`, `i8 = -74`) becomes 0b01101101 (`u8 = 109`, `i8 = 109`).

## @byteOffsetOf §

    @byteOffsetOf(comptime T: type, comptime field_name: []const u8) comptime_int

Returns the byte offset of a field relative to its containing struct.

See also:

- @bitOffsetOf

## @call §

    @call(options: std.builtin.CallOptions, function: var, args: var) var

Calls a function, in the same way that invoking an expression with parentheses does:

call.zig

    const assert = @import("std").debug.assert;

    test "noinline function call" {
        assert(@call(.{}, add, .{3, 9}) == 12);
    }

    fn add(a: i32, b: i32) i32 {
        return a + b;
    }

    $ zig test call.zig
    1/1 test "noinline function call"...OK
    All 1 tests passed.

`@call` allows more flexibility than normal function call syntax does. The `CallOptions` struct is reproduced here:

    pub const CallOptions = struct {
        modifier: Modifier = .auto,
        stack: ?[]align(std.Target.stack_align) u8 = null,

        pub const Modifier = enum {
            /// Equivalent to function call syntax.
            auto,

            /// Equivalent to async keyword used with function call syntax.
            async_kw,

            /// Prevents tail call optimization. This guarantees that the return
            /// address will point to the callsite, as opposed to the callsite's
            /// callsite. If the call is otherwise required to be tail-called
            /// or inlined, a compile error is emitted instead.
            never_tail,

            /// Guarantees that the call will not be inlined. If the call is
            /// otherwise required to be inlined, a compile error is emitted instead.
            never_inline,

            /// Asserts that the function call will not suspend. This allows a
            /// non-async function to call an async function.
            no_async,

            /// Guarantees that the call will be generated with tail call optimization.
            /// If this is not possible, a compile error is emitted instead.
            always_tail,

            /// Guarantees that the call will inlined at the callsite.
            /// If this is not possible, a compile error is emitted instead.
            always_inline,

            /// Evaluates the call at compile-time. If the call cannot be completed at
            /// compile-time, a compile error is emitted instead.
            compile_time,
        };
    };

## @cDefine §

    @cDefine(comptime name: []u8, value)

This function can only occur inside `@cImport`.

This appends `#define $name $value` to the `@cImport` temporary buffer.

To define without a value, like this:

    #define _GNU_SOURCE

Use the void value, like this:

    @cDefine("_GNU_SOURCE", {})

See also:

- Import from C Header File
- @cInclude
- @cImport
- @cUndef
- void

## @cImport §

    @cImport(expression) type

This function parses C code and imports the functions, types, variables, and compatible macro definitions into a new empty struct type, and then returns that type.

`expression` is interpreted at compile time. The builtin functions `@cInclude`, `@cDefine`, and `@cUndef` work within this expression, appending to a temporary buffer which is then parsed as C code.

Usually you should only have one `@cImport` in your entire application, because it saves the compiler from invoking clang multiple times, and prevents inline functions from being duplicated.

Reasons for having multiple `@cImport` expressions would be:

- To avoid a symbol collision, for example if foo.h and bar.h both `#define CONNECTION_COUNT`
- To analyze the C code with different preprocessor defines

See also:

- Import from C Header File
- @cInclude
- @cDefine
- @cUndef

## @cInclude §

    @cInclude(comptime path: []u8)

This function can only occur inside `@cImport`.

This appends `#include <$path>\n` to the `c_import` temporary buffer.

See also:

- Import from C Header File
- @cImport
- @cDefine
- @cUndef

## @clz §

    @clz(comptime T: type, integer: T)

This function counts the number of most-significant (leading in a big-Endian sense) zeroes in `integer`.

If `integer` is known at comptime, the return type is `comptime_int`. Otherwise, the return type is an unsigned integer with the minimum number of bits that can represent the bit count of the integer type.

If `integer` is zero, `@clz` returns the bit width of integer type `T`.

See also:

- @ctz
- @popCount

## @cmpxchgStrong §

    @cmpxchgStrong(comptime T: type, ptr: *T, expected_value: T, new_value: T, success_order: AtomicOrder, fail_order: AtomicOrder) ?T

This function performs a strong atomic compare exchange operation. It's the equivalent of this code, except atomic:

    fn cmpxchgStrongButNotAtomic(comptime T: type, ptr: *T, expected_value: T, new_value: T) ?T {
        const old_value = ptr.*;
        if (old_value == expected_value) {
            ptr.* = new_value;
            return null;
        } else {
            return old_value;
        }
    }

If you are using cmpxchg in a loop, @cmpxchgWeak is the better choice, because it can be implemented more efficiently in machine instructions.

`T` must be a `bool`, a float, an integer or an enum.

`@TypeOf(ptr).alignment` must be `>= @sizeOf(T).`

See also:

- Compile Variables
- cmpxchgWeak

## @cmpxchgWeak §

    @cmpxchgWeak(comptime T: type, ptr: *T, expected_value: T, new_value: T, success_order: AtomicOrder, fail_order: AtomicOrder) ?T

This function performs a weak atomic compare exchange operation. It's the equivalent of this code, except atomic:

    fn cmpxchgWeakButNotAtomic(comptime T: type, ptr: *T, expected_value: T, new_value: T) ?T {
        const old_value = ptr.*;
        if (old_value == expected_value and usuallyTrueButSometimesFalse()) {
            ptr.* = new_value;
            return null;
        } else {
            return old_value;
        }
    }

If you are using cmpxchg in a loop, the sporadic failure will be no problem, and `cmpxchgWeak` is the better choice, because it can be implemented more efficiently in machine instructions. However if you need a stronger guarantee, use @cmpxchgStrong.

`T` must be a `bool`, a float, an integer or an enum.

`@TypeOf(ptr).alignment` must be `>= @sizeOf(T).`

See also:

- Compile Variables
- cmpxchgStrong

## @compileError §

    @compileError(comptime msg: []u8)

This function, when semantically analyzed, causes a compile error with the message `msg`.

There are several ways that code avoids being semantically checked, such as using `if` or `switch` with compile time constants, and `comptime` functions.

## @compileLog §

    @compileLog(args: ...)

This function prints the arguments passed to it at compile-time.

To prevent accidentally leaving compile log statements in a codebase, a compilation error is added to the build, pointing to the compile log statement. This error prevents code from being generated, but does not otherwise interfere with analysis.

This function can be used to do "printf debugging" on compile-time executing code.

test.zig

    const warn = @import("std").debug.warn;

    const num1 = blk: {
        var val1: i32 = 99;
        @compileLog("comptime val1 = ", val1);
        val1 = val1 + 1;
        break :blk val1;
    };

    test "main" {
        @compileLog("comptime in main");

        warn("Runtime in main, num1 = {}.\n", .{num1});
    }

    $ zig test test.zig
    | *"comptime in main"
    | *"comptime val1 = ", 99
    ./docgen_tmp/test.zig:11:5: error: found compile log statement
        @compileLog("comptime in main");
        ^
    ./docgen_tmp/test.zig:1:34: note: referenced here
    const warn = @import("std").debug.warn;
                                     ^
    ./docgen_tmp/test.zig:13:5: note: referenced here
        warn("Runtime in main, num1 = {}.\n", .{num1});
        ^
    ./docgen_tmp/test.zig:5:5: error: found compile log statement
        @compileLog("comptime val1 = ", val1);
        ^
    ./docgen_tmp/test.zig:13:45: note: referenced here
        warn("Runtime in main, num1 = {}.\n", .{num1});
                                                ^

will ouput:

If all `@compileLog` calls are removed or not encountered by analysis, the program compiles successfully and the generated executable prints:

test.zig

    const warn = @import("std").debug.warn;

    const num1 = blk: {
        var val1: i32 = 99;
        val1 = val1 + 1;
        break :blk val1;
    };

    test "main" {
        warn("Runtime in main, num1 = {}.\n", .{num1});
    }

    $ zig test test.zig
    1/1 test "main"...Runtime in main, num1 = 100.
    OK
    All 1 tests passed.

## @ctz §

    @ctz(comptime T: type, integer: T)

This function counts the number of least-significant (trailing in a big-Endian sense) zeroes in `integer`.

If `integer` is known at comptime, the return type is `comptime_int`. Otherwise, the return type is an unsigned integer with the minimum number of bits that can represent the bit count of the integer type.

If `integer` is zero, `@ctz` returns the bit width of integer type `T`.

See also:

- @clz
- @popCount

## @cUndef §

    @cUndef(comptime name: []u8)

This function can only occur inside `@cImport`.

This appends `#undef $name` to the `@cImport` temporary buffer.

See also:

- Import from C Header File
- @cImport
- @cDefine
- @cInclude

## @divExact §

    @divExact(numerator: T, denominator: T) T

Exact division. Caller guarantees `denominator != 0` and `@divTrunc(numerator, denominator) * denominator == numerator`.

- `@divExact(6, 3) == 2`
- `@divExact(a, b) * b == a`

For a function that returns a possible error code, use `@import("std").math.divExact`.

See also:

- @divTrunc
- @divFloor

## @divFloor §

    @divFloor(numerator: T, denominator: T) T

Floored division. Rounds toward negative infinity. For unsigned integers it is the same as `numerator / denominator`. Caller guarantees `denominator != 0` and `!(@typeInfo(T) == .Int and T.is_signed and numerator == std.math.minInt(T) and denominator == -1)`.

- `@divFloor(-5, 3) == -2`
- `@divFloor(a, b) + @mod(a, b) == a`

For a function that returns a possible error code, use `@import("std").math.divFloor`.

See also:

- @divTrunc
- @divExact

## @divTrunc §

    @divTrunc(numerator: T, denominator: T) T

Truncated division. Rounds toward zero. For unsigned integers it is the same as `numerator / denominator`. Caller guarantees `denominator != 0` and `!(@typeInfo(T) == .Int and T.is_signed and numerator == std.math.minInt(T) and denominator == -1)`.

- `@divTrunc(-5, 3) == -1`
- `@divTrunc(a, b) + @rem(a, b) == a`

For a function that returns a possible error code, use `@import("std").math.divTrunc`.

See also:

- @divFloor
- @divExact

## @embedFile §

    @embedFile(comptime path: []const u8) *const [X:0]u8

This function returns a compile time constant pointer to null-terminated, fixed-size array with length equal to the byte count of the file given by `path`. The contents of the array are the contents of the file. This is equivalent to a string literal with the file contents.

`path` is absolute or relative to the current file, just like `@import`.

See also:

- @import

## @enumToInt §

    @enumToInt(enum_or_tagged_union: var) var

Converts an enumeration value into its integer tag type. When a tagged union is passed, the tag value is used as the enumeration value.

If there is only one possible enum value, the resut is a `comptime_int` known at comptime.

See also:

- @intToEnum

## @errorName §

    @errorName(err: anyerror) []const u8

This function returns the string representation of an error. The string representation of `error.OutOfMem` is `"OutOfMem"`.

If there are no calls to `@errorName` in an entire application, or all calls have a compile-time known value for `err`, then no error name table will be generated.

## @errorReturnTrace §

    @errorReturnTrace() ?*builtin.StackTrace

If the binary is built with error return tracing, and this function is invoked in a function that calls a function with an error or error union return type, returns a stack trace object. Otherwise returns `null`.

## @errorToInt §

    @errorToInt(err: var) std.meta.IntType(false, @sizeOf(anyerror) * 8)

Supports the following types:

- The Global Error Set
- Error Set Type
- Error Union Type

Converts an error to the integer representation of an error.

It is generally recommended to avoid this cast, as the integer representation of an error is not stable across source code changes.

See also:

- @intToError

## @errSetCast §

    @errSetCast(comptime T: DestType, value: var) DestType

Converts an error value from one error set to another error set. Attempting to convert an error which is not in the destination error set results in safety-protected Undefined Behavior.

## @export §

    @export(target: var, comptime options: std.builtin.ExportOptions) void

Creates a symbol in the output object file.

This function can be called from a comptime block to conditionally export symbols. When `target` is a function with the C calling convention and `options.linkage` is `Strong`, this is equivalent to the `export` keyword used on a function:

test.zig

    comptime {
        @export(internalName, .{ .name = "foo", .linkage = .Strong });
    }

    fn internalName() callconv(.C) void {}

    $ zig build-obj test.zig

This is equivalent to:

test.zig

    export fn foo() void {}

    $ zig build-obj test.zig

Note that even when using `export`, `@"foo"` syntax can be used to choose any string for the symbol name:

test.zig

    export fn @"A function name that is a complete sentence."() void {}

    $ zig build-obj test.zig

When looking at the resulting object, you can see the symbol is used verbatim:

    00000000000001f0 T A function name that is a complete sentence.

See also:

- Exporting a C Library

## @fence §

    @fence(order: AtomicOrder)

The `fence` function is used to introduce happens-before edges between operations.

`AtomicOrder` can be found with `@import("builtin").AtomicOrder`.

See also:

- Compile Variables

## @field §

    @field(lhs: var, comptime field_name: []const u8) (field)

Performs field access by a compile-time string.

test.zig

    const std = @import("std");

    const Point = struct {
        x: u32,
        y: u32
    };

    test "field access by string" {
        const assert = std.debug.assert;
        var p = Point {.x = 0, .y = 0};

        @field(p, "x") = 4;
        @field(p, "y") = @field(p, "x") + 1;

        assert(@field(p, "x") == 4);
        assert(@field(p, "y") == 5);
    }

    $ zig test test.zig
    1/1 test "field access by string"...OK
    All 1 tests passed.

## @fieldParentPtr §

    @fieldParentPtr(comptime ParentType: type, comptime field_name: []const u8,
        field_ptr: *T) *ParentType

Given a pointer to a field, returns the base pointer of a struct.

## @floatCast §

    @floatCast(comptime DestType: type, value: var) DestType

Convert from one float type to another. This cast is safe, but may cause the numeric value to lose precision.

## @floatToInt §

    @floatToInt(comptime DestType: type, float: var) DestType

Converts the integer part of a floating point number to the destination type.

If the integer part of the floating point number cannot fit in the destination type, it invokes safety-checked Undefined Behavior.

See also:

- @intToFloat

## @frame §

    @frame() *@Frame(func)

This function returns a pointer to the frame for a given function. This type can be coerced to `anyframe->T` and to `anyframe`, where `T` is the return type of the function in scope.

This function does not mark a suspension point, but it does cause the function in scope to become an async function.

## @Frame §

    @Frame(func: var) type

This function returns the frame type of a function. This works for Async Functions as well as any function without a specific calling convention.

This type is suitable to be used as the return type of async which allows one to, for example, heap-allocate an async function frame:

test.zig

    const std = @import("std");

    test "heap allocated frame" {
        const frame = try std.heap.page_allocator.create(@Frame(func));
        frame.* = async func();
    }

    fn func() void {
        suspend;
    }

    $ zig test test.zig
    1/1 test "heap allocated frame"...OK
    All 1 tests passed.

## @frameAddress §

    @frameAddress() usize

This function returns the base pointer of the current stack frame.

The implications of this are target specific and not consistent across all platforms. The frame address may not be available in release mode due to aggressive optimizations.

This function is only valid within function scope.

## @frameSize §

    @frameSize() usize

This is the same as `@sizeOf(@Frame(func))`, where `func` may be runtime-known.

This function is typically used in conjunction with @asyncCall.

## @hasDecl §

    @hasDecl(comptime Container: type, comptime name: []const u8) bool

Returns whether or not a struct, enum, or union has a declaration matching `name`.

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    const Foo = struct {
        nope: i32,

        pub var blah = "xxx";
        const hi = 1;
    };

    test "@hasDecl" {
        assert(@hasDecl(Foo, "blah"));

        // Even though `hi` is private, @hasDecl returns true because this test is
        // in the same file scope as Foo. It would return false if Foo was declared
        // in a different file.
        assert(@hasDecl(Foo, "hi"));

        // @hasDecl is for declarations; not fields.
        assert(!@hasDecl(Foo, "nope"));
        assert(!@hasDecl(Foo, "nope1234"));
    }

    $ zig test test.zig
    1/1 test "@hasDecl"...OK
    All 1 tests passed.

See also:

- @hasField

## @hasField §

    @hasField(comptime Container: type, comptime name: []const u8) bool

Returns whether the field name of a struct, union, or enum exists.

The result is a compile time constant.

It does not include functions, variables, or constants.

See also:

- @hasDecl

## @import §

    @import(comptime path: []u8) type

This function finds a zig file corresponding to `path` and adds it to the build, if it is not already added.

Zig source files are implicitly structs, with a name equal to the file's basename with the extension truncated. `@import` returns the struct type corresponding to the file.

Declarations which have the `pub` keyword may be referenced from a different source file than the one they are declared in.

`path` can be a relative or absolute path, or it can be the name of a package. If it is a relative path, it is relative to the file that contains the `@import` function call.

The following packages are always available:

- `@import("std")` - Zig Standard Library
- `@import("builtin")` - Compiler-provided types and variables. The command `zig builtin` outputs the source to stdout for reference.

See also:

- Compile Variables
- @embedFile

## @intCast §

    @intCast(comptime DestType: type, int: var) DestType

Converts an integer to another integer while keeping the same numerical value. Attempting to convert a number which is out of range of the destination type results in safety-protected Undefined Behavior.

If `T` is `comptime_int`, then this is semantically equivalent to Type Coercion.

## @intToEnum §

    @intToEnum(comptime DestType: type, int_value: @TagType(DestType)) DestType

Converts an integer into an enum value.

Attempting to convert an integer which represents no value in the chosen enum type invokes safety-checked Undefined Behavior.

See also:

- @enumToInt

## @intToError §

    @intToError(value: std.meta.IntType(false, @sizeOf(anyerror) * 8)) anyerror

Converts from the integer representation of an error into The Global Error Set type.

It is generally recommended to avoid this cast, as the integer representation of an error is not stable across source code changes.

Attempting to convert an integer that does not correspond to any error results in safety-protected Undefined Behavior.

See also:

- @errorToInt

## @intToFloat §

    @intToFloat(comptime DestType: type, int: var) DestType

Converts an integer to the closest floating point representation. To convert the other way, use @floatToInt. This cast is always safe.

## @intToPtr §

    @intToPtr(comptime DestType: type, address: usize) DestType

Converts an integer to a pointer. To convert the other way, use @ptrToInt.

If the destination pointer type does not allow address zero and `address` is zero, this invokes safety-checked Undefined Behavior.

## @memcpy §

    @memcpy(noalias dest: [*]u8, noalias source: [*]const u8, byte_count: usize)

This function copies bytes from one region of memory to another. `dest` and `source` are both pointers and must not overlap.

This function is a low level intrinsic with no safety mechanisms. Most code should not use this function, instead using something like this:

    for (source[0..byte_count]) |b, i| dest[i] = b;

The optimizer is intelligent enough to turn the above snippet into a memcpy.

There is also a standard library function for this:

    const mem = @import("std").mem;
    mem.copy(u8, dest[0..byte_count], source[0..byte_count]);

## @memset §

    @memset(dest: [*]u8, c: u8, byte_count: usize)

This function sets a region of memory to `c`. `dest` is a pointer.

This function is a low level intrinsic with no safety mechanisms. Most code should not use this function, instead using something like this:

    for (dest[0..byte_count]) |*b| b.* = c;

The optimizer is intelligent enough to turn the above snippet into a memset.

There is also a standard library function for this:

    const mem = @import("std").mem;
    mem.set(u8, dest, c);

## @mod §

    @mod(numerator: T, denominator: T) T

Modulus division. For unsigned integers this is the same as `numerator % denominator`. Caller guarantees `denominator > 0`.

- `@mod(-5, 3) == 1`
- `@divFloor(a, b) + @mod(a, b) == a`

For a function that returns an error code, see `@import("std").math.mod`.

See also:

- @rem

## @mulWithOverflow §

    @mulWithOverflow(comptime T: type, a: T, b: T, result: *T) bool

Performs `result.* = a * b`. If overflow or underflow occurs, stores the overflowed bits in `result` and returns `true`. If no overflow or underflow occurs, returns `false`.

## @OpaqueType §

    @OpaqueType() type

Creates a new type with an unknown (but non-zero) size and alignment.

This is typically used for type safety when interacting with C code that does not expose struct details. Example:

test.zig

    const Derp = @OpaqueType();
    const Wat = @OpaqueType();

    extern fn bar(d: *Derp) void;
    fn foo(w: *Wat) callconv(.C) void {
        bar(w);
    }

    test "call foo" {
        foo(undefined);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:6:9: error: expected type '*Derp', found '*Wat'
        bar(w);
            ^
    ./docgen_tmp/test.zig:6:9: note: pointer type child 'Wat' cannot cast into pointer type child 'Derp'
        bar(w);
            ^

## @panic §

    @panic(message: []const u8) noreturn

Invokes the panic handler function. By default the panic handler function calls the public `panic` function exposed in the root source file, or if there is not one specified, the `std.builtin.default_panic` function from `std/builtin.zig`.

Generally it is better to use `@import("std").debug.panic`. However, `@panic` can be useful for 2 scenarios:

- From library code, calling the programmer's panic function if they exposed one in the root source file.
- When mixing C and Zig code, calling the canonical panic implementation across multiple .o files.

See also:

- Root Source File

## @popCount §

    @popCount(comptime T: type, integer: T)

Counts the number of bits set in an integer.

If `integer` is known at comptime, the return type is `comptime_int`. Otherwise, the return type is an unsigned integer with the minimum number of bits that can represent the bit count of the integer type.

See also:

- @ctz
- @clz

## @ptrCast §

    @ptrCast(comptime DestType: type, value: var) DestType

Converts a pointer of one type to a pointer of another type.

Optional Pointers are allowed. Casting an optional pointer which is null to a non-optional pointer invokes safety-checked Undefined Behavior.

## @ptrToInt §

    @ptrToInt(value: var) usize

Converts `value` to a `usize` which is the address of the pointer. `value` can be one of these types:

- `*T`
- `?*T`
- `fn()`
- `?fn()`

To convert the other way, use @intToPtr

## @rem §

    @rem(numerator: T, denominator: T) T

Remainder division. For unsigned integers this is the same as `numerator % denominator`. Caller guarantees `denominator > 0`.

- `@rem(-5, 3) == -2`
- `@divTrunc(a, b) + @rem(a, b) == a`

For a function that returns an error code, see `@import("std").math.rem`.

See also:

- @mod

## @returnAddress §

    @returnAddress() usize

This function returns the address of the next machine code instruction that will be executed when the current function returns.

The implications of this are target specific and not consistent across all platforms.

This function is only valid within function scope. If the function gets inlined into a calling function, the returned address will apply to the calling function.

## @setAlignStack §

    @setAlignStack(comptime alignment: u29)

Ensures that a function will have a stack alignment of at least `alignment` bytes.

## @setCold §

    @setCold(is_cold: bool)

Tells the optimizer that a function is rarely called.

## @setEvalBranchQuota §

    @setEvalBranchQuota(new_quota: usize)

Changes the maximum number of backwards branches that compile-time code execution can use before giving up and making a compile error.

If the `new_quota` is smaller than the default quota (`1000`) or a previously explicitly set quota, it is ignored.

Example:

test.zig

    test "foo" {
        comptime {
            var i = 0;
            while (i < 1001) : (i += 1) {}
        }
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:4:9: error: evaluation exceeded 1000 backwards branches
            while (i < 1001) : (i += 1) {}
            ^
    ./docgen_tmp/test.zig:1:12: note: referenced here
    test "foo" {
               ^

Now we use `@setEvalBranchQuota`:

test.zig

    test "foo" {
        comptime {
            @setEvalBranchQuota(1001);
            var i = 0;
            while (i < 1001) : (i += 1) {}
        }
    }

    $ zig test test.zig
    1/1 test "foo"...OK
    All 1 tests passed.

See also:

- comptime

## @setFloatMode §

    @setFloatMode(mode: @import("builtin").FloatMode)

Sets the floating point mode of the current scope. Possible values are:

    pub const FloatMode = enum {
        Strict,
        Optimized,
    };

- `Strict` (default) - Floating point operations follow strict IEEE compliance.
- `Optimized` - Floating point operations may do all of the following:
  - Assume the arguments and result are not NaN. Optimizations are required to retain defined behavior over NaNs, but the value of the result is undefined.
  - Assume the arguments and result are not +/-Inf. Optimizations are required to retain defined behavior over +/-Inf, but the value of the result is undefined.
  - Treat the sign of a zero argument or result as insignificant.
  - Use the reciprocal of an argument rather than perform division.
  - Perform floating-point contraction (e.g. fusing a multiply followed by an addition into a fused multiply-and-add).
  - Perform algebraically equivalent transformations that may change results in floating point (e.g. reassociate).
  This is equivalent to `-ffast-math` in GCC.

The floating point mode is inherited by child scopes, and can be overridden in any scope. You can set the floating point mode in a struct or module scope by using a comptime block.

See also:

- Floating Point Operations

## @setRuntimeSafety §

    @setRuntimeSafety(safety_on: bool)

Sets whether runtime safety checks are enabled for the scope that contains the function call.

test.zig

    test "@setRuntimeSafety" {
        // The builtin applies to the scope that it is called in. So here, integer overflow
        // will not be caught in ReleaseFast and ReleaseSmall modes:
        // var x: u8 = 255;
        // x += 1; // undefined behavior in ReleaseFast/ReleaseSmall modes.
        {
            // However this block has safety enabled, so safety checks happen here,
            // even in ReleaseFast and ReleaseSmall modes.
            @setRuntimeSafety(true);
            var x: u8 = 255;
            x += 1;

            {
                // The value can be overridden at any scope. So here integer overflow
                // would not be caught in any build mode.
                @setRuntimeSafety(false);
                // var x: u8 = 255;
                // x += 1; // undefined behavior in all build modes.
            }
        }
    }

    $ zig test test.zig --release-fast
    1/1 test "@setRuntimeSafety"...integer overflow

    Tests failed. Use the following command to reproduce the failure:
    /home/andy/dev/zig/docgen_tmp/test

Note: it is planned to replace `@setRuntimeSafety` with `@optimizeFor`

## @shlExact §

    @shlExact(value: T, shift_amt: Log2T) T

Performs the left shift operation (`<<`). Caller guarantees that the shift will not shift any 1 bits out.

The type of `shift_amt` is an unsigned integer with `log2(T.bit_count)` bits. This is because `shift_amt >= T.bit_count` is undefined behavior.

See also:

- @shrExact
- @shlWithOverflow

## @shlWithOverflow §

    @shlWithOverflow(comptime T: type, a: T, shift_amt: Log2T, result: *T) bool

Performs `result.* = a << b`. If overflow or underflow occurs, stores the overflowed bits in `result` and returns `true`. If no overflow or underflow occurs, returns `false`.

The type of `shift_amt` is an unsigned integer with `log2(T.bit_count)` bits. This is because `shift_amt >= T.bit_count` is undefined behavior.

See also:

- @shlExact
- @shrExact

## @shrExact §

    @shrExact(value: T, shift_amt: Log2T) T

Performs the right shift operation (`>>`). Caller guarantees that the shift will not shift any 1 bits out.

The type of `shift_amt` is an unsigned integer with `log2(T.bit_count)` bits. This is because `shift_amt >= T.bit_count` is undefined behavior.

See also:

- @shlExact
- @shlWithOverflow

## @shuffle §

    @shuffle(comptime E: type, a: @Vector(a_len, E), b: @Vector(b_len, E), comptime mask: @Vector(mask_len, i32)) @Vector(mask_len, E)

Constructs a new vector by selecting elements from `a` and `b` based on `mask`.

Each element in `mask` selects an element from either `a` or `b`. Positive numbers select from `a` starting at 0. Negative values select from `b`, starting at `-1` and going down. It is recommended to use the `~` operator from indexes from `b` so that both indexes can start from `0` (i.e. `~@as(i32, 0)` is `-1`).

For each element of `mask`, if it or the selected value from `a` or `b` is `undefined`, then the resulting element is `undefined`.

`a_len` and `b_len` may differ in length. Out-of-bounds element indexes in `mask` result in compile errors.

If `a` or `b` is `undefined`, it is equivalent to a vector of all `undefined` with the same length as the other vector. If both vectors are `undefined`, `@shuffle` returns a vector with all elements `undefined`.

`E` must be an integer, float, pointer, or `bool`. The mask may be any vector length, and its length determines the result length.

See also:

- SIMD

## @sizeOf §

    @sizeOf(comptime T: type) comptime_int

This function returns the number of bytes it takes to store `T` in memory. The result is a target-specific compile time constant.

This size may contain padding bytes. If there were two consecutive T in memory, this would be the offset in bytes between element at index 0 and the element at index 1. For integer, consider whether you want to use `@sizeOf(T)` or `@typeInfo(T).Int.bits`.

This function measures the size at runtime. For types that are disallowed at runtime, such as `comptime_int` and `type`, the result is `0`.

See also:

- @bitSizeOf
- @typeInfo

## @splat §

    @splat(comptime len: u32, scalar: var) @Vector(len, @TypeOf(scalar))

Produces a vector of length `len` where each element is the value `scalar`:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "vector @splat" {
        const scalar: u32 = 5;
        const result = @splat(4, scalar);
        comptime assert(@TypeOf(result) == @Vector(4, u32));
        assert(std.mem.eql(u32, &@as([4]u32, result), &[_]u32{ 5, 5, 5, 5 }));
    }

    $ zig test test.zig
    1/1 test "vector @splat"...OK
    All 1 tests passed.

`scalar` must be an integer, bool, float, or pointer.

See also:

- Vectors
- @shuffle

## @sqrt §

    @sqrt(value: var) @TypeOf(value)

Performs the square root of a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @sin §

    @sin(value: var) @TypeOf(value)

Sine trigometric function on a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @cos §

    @cos(value: var) @TypeOf(value)

Cosine trigometric function on a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @exp §

    @exp(value: var) @TypeOf(value)

Base-e exponential function on a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @exp2 §

    @exp2(value: var) @TypeOf(value)

Base-2 exponential function on a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @log §

    @log(value: var) @TypeOf(value)

Returns the natural logarithm of a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @log2 §

    @log2(value: var) @TypeOf(value)

Returns the logarithm to the base 2 of a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @log10 §

    @log10(value: var) @TypeOf(value)

Returns the logarithm to the base 10 of a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @fabs §

    @fabs(value: var) @TypeOf(value)

Returns the absolute value of a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @floor §

    @floor(value: var) @TypeOf(value)

Returns the largest integral value not greater than the given floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @ceil §

    @ceil(value: var) @TypeOf(value)

Returns the largest integral value not less than the given floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @trunc §

    @trunc(value: var) @TypeOf(value)

Rounds the given floating point number to an integer, towards zero. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @round §

    @round(value: var) @TypeOf(value)

Rounds the given floating point number to an integer, away from zero. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

## @subWithOverflow §

    @subWithOverflow(comptime T: type, a: T, b: T, result: *T) bool

Performs `result.* = a - b`. If overflow or underflow occurs, stores the overflowed bits in `result` and returns `true`. If no overflow or underflow occurs, returns `false`.

## @tagName §

    @tagName(value: var) []const u8

Converts an enum value or union value to a slice of bytes representing the name.

If the enum is non-exhaustive and the tag value does not map to a name, it invokes safety-checked Undefined Behavior.

## @TagType §

    @TagType(T: type) type

For an enum, returns the integer type that is used to store the enumeration value.

For a union, returns the enum type that is used to store the tag value.

## @This §

    @This() type

Returns the innermost struct or union that this function call is inside. This can be useful for an anonymous struct that needs to refer to itself:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "@This()" {
        var items = [_]i32{ 1, 2, 3, 4 };
        const list = List(i32){ .items = items[0..] };
        assert(list.length() == 4);
    }

    fn List(comptime T: type) type {
        return struct {
            const Self = @This();

            items: []T,

            fn length(self: Self) usize {
                return self.items.len;
            }
        };
    }

    $ zig test test.zig
    1/1 test "@This()"...OK
    All 1 tests passed.

When `@This()` is used at global scope, it returns a reference to the current import. There is a proposal to remove the import type and use an empty struct type instead. See #1047 for details.

## @truncate §

    @truncate(comptime T: type, integer: var) T

This function truncates bits from an integer type, resulting in a smaller or same-sized integer type.

The following produces safety-checked Undefined Behavior:

test.zig

    test "integer cast panic" {
        var a: u16 = 0xabcd;
        var b: u8 = @intCast(u8, a);
    }

    $ zig test test.zig
    1/1 test "integer cast panic"...integer cast truncated bits
    /home/andy/dev/zig/docgen_tmp/test.zig:3:17: 0x204bd5 in test "integer cast panic" (test)
        var b: u8 = @intCast(u8, a);
                    ^
    /home/andy/dev/zig/lib/std/special/test_runner.zig:47:28: 0x22bade in std.special.main (test)
            } else test_fn.func();
                               ^
    /home/andy/dev/zig/lib/std/start.zig:253:37: 0x20560d in std.start.posixCallMainAndExit (test)
                const result = root.main() catch |err| {
                                        ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20534f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^

    Tests failed. Use the following command to reproduce the failure:
    /home/andy/dev/zig/docgen_tmp/test

However this is well defined and working code:

truncate.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "integer truncation" {
        var a: u16 = 0xabcd;
        var b: u8 = @truncate(u8, a);
        assert(b == 0xcd);
    }

    $ zig test truncate.zig
    1/1 test "integer truncation"...OK
    All 1 tests passed.

This function always truncates the significant bits of the integer, regardless of endianness on the target platform.

## @Type §

    @Type(comptime info: @import("builtin").TypeInfo) type

This function is the inverse of @typeInfo. It reifies type information into a `type`.

It is available for the following types:

- `type`
- `noreturn`
- `void`
- `bool`
- Integers - The maximum bit count for an integer type is `65535`.
- Floats
- Pointers
- `comptime_int`
- `comptime_float`
- `@TypeOf(undefined)`
- `@TypeOf(null)`

For these types it is a TODO in the compiler to implement:

- Array
- Optional
- ErrorUnion
- ErrorSet
- Enum
- Opaque
- FnFrame
- AnyFrame
- Vector
- EnumLiteral

For these types, `@Type` is not available. There is an open proposal to allow unions and structs.

- union
- Functions
- BoundFn
- struct

## @typeInfo §

    @typeInfo(comptime T: type) @import("std").builtin.TypeInfo

Provides type reflection.

For structs, unions, enums, and error sets, the fields are guaranteed to be in the same order as declared. For declarations, the order is unspecified.

## @typeName §

    @typeName(T: type) [N]u8

This function returns the string representation of a type, as an array. It is equivalent to a string literal of the type name.

## @TypeOf §

    @TypeOf(...) type

`@TypeOf` is a special builtin function that takes any (nonzero) number of expressions as parameters and returns the type of the result, using Peer Type Resolution.

The expressions are evaluated, however they are guaranteed to have no runtime side-effects:

test.zig

    const std = @import("std");
    const assert = std.debug.assert;

    test "no runtime side effects" {
        var data: i32 = 0;
        const T = @TypeOf(foo(i32, &data));
        comptime assert(T == i32);
        assert(data == 0);
    }

    fn foo(comptime T: type, ptr: *T) T {
        ptr.* += 1;
        return ptr.*;
    }

    $ zig test test.zig
    1/1 test "no runtime side effects"...OK
    All 1 tests passed.

## @unionInit §

    @unionInit(comptime Union: type, comptime active_field_name: []const u8, init_expr) Union

This is the same thing as union initialization syntax, except that the field name is a comptime-known value rather than an identifier token.

`@unionInit` forwards its result location to `init_expr`.

## @Vector §

    @Vector(comptime len: u32, comptime ElemType: type) type

This function returns a vector type for SIMD.

`ElemType` must be an integer, a float, or a pointer.

# Build Mode §

Zig has four build modes:

- Debug (default)
- ReleaseFast
- ReleaseSafe
- ReleaseSmall

To add standard build options to a `build.zig` file:

    const Builder = @import("std").build.Builder;

    pub fn build(b: *Builder) void {
        const exe = b.addExecutable("example", "example.zig");
        exe.setBuildMode(b.standardReleaseOptions());
        b.default_step.dependOn(&exe.step);
    }

This causes these options to be available:

      -Drelease-safe=[bool] optimizations on and safety on
      -Drelease-fast=[bool] optimizations on and safety off
      -Drelease-small=[bool] size optimizations on and safety off

## Debug §

    $ zig build-exe example.zig

- Fast compilation speed
- Safety checks enabled
- Slow runtime performance
- Large binary size
- No reproducible build requirement

## ReleaseFast §

    $ zig build-exe example.zig --release-fast

- Fast runtime performance
- Safety checks disabled
- Slow compilation speed
- Large binary size
- Reproducible build

## ReleaseSafe §

    $ zig build-exe example.zig --release-safe

- Medium runtime performance
- Safety checks enabled
- Slow compilation speed
- Large binary size
- Reproducible build

## ReleaseSmall §

    $ zig build-exe example.zig --release-small

- Medium runtime performance
- Safety checks disabled
- Slow compilation speed
- Small binary size
- Reproducible build

See also:

- Compile Variables
- Zig Build System
- Undefined Behavior

# Single Threaded Builds §

Zig has a compile option `--single-threaded` which has the following effects:

- All Thread Local Variables are treated as Global Variables.
- The overhead of Async Functions becomes equivalent to function call overhead.
- The `@import("builtin").single_threaded` becomes `true` and therefore various userland APIs which read this variable become more efficient. For example `std.Mutex` becomes an empty data structure and all of its functions become no-ops.

# Undefined Behavior §

Zig has many instances of undefined behavior. If undefined behavior is detected at compile-time, Zig emits a compile error and refuses to continue. Most undefined behavior that cannot be detected at compile-time can be detected at runtime. In these cases, Zig has safety checks. Safety checks can be disabled on a per-block basis with @setRuntimeSafety. The ReleaseFast and ReleaseSmall build modes disable all safety checks (except where overridden by @setRuntimeSafety) in order to facilitate optimizations.

When a safety check fails, Zig crashes with a stack trace, like this:

test.zig

    test "safety check" {
        unreachable;
    }

    $ zig test test.zig
    1/1 test "safety check"...reached unreachable code
    /home/andy/dev/zig/docgen_tmp/test.zig:2:5: 0x204b9a in test "safety check" (test)
        unreachable;
        ^
    /home/andy/dev/zig/lib/std/special/test_runner.zig:47:28: 0x22ba9e in std.special.main (test)
            } else test_fn.func();
                               ^
    /home/andy/dev/zig/lib/std/start.zig:253:37: 0x2055cd in std.start.posixCallMainAndExit (test)
                const result = root.main() catch |err| {
                                        ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20530f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^

    Tests failed. Use the following command to reproduce the failure:
    /home/andy/dev/zig/docgen_tmp/test

## Reaching Unreachable Code §

At compile-time:

test.zig

    comptime {
        assert(false);
    }
    fn assert(ok: bool) void {
        if (!ok) unreachable; // assertion failure
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:5:14: error: unable to evaluate constant expression
        if (!ok) unreachable; // assertion failure
                 ^
    ./docgen_tmp/test.zig:2:11: note: called from here
        assert(false);
              ^
    ./docgen_tmp/test.zig:1:10: note: called from here
    comptime {
             ^
    ./docgen_tmp/test.zig:2:11: note: referenced here
        assert(false);
              ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        std.debug.assert(false);
    }

    $ zig build-exe test.zig
    $ ./test
    reached unreachable code
    /home/andy/dev/zig/lib/std/debug.zig:228:14: 0x203cbb in std.debug.assert (test)
        if (!ok) unreachable; // assertion failure
                 ^
    /home/andy/dev/zig/docgen_tmp/test.zig:4:21: 0x22a7da in main (test)
        std.debug.assert(false);
                        ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x2046df in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x2044bf in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

## Index out of Bounds §

At compile-time:

test.zig

    comptime {
        const array: [5]u8 = "hello".*;
        const garbage = array[5];
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:3:26: error: index 5 outside array of size 5
        const garbage = array[5];
                             ^

At runtime:

test.zig

    pub fn main() void {
        var x = foo("hello");
    }

    fn foo(x: []const u8) u8 {
        return x[5];
    }

    $ zig build-exe test.zig
    $ ./test
    index out of bounds
    /home/andy/dev/zig/docgen_tmp/test.zig:6:13: 0x22e969 in foo (test)
        return x[5];
                ^
    /home/andy/dev/zig/docgen_tmp/test.zig:2:16: 0x22a7e6 in main (test)
        var x = foo("hello");
                   ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x2046df in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x2044bf in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

## Cast Negative Number to Unsigned Integer §

At compile-time:

test.zig

    comptime {
        const value: i32 = -1;
        const unsigned = @intCast(u32, value);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:3:36: error: cannot cast negative value -1 to unsigned integer type 'u32'
        const unsigned = @intCast(u32, value);
                                       ^
    ./docgen_tmp/test.zig:3:22: note: referenced here
        const unsigned = @intCast(u32, value);
                         ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        var value: i32 = -1;
        var unsigned = @intCast(u32, value);
        std.debug.warn("value: {}\n", .{unsigned});
    }

    $ zig build-exe test.zig
    $ ./test
    attempt to cast negative value to unsigned integer
    /home/andy/dev/zig/docgen_tmp/test.zig:5:20: 0x22a867 in main (test)
        var unsigned = @intCast(u32, value);
                       ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

To obtain the maximum value of an unsigned integer, use `std.math.maxInt`.

## Cast Truncates Data §

At compile-time:

test.zig

    comptime {
        const spartan_count: u16 = 300;
        const byte = @intCast(u8, spartan_count);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:3:31: error: integer value 300 cannot be coerced to type 'u8'
        const byte = @intCast(u8, spartan_count);
                                  ^
    ./docgen_tmp/test.zig:3:18: note: referenced here
        const byte = @intCast(u8, spartan_count);
                     ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        var spartan_count: u16 = 300;
        const byte = @intCast(u8, spartan_count);
        std.debug.warn("value: {}\n", .{byte});
    }

    $ zig build-exe test.zig
    $ ./test
    integer cast truncated bits
    /home/andy/dev/zig/docgen_tmp/test.zig:5:18: 0x22a86c in main (test)
        const byte = @intCast(u8, spartan_count);
                     ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

To truncate bits, use @truncate.

## Integer Overflow §

### Default Operations §

The following operators can cause integer overflow:

- `+` (addition)
- `-` (subtraction)
- `-` (negation)
- `*` (multiplication)
- `/` (division)
- @divTrunc (division)
- @divFloor (division)
- @divExact (division)

Example with addition at compile-time:

test.zig

    comptime {
        var byte: u8 = 255;
        byte += 1;
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:3:10: error: operation caused overflow
        byte += 1;
             ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        var byte: u8 = 255;
        byte += 1;
        std.debug.warn("value: {}\n", .{byte});
    }

    $ zig build-exe test.zig
    $ ./test
    integer overflow
    /home/andy/dev/zig/docgen_tmp/test.zig:5:10: 0x22a850 in main (test)
        byte += 1;
             ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

### Standard Library Math Functions §

These functions provided by the standard library return possible errors.

- `@import("std").math.add`
- `@import("std").math.sub`
- `@import("std").math.mul`
- `@import("std").math.divTrunc`
- `@import("std").math.divFloor`
- `@import("std").math.divExact`
- `@import("std").math.shl`

Example of catching an overflow for addition:

test.zig

    const math = @import("std").math;
    const warn = @import("std").debug.warn;
    pub fn main() !void {
        var byte: u8 = 255;

        byte = if (math.add(u8, byte, 1)) |result| result else |err| {
            warn("unable to add one: {}\n", .{@errorName(err)});
            return err;
        };

        warn("result: {}\n", .{byte});
    }

    $ zig build-exe test.zig
    $ ./test
    unable to add one: Overflow
    error: Overflow
    /home/andy/dev/zig/lib/std/math.zig:342:5: 0x22ee70 in std.math.add (test)
        return if (@addWithOverflow(T, a, b, &answer)) error.Overflow else answer;
        ^
    /home/andy/dev/zig/docgen_tmp/test.zig:8:9: 0x22ab3b in main (test)
            return err;
            ^

### Builtin Overflow Functions §

These builtins return a `bool` of whether or not overflow occurred, as well as returning the overflowed bits:

- @addWithOverflow
- @subWithOverflow
- @mulWithOverflow
- @shlWithOverflow

Example of @addWithOverflow:

test.zig

    const warn = @import("std").debug.warn;
    pub fn main() void {
        var byte: u8 = 255;

        var result: u8 = undefined;
        if (@addWithOverflow(u8, byte, 10, &result)) {
            warn("overflowed result: {}\n", .{result});
        } else {
            warn("result: {}\n", .{result});
        }
    }

    $ zig build-exe test.zig
    $ ./test
    overflowed result: 9

### Wrapping Operations §

These operations have guaranteed wraparound semantics.

- `+%` (wraparound addition)
- `-%` (wraparound subtraction)
- `-%` (wraparound negation)
- `*%` (wraparound multiplication)

test.zig

    const std = @import("std");
    const assert = std.debug.assert;
    const minInt = std.math.minInt;
    const maxInt = std.math.maxInt;

    test "wraparound addition and subtraction" {
        const x: i32 = maxInt(i32);
        const min_val = x +% 1;
        assert(min_val == minInt(i32));
        const max_val = min_val -% 1;
        assert(max_val == maxInt(i32));
    }

    $ zig test test.zig
    1/1 test "wraparound addition and subtraction"...OK
    All 1 tests passed.

## Exact Left Shift Overflow §

At compile-time:

test.zig

    comptime {
        const x = @shlExact(@as(u8, 0b01010101), 2);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:2:15: error: operation caused overflow
        const x = @shlExact(@as(u8, 0b01010101), 2);
                  ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        var x: u8 = 0b01010101;
        var y = @shlExact(x, 2);
        std.debug.warn("value: {}\n", .{y});
    }

    $ zig build-exe test.zig
    $ ./test
    left shift overflowed bits
    /home/andy/dev/zig/docgen_tmp/test.zig:5:13: 0x22a89d in main (test)
        var y = @shlExact(x, 2);
                ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20475f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20453f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

## Exact Right Shift Overflow §

At compile-time:

test.zig

    comptime {
        const x = @shrExact(@as(u8, 0b10101010), 2);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:2:15: error: exact shift shifted out 1 bits
        const x = @shrExact(@as(u8, 0b10101010), 2);
                  ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        var x: u8 = 0b10101010;
        var y = @shrExact(x, 2);
        std.debug.warn("value: {}\n", .{y});
    }

    $ zig build-exe test.zig
    $ ./test
    right shift overflowed bits
    /home/andy/dev/zig/docgen_tmp/test.zig:5:13: 0x22a89d in main (test)
        var y = @shrExact(x, 2);
                ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20475f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20453f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

## Division by Zero §

At compile-time:

test.zig

    comptime {
        const a: i32 = 1;
        const b: i32 = 0;
        const c = a / b;
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:4:17: error: division by zero
        const c = a / b;
                    ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        var a: u32 = 1;
        var b: u32 = 0;
        var c = a / b;
        std.debug.warn("value: {}\n", .{c});
    }

    $ zig build-exe test.zig
    $ ./test
    division by zero
    /home/andy/dev/zig/docgen_tmp/test.zig:6:15: 0x22a859 in main (test)
        var c = a / b;
                  ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

## Remainder Division by Zero §

At compile-time:

test.zig

    comptime {
        const a: i32 = 10;
        const b: i32 = 0;
        const c = a % b;
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:4:17: error: division by zero
        const c = a % b;
                    ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        var a: u32 = 10;
        var b: u32 = 0;
        var c = a % b;
        std.debug.warn("value: {}\n", .{c});
    }

    $ zig build-exe test.zig
    $ ./test
    remainder division by zero or negative value
    /home/andy/dev/zig/docgen_tmp/test.zig:6:15: 0x22a87b in main (test)
        var c = a % b;
                  ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

## Exact Division Remainder §

At compile-time:

test.zig

    comptime {
        const a: u32 = 10;
        const b: u32 = 3;
        const c = @divExact(a, b);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:4:15: error: exact division had a remainder
        const c = @divExact(a, b);
                  ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        var a: u32 = 10;
        var b: u32 = 3;
        var c = @divExact(a, b);
        std.debug.warn("value: {}\n", .{c});
    }

    $ zig build-exe test.zig
    $ ./test
    exact division produced remainder
    /home/andy/dev/zig/docgen_tmp/test.zig:6:13: 0x22a89d in main (test)
        var c = @divExact(a, b);
                ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

## Attempt to Unwrap Null §

At compile-time:

test.zig

    comptime {
        const optional_number: ?i32 = null;
        const number = optional_number.?;
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:3:35: error: unable to unwrap null
        const number = optional_number.?;
                                      ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        var optional_number: ?i32 = null;
        var number = optional_number.?;
        std.debug.warn("value: {}\n", .{number});
    }

    $ zig build-exe test.zig
    $ ./test
    attempt to use null value
    /home/andy/dev/zig/docgen_tmp/test.zig:5:33: 0x22a86c in main (test)
        var number = optional_number.?;
                                    ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20474f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20452f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

One way to avoid this crash is to test for null instead of assuming non-null, with the `if` expression:

test.zig

    const warn = @import("std").debug.warn;
    pub fn main() void {
        const optional_number: ?i32 = null;

        if (optional_number) |number| {
            warn("got number: {}\n", .{number});
        } else {
            warn("it's null\n", .{});
        }
    }

    $ zig build-exe test.zig
    $ ./test
    it's null

See also:

- Optionals

## Attempt to Unwrap Error §

At compile-time:

test.zig

    comptime {
        const number = getNumberOrFail() catch unreachable;
    }

    fn getNumberOrFail() !i32 {
        return error.UnableToReturnNumber;
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:2:38: error: caught unexpected error 'UnableToReturnNumber'
        const number = getNumberOrFail() catch unreachable;
                                         ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        const number = getNumberOrFail() catch unreachable;
        std.debug.warn("value: {}\n", .{number});
    }

    fn getNumberOrFail() !i32 {
        return error.UnableToReturnNumber;
    }

    $ zig build-exe test.zig
    $ ./test
    attempt to unwrap error: UnableToReturnNumber
    /home/andy/dev/zig/docgen_tmp/test.zig:9:5: 0x22ea6c in getNumberOrFail (test)
        return error.UnableToReturnNumber;
        ^
    ???:?:?: 0x20698c in ??? (???)
    /home/andy/dev/zig/docgen_tmp/test.zig:4:38: 0x22a8cb in main (test)
        const number = getNumberOrFail() catch unreachable;
                                         ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20476f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20454f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

One way to avoid this crash is to test for an error instead of assuming a successful result, with the `if` expression:

test.zig

    const warn = @import("std").debug.warn;

    pub fn main() void {
        const result = getNumberOrFail();

        if (result) |number| {
            warn("got number: {}\n", .{number});
        } else |err| {
            warn("got error: {}\n", .{@errorName(err)});
        }
    }

    fn getNumberOrFail() !i32 {
        return error.UnableToReturnNumber;
    }

    $ zig build-exe test.zig
    $ ./test
    got error: UnableToReturnNumber

See also:

- Errors

## Invalid Error Code §

At compile-time:

test.zig

    comptime {
        const err = error.AnError;
        const number = @errorToInt(err) + 10;
        const invalid_err = @intToError(number);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:4:25: error: integer value 11 represents no error
        const invalid_err = @intToError(number);
                            ^

At runtime:

test.zig

    const std = @import("std");

    pub fn main() void {
        var err = error.AnError;
        var number = @errorToInt(err) + 500;
        var invalid_err = @intToError(number);
        std.debug.warn("value: {}\n", .{number});
    }

    $ zig build-exe test.zig
    $ ./test
    invalid error code
    /home/andy/dev/zig/docgen_tmp/test.zig:6:23: 0x22a8b4 in main (test)
        var invalid_err = @intToError(number);
                          ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20475f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20453f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

## Invalid Enum Cast §

At compile-time:

test.zig

    const Foo = enum {
        A,
        B,
        C,
    };
    comptime {
        const a: u2 = 3;
        const b = @intToEnum(Foo, a);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:8:15: error: enum 'Foo' has no tag matching integer value 3
        const b = @intToEnum(Foo, a);
                  ^
    ./docgen_tmp/test.zig:1:13: note: 'Foo' declared here
    const Foo = enum {
                ^

At runtime:

test.zig

    const std = @import("std");

    const Foo = enum {
        A,
        B,
        C,
    };

    pub fn main() void {
        var a: u2 = 3;
        var b = @intToEnum(Foo, a);
        std.debug.warn("value: {}\n", .{@tagName(b)});
    }

    $ zig build-exe test.zig
    $ ./test
    invalid enum value
    /home/andy/dev/zig/docgen_tmp/test.zig:11:13: 0x22a8cc in main (test)
        var b = @intToEnum(Foo, a);
                ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20479f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20457f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

## Invalid Error Set Cast §

At compile-time:

test.zig

    const Set1 = error{
        A,
        B,
    };
    const Set2 = error{
        A,
        C,
    };
    comptime {
        _ = @errSetCast(Set2, Set1.B);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:10:9: error: error.B not a member of error set 'Set2'
        _ = @errSetCast(Set2, Set1.B);
            ^

At runtime:

test.zig

    const std = @import("std");

    const Set1 = error{
        A,
        B,
    };
    const Set2 = error{
        A,
        C,
    };
    pub fn main() void {
        foo(Set1.B);
    }
    fn foo(set1: Set1) void {
        const x = @errSetCast(Set2, set1);
        std.debug.warn("value: {}\n", .{x});
    }

    $ zig build-exe test.zig
    $ ./test
    invalid error code
    /home/andy/dev/zig/docgen_tmp/test.zig:15:15: 0x22ea4c in foo (test)
        const x = @errSetCast(Set2, set1);
                  ^
    /home/andy/dev/zig/docgen_tmp/test.zig:12:8: 0x22a89d in main (test)
        foo(Set1.B);
           ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x20479f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20457f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

## Incorrect Pointer Alignment §

At compile-time:

test.zig

    comptime {
        const ptr = @intToPtr(*align(1) i32, 0x1);
        const aligned = @alignCast(4, ptr);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:3:35: error: pointer address 0x1 is not aligned to 4 bytes
        const aligned = @alignCast(4, ptr);
                                      ^
    ./docgen_tmp/test.zig:3:21: note: referenced here
        const aligned = @alignCast(4, ptr);
                        ^

At runtime:

test.zig

    const mem = @import("std").mem;
    pub fn main() !void {
        var array align(4) = [_]u32{ 0x11111111, 0x11111111 };
        const bytes = mem.sliceAsBytes(array[0..]);
        if (foo(bytes) != 0x11111111) return error.Wrong;
    }
    fn foo(bytes: []u8) u32 {
        const slice4 = bytes[1..5];
        const int_slice = mem.bytesAsSlice(u32, @alignCast(4, slice4));
        return int_slice[0];
    }

    $ zig build-exe test.zig
    $ ./test
    incorrect alignment
    /home/andy/dev/zig/docgen_tmp/test.zig:9:59: 0x22edbb in foo (test)
        const int_slice = mem.bytesAsSlice(u32, @alignCast(4, slice4));
                                                              ^
    /home/andy/dev/zig/docgen_tmp/test.zig:5:12: 0x22aa49 in main (test)
        if (foo(bytes) != 0x11111111) return error.Wrong;
               ^
    /home/andy/dev/zig/lib/std/start.zig:253:37: 0x2047ed in std.start.posixCallMainAndExit (test)
                const result = root.main() catch |err| {
                                        ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x20452f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

## Wrong Union Field Access §

At compile-time:

test.zig

    comptime {
        var f = Foo{ .int = 42 };
        f.float = 12.34;
    }

    const Foo = union {
        float: f32,
        int: u32,
    };

    $ zig test test.zig
    ./docgen_tmp/test.zig:3:6: error: accessing union field 'float' while field 'int' is set
        f.float = 12.34;
         ^

At runtime:

test.zig

    const std = @import("std");

    const Foo = union {
        float: f32,
        int: u32,
    };

    pub fn main() void {
        var f = Foo{ .int = 42 };
        bar(&f);
    }

    fn bar(f: *Foo) void {
        f.float = 12.34;
        std.debug.warn("value: {}\n", .{f.float});
    }

    $ zig build-exe test.zig
    $ ./test
    access of inactive union field
    /home/andy/dev/zig/docgen_tmp/test.zig:14:6: 0x23c21a in bar (test)
        f.float = 12.34;
         ^
    /home/andy/dev/zig/docgen_tmp/test.zig:10:8: 0x23806c in main (test)
        bar(&f);
           ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x211f5f in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x211d3f in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

This safety is not available for `extern` or `packed` unions.

To change the active field of a union, assign the entire union, like this:

test.zig

    const std = @import("std");

    const Foo = union {
        float: f32,
        int: u32,
    };

    pub fn main() void {
        var f = Foo{ .int = 42 };
        bar(&f);
    }

    fn bar(f: *Foo) void {
        f.* = Foo{ .float = 12.34 };
        std.debug.warn("value: {}\n", .{f.float});
    }

    $ zig build-exe test.zig
    $ ./test
    value: 1.23400001e+01

To change the active field of a union when a meaningful value for the field is not known, use undefined, like this:

test.zig

    const std = @import("std");

    const Foo = union {
        float: f32,
        int: u32,
    };

    pub fn main() void {
        var f = Foo{ .int = 42 };
        f = Foo{ .float = undefined };
        bar(&f);
        std.debug.warn("value: {}\n", .{f.float});
    }

    fn bar(f: *Foo) void {
        f.float = 12.34;
    }

    $ zig build-exe test.zig
    $ ./test
    value: 1.23400001e+01

See also:

- union
- extern union

## Out of Bounds Float to Integer Cast §

TODO

## Pointer Cast Invalid Null §

This happens when casting a pointer with the address 0 to a pointer which may not have the address 0. For example, C Pointers, Optional Pointers, and allowzero pointers allow address zero, but normal Pointers do not.

At compile-time:

test.zig

    comptime {
        const opt_ptr: ?*i32 = null;
        const ptr = @ptrCast(*i32, opt_ptr);
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:3:17: error: null pointer casted to type '*i32'
        const ptr = @ptrCast(*i32, opt_ptr);
                    ^

At runtime:

test.zig

    pub fn main() void {
        var opt_ptr: ?*i32 = null;
        var ptr = @ptrCast(*i32, opt_ptr);
    }

    $ zig build-exe test.zig
    $ ./test
    cast causes pointer to be null
    /home/andy/dev/zig/docgen_tmp/test.zig:3:15: 0x22a800 in main (test)
        var ptr = @ptrCast(*i32, opt_ptr);
                  ^
    /home/andy/dev/zig/lib/std/start.zig:243:22: 0x2046df in std.start.posixCallMainAndExit (test)
                root.main();
                         ^
    /home/andy/dev/zig/lib/std/start.zig:123:5: 0x2044bf in std.start._start (test)
        @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
        ^
    (process terminated by signal)

# Memory §

The Zig language performs no memory management on behalf of the programmer. This is why Zig has no runtime, and why Zig code works seamlessly in so many environments, including real-time software, operating system kernels, embedded devices, and low latency servers. As a consequence, Zig programmers must always be able to answer the question:

Where are the bytes?

Like Zig, the C programming language has manual memory management. However, unlike Zig, C has a default allocator - `malloc`, `realloc`, and `free`. When linking against libc, Zig exposes this allocator with `std.heap.c_allocator`. However, by convention, there is no default allocator in Zig. Instead, functions which need to allocate accept an `*Allocator` parameter. Likewise, data structures such as `std.ArrayList` accept an `*Allocator` parameter in their initialization functions:

allocator.zig

    const std = @import("std");
    const Allocator = std.mem.Allocator;
    const assert = std.debug.assert;

    test "using an allocator" {
        var buffer: [100]u8 = undefined;
        const allocator = &std.heap.FixedBufferAllocator.init(&buffer).allocator;
        const result = try concat(allocator, "foo", "bar");
        assert(std.mem.eql(u8, "foobar", result));
    }

    fn concat(allocator: *Allocator, a: []const u8, b: []const u8) ![]u8 {
        const result = try allocator.alloc(u8, a.len + b.len);
        std.mem.copy(u8, result, a);
        std.mem.copy(u8, result[a.len..], b);
        return result;
    }

    $ zig test allocator.zig
    1/1 test "using an allocator"...OK
    All 1 tests passed.

In the above example, 100 bytes of stack memory are used to initialize a `FixedBufferAllocator`, which is then passed to a function. As a convenience there is a global `FixedBufferAllocator` available for quick tests at `std.testing.allocator`, which will also do perform basic leak detection.

Currently Zig has no general purpose allocator, but there is one under active development. Once it is merged into the Zig standard library it will become available to import with `std.heap.default_allocator`. However, it will still be recommended to follow the Choosing an Allocator guide.

## Choosing an Allocator §

What allocator to use depends on a number of factors. Here is a flow chart to help you decide:

1. Are you making a library? In this case, best to accept an `*Allocator` as a parameter and allow your library's users to decide what allocator to use.
2. Are you linking libc? In this case, `std.heap.c_allocator` is likely the right choice, at least for your main allocator.
3. Is the maximum number of bytes that you will need bounded by a number known at comptime? In this case, use `std.heap.FixedBufferAllocator` or `std.heap.ThreadSafeFixedBufferAllocator` depending on whether you need thread-safety or not.
4. Is your program a command line application which runs from start to end without any fundamental cyclical pattern (such as a video game main loop, or a web server request handler), such that it would make sense to free everything at once at the end? In this case, it is recommended to follow this pattern:

   cli_allocation.zig

       const std = @import("std");

       pub fn main() !void {
           var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
           defer arena.deinit();

           const allocator = &arena.allocator;

           const ptr = try allocator.create(i32);
           std.debug.warn("ptr={*}\n", .{ptr});
       }

       $ zig build-exe cli_allocation.zig
       $ ./cli_allocation
       ptr=i32@7fd997221018

   When using this kind of allocator, there is no need to free anything manually. Everything gets freed at once with the call to `arena.deinit()`.
5. Are the allocations part of a cyclical pattern such as a video game main loop, or a web server request handler? If the allocations can all be freed at once, at the end of the cycle, for example once the video game frame has been fully rendered, or the web server request has been served, then `std.heap.ArenaAllocator` is a great candidate. As demonstrated in the previous bullet point, this allows you to free entire arenas at once. Note also that if an upper bound of memory can be established, then `std.heap.FixedBufferAllocator` can be used as a further optimization.
6. Are you writing a test, and you want to make sure `error.OutOfMemory` is handled correctly? In this case, use `std.testing.FailingAllocator`.
7. Finally, if none of the above apply, you need a general purpose allocator. Zig does not yet have a general purpose allocator in the standard library, but one is being actively developed. You can also consider Implementing an Allocator.

## Where are the bytes? §

String literals such as `"foo"` are in the global constant data section. This is why it is an error to pass a string literal to a mutable slice, like this:

test.zig

    fn foo(s: []u8) void {}

    test "string literal to mutable slice" {
        foo("hello");
    }

    $ zig test test.zig
    ./docgen_tmp/test.zig:4:9: error: expected type '[]u8', found '*const [5:0]u8'
        foo("hello");
            ^

However if you make the slice constant, then it works:

strlit.zig

    fn foo(s: []const u8) void {}

    test "string literal to constant slice" {
        foo("hello");
    }

    $ zig test strlit.zig
    1/1 test "string literal to constant slice"...OK
    All 1 tests passed.

Just like string literals, `const` declarations, when the value is known at comptime, are stored in the global constant data section. Also Compile Time Variables are stored in the global constant data section.

`var` declarations inside functions are stored in the function's stack frame. Once a function returns, any Pointers to variables in the function's stack frame become invalid references, and dereferencing them becomes unchecked Undefined Behavior.

`var` declarations at the top level or in struct declarations are stored in the global data section.

The location of memory allocated with `allocator.alloc` or `allocator.create` is determined by the allocator's implementation.

TODO: thread local variables

## Implementing an Allocator §

Zig programmers can implement their own allocators by fulfilling the Allocator interface. In order to do this one must read carefully the documentation comments in std/mem.zig and then supply a `reallocFn` and a `shrinkFn`.

There are many example allocators to look at for inspiration. Look at std/heap.zig and at this work-in-progress general purpose allocator. TODO: once #21 is done, link to the docs here.

## Heap Allocation Failure §

Many programming languages choose to handle the possibility of heap allocation failure by unconditionally crashing. By convention, Zig programmers do not consider this to be a satisfactory solution. Instead, `error.OutOfMemory` represents heap allocation failure, and Zig libraries return this error code whenever heap allocation failure prevented an operation from completing successfully.

Some have argued that because some operating systems such as Linux have memory overcommit enabled by default, it is pointless to handle heap allocation failure. There are many problems with this reasoning:

- Only some operating systems have an overcommit feature.
  - Linux has it enabled by default, but it is configurable.
  - Windows does not overcommit.
  - Embedded systems do not have overcommit.
  - Hobby operating systems may or may not have overcommit.
- For real-time systems, not only is there no overcommit, but typically the maximum amount of memory per application is determined ahead of time.
- When writing a library, one of the main goals is code reuse. By making code handle allocation failure correctly, a library becomes eligible to be reused in more contexts.
- Although some software has grown to depend on overcommit being enabled, its existence is the source of countless user experience disasters. When a system with overcommit enabled, such as Linux on default settings, comes close to memory exhaustion, the system locks up and becomes unusable. At this point, the OOM Killer selects an application to kill based on heuristics. This non-deterministic decision often results in an important process being killed, and often fails to return the system back to working order.

## Recursion §

Recursion is a fundamental tool in modeling software. However it has an often-overlooked problem: unbounded memory allocation.

Recursion is an area of active experimentation in Zig and so the documentation here is not final. You can read a summary of recursion status in the 0.3.0 release notes.

The short summary is that currently recursion works normally as you would expect. Although Zig code is not yet protected from stack overflow, it is planned that a future version of Zig will provide such protection, with some degree of cooperation from Zig code required.

## Lifetime and Ownership §

It is the Zig programmer's responsibility to ensure that a pointer is not accessed when the memory pointed to is no longer available. Note that a slice is a form of pointer, in that it references other memory.

In order to prevent bugs, there are some helpful conventions to follow when dealing with pointers. In general, when a function returns a pointer, the documentation for the function should explain who "owns" the pointer. This concept helps the programmer decide when it is appropriate, if ever, to free the pointer.

For example, the function's documentation may say "caller owns the returned memory", in which case the code that calls the function must have a plan for when to free that memory. Probably in this situation, the function will accept an `*Allocator` parameter.

Sometimes the lifetime of a pointer may be more complicated. For example, when using `std.ArrayList(T).span()`, the returned slice has a lifetime that remains valid until the next time the list is resized, such as by appending new elements.

The API documentation for functions and data structures should take great care to explain the ownership and lifetime semantics of pointers. Ownership determines whose responsibility it is to free the memory referenced by the pointer, and lifetime determines the point at which the memory becomes inaccessible (lest Undefined Behavior occur).

# Compile Variables §

Compile variables are accessible by importing the `"builtin"` package, which the compiler makes available to every Zig source file. It contains compile-time constants such as the current target, endianness, and release mode.

    const builtin = @import("builtin");
    const separator = if (builtin.os == builtin.Os.windows) '\\' else '/';

Example of what is imported with `@import("builtin")`:

    usingnamespace @import("std").builtin;

    pub const endian = Endian.Little;
    pub const output_mode = OutputMode.Obj;
    pub const link_mode = LinkMode.Static;
    pub const is_test = false;
    pub const single_threaded = false;
    /// Deprecated: use `std.Target.cpu.arch`
    pub const arch = Arch.x86_64;
    pub const abi = Abi.gnu;
    pub const cpu: Cpu = Cpu{
        .arch = .x86_64,
        .model = &Target.x86.cpu.skylake,
        .features = Target.x86.featureSet(&[_]Target.x86.Feature{
            .@"64bit",
            .@"adx",
            .@"aes",
            .@"avx",
            .@"avx2",
            .@"bmi",
            .@"bmi2",
            .@"clflushopt",
            .@"cmov",
            .@"cx16",
            .@"cx8",
            .@"ermsb",
            .@"f16c",
            .@"false_deps_popcnt",
            .@"fast_gather",
            .@"fast_scalar_fsqrt",
            .@"fast_shld_rotate",
            .@"fast_variable_shuffle",
            .@"fast_vector_fsqrt",
            .@"fma",
            .@"fsgsbase",
            .@"fxsr",
            .@"idivq_to_divl",
            .@"invpcid",
            .@"lzcnt",
            .@"macrofusion",
            .@"merge_to_threeway_branch",
            .@"mmx",
            .@"movbe",
            .@"nopl",
            .@"pclmul",
            .@"popcnt",
            .@"prfchw",
            .@"rdrnd",
            .@"rdseed",
            .@"rtm",
            .@"sahf",
            .@"sgx",
            .@"slow_3ops_lea",
            .@"sse",
            .@"sse2",
            .@"sse3",
            .@"sse4_1",
            .@"sse4_2",
            .@"ssse3",
            .@"vzeroupper",
            .@"x87",
            .@"xsave",
            .@"xsavec",
            .@"xsaveopt",
            .@"xsaves",
        }),
    };
    pub const os = Os{
        .tag = .linux,
        .version_range = .{ .linux = .{
            .range = .{
                .min = .{
                    .major = 5,
                    .minor = 4,
                    .patch = 15,
                },
                .max = .{
                    .major = 5,
                    .minor = 4,
                    .patch = 15,
                },
            },
            .glibc = .{
                .major = 2,
                .minor = 27,
                .patch = 0,
            },
        }},
    };
    pub const object_format = ObjectFormat.elf;
    pub const mode = Mode.Debug;
    pub const link_libc = false;
    pub const link_libcpp = false;
    pub const have_error_return_tracing = true;
    pub const valgrind_support = true;
    pub const position_independent_code = false;
    pub const strip_debug_info = false;
    pub const code_model = CodeModel.default;

See also:

- Build Mode

# Root Source File §

TODO: explain how root source file finds other files

TODO: pub fn main

TODO: pub fn panic

TODO: if linking with libc you can use export fn main

TODO: order independent top level declarations

TODO: lazy analysis

TODO: using comptime { _ = @import() }

# Zig Test §

`zig test` is a tool that can be used to quickly build and run Zig code to make sure behavior meets expectations. `@import("builtin").is_test` is available for code to detect whether the current build is a test build.

detect_test.zig

    const std = @import("std");
    const builtin = std.builtin;
    const assert = std.debug.assert;

    test "builtin.is_test" {
        assert(builtin.is_test);
    }

    $ zig test detect_test.zig
    1/1 test "builtin.is_test"...OK
    All 1 tests passed.

Zig has lazy top level declaration analysis, which means that if a function is not called, or otherwise used, it is not analyzed. This means that there may be an undiscovered compile error in a function because it is never called.

unused_fn.zig

    fn unused() i32 {
        return "wrong return type";
    }
    test "unused function" { }

    $ zig test unused_fn.zig
    1/1 test "unused function"...OK
    All 1 tests passed.

Note that, while in Debug and ReleaseSafe modes, unreachable emits a call to @panic, in ReleaseFast and ReleaseSmall modes, it is really undefined behavior. The implementation of `std.debug.assert` is as simple as:

    pub fn assert(ok: bool) void {
        if (!ok) unreachable;
    }

This means that when testing in ReleaseFast or ReleaseSmall mode, `assert` is not sufficient to check the result of a computation:

    const std = @import("std");
    const assert = std.debug.assert;

    test "assert in release fast mode" {
        assert(false);
    }

When compiling this test in ReleaseFast mode, it invokes unchecked Undefined Behavior. Since that could do anything, this documentation cannot show you the output.

Better practice for checking the output when testing is to use `std.testing.expect`:

test.zig

    const std = @import("std");
    const expect = std.testing.expect;

    test "assert in release fast mode" {
        expect(false);
    }

    $ zig test test.zig --release-fast
    1/1 test "assert in release fast mode"...test failure

    Tests failed. Use the following command to reproduce the failure:
    /home/andy/dev/zig/docgen_tmp/test

See the rest of the `std.testing` namespace for more available functions.

`zig test` has a few command line parameters which affect the compilation. See `zig --help` for a full list. The most interesting one is `--test-filter [text]`. This makes the test build only include tests whose name contains the supplied filter text. Again, thanks to lazy analysis, this can allow you to narrow a build to only a few functions in isolation.

# Zig Build System §

TODO: explain purpose, it's supposed to replace make/cmake

TODO: example of building a zig executable

TODO: example of building a C library

# C §

Although Zig is independent of C, and, unlike most other languages, does not depend on libc, Zig acknowledges the importance of interacting with existing C code.

There are a few ways that Zig facilitates C interop.

## C Type Primitives §

These have guaranteed C ABI compatibility and can be used like any other type.

- `c_short`
- `c_ushort`
- `c_int`
- `c_uint`
- `c_long`
- `c_ulong`
- `c_longlong`
- `c_ulonglong`
- `c_longdouble`
- `c_void`

See also:

- Primitive Types

## Import from C Header File §

The `@cImport` builtin function can be used to directly import symbols from .h files:

test.zig

    const c = @cImport({
        // See https://github.com/ziglang/zig/issues/515
        @cDefine("_NO_CRT_STDIO_INLINE", "1");
        @cInclude("stdio.h");
    });
    pub fn main() void {
        _ = c.printf("hello\n");
    }

    $ zig build-exe test.zig -lc
    $ ./test
    hello

The `@cImport` function takes an expression as a parameter. This expression is evaluated at compile-time and is used to control preprocessor directives and include multiple .h files:

    const builtin = @import("builtin");

    const c = @cImport({
        @cDefine("NDEBUG", builtin.mode == .ReleaseFast);
        if (something) {
            @cDefine("_GNU_SOURCE", {});
        }
        @cInclude("stdlib.h");
        if (something) {
            @cUndef("_GNU_SOURCE");
        }
        @cInclude("soundio.h");
    });

See also:

- @cImport
- @cInclude
- @cDefine
- @cUndef
- @import

## C Pointers §

This type is to be avoided whenever possible. The only valid reason for using a C pointer is in auto-generated code from translating C code.

When importing C header files, it is ambiguous whether pointers should be translated as single-item pointers (`*T`) or unknown-length pointers (`[*]T`). C pointers are a compromise so that Zig code can utilize translated header files directly.

`[*c]T` - C pointer.

- Supports all the syntax of the other two pointer types.
- Coerces to other pointer types, as well as Optional Pointers. When a C pointer is coerced to a non-optional pointer, safety-checked Undefined Behavior occurs if the address is 0.
- Allows address 0. On non-freestanding targets, dereferencing address 0 is safety-checked Undefined Behavior. Optional C pointers introduce another bit to keep track of null, just like `?usize`. Note that creating an optional C pointer is unnecessary as one can use normal Optional Pointers.
- Supports Type Coercion to and from integers.
- Supports comparison with integers.
- Does not support Zig-only pointer attributes such as alignment. Use normal Pointers please!

When a C pointer is pointing to a single struct (not an array), deference the C pointer to access to the struct's fields or member data. That syntax looks like this:

`ptr_to_struct.*.struct_member`

This is comparable to doing `->` in C.

When a C pointer is pointing to an array of structs, the syntax reverts to this:

`ptr_to_struct_array[index].struct_member`

## Exporting a C Library §

One of the primary use cases for Zig is exporting a library with the C ABI for other programming languages to call into. The `export` keyword in front of functions, variables, and types causes them to be part of the library API:

mathtest.zig

    export fn add(a: i32, b: i32) i32 {
        return a + b;
    }

To make a static library:

    $ zig build-lib mathtest.zig

To make a shared library:

    $ zig build-lib mathtest.zig -dynamic

Here is an example with the Zig Build System:

test.c

    // This header is generated by zig from mathtest.zig
    #include "mathtest.h"
    #include <stdio.h>

    int main(int argc, char **argv) {
        int32_t result = add(42, 1337);
        printf("%d\n", result);
        return 0;
    }

build.zig

    const Builder = @import("std").build.Builder;

    pub fn build(b: *Builder) void {
        const lib = b.addSharedLibrary("mathtest", "mathtest.zig", b.version(1, 0, 0));

        const exe = b.addExecutable("test", null);
        exe.addCSourceFile("test.c", &[_][]const u8{"-std=c99"});
        exe.linkLibrary(lib);
        exe.linkSystemLibrary("c");

        b.default_step.dependOn(&exe.step);

        const run_cmd = exe.run();

        const test_step = b.step("test", "Test the program");
        test_step.dependOn(&run_cmd.step);
    }

terminal

    $ zig build test
    1379

See also:

- export

## Mixing Object Files §

You can mix Zig object files with any other object files that respect the C ABI. Example:

base64.zig

    const base64 = @import("std").base64;

    export fn decode_base_64(
        dest_ptr: [*]u8,
        dest_len: usize,
        source_ptr: [*]const u8,
        source_len: usize,
    ) usize {
        const src = source_ptr[0..source_len];
        const dest = dest_ptr[0..dest_len];
        const base64_decoder = base64.standard_decoder_unsafe;
        const decoded_size = base64_decoder.calcSize(src);
        base64_decoder.decode(dest[0..decoded_size], src);
        return decoded_size;
    }

test.c

    // This header is generated by zig from base64.zig
    #include "base64.h"

    #include <string.h>
    #include <stdio.h>

    int main(int argc, char **argv) {
        const char *encoded = "YWxsIHlvdXIgYmFzZSBhcmUgYmVsb25nIHRvIHVz";
        char buf[200];

        size_t len = decode_base_64(buf, 200, encoded, strlen(encoded));
        buf[len] = 0;
        puts(buf);

        return 0;
    }

build.zig

    const Builder = @import("std").build.Builder;

    pub fn build(b: *Builder) void {
        const obj = b.addObject("base64", "base64.zig");

        const exe = b.addExecutable("test", null);
        exe.addCSourceFile("test.c", &[_][]const u8{"-std=c99"});
        exe.addObject(obj);
        exe.linkSystemLibrary("c");
        exe.install();
    }

terminal

    $ zig build
    $ ./zig-cache/bin/test
    all your base are belong to us

See also:

- Targets
- Zig Build System

# WebAssembly §

Zig supports building for WebAssembly out of the box.

## Freestanding §

For host environments like the web browser and nodejs, build as a library using the freestanding OS target. Here's an example of running Zig code compiled to WebAssembly with nodejs.

math.zig

    extern fn print(i32) void;

    export fn add(a: i32, b: i32) void {
        print(a + b);
    }

    $ zig build-lib math.zig -target wasm32-freestanding

test.js

    const fs = require('fs');
    const source = fs.readFileSync("./math.wasm");
    const typedArray = new Uint8Array(source);

    WebAssembly.instantiate(typedArray, {
      env: {
        print: (result) => { console.log(`The result is ${result}`); }
      }}).then(result => {
      const add = result.instance.exports.add;
      add(1, 2);
    });

    $ node test.js
    The result is 3

## WASI §

Zig's support for WebAssembly System Interface (WASI) is under active development. Example of using the standard library and reading command line arguments:

wasi.zig

    const std = @import("std");

    pub fn main() !void {
        // TODO a better default allocator that isn't as wasteful!
        const args = try std.process.argsAlloc(std.heap.page_allocator);
        defer std.process.argsFree(std.heap.page_allocator, args);

        for (args) |arg, i| {
            std.debug.warn("{}: {}\n", .{i, arg});
        }
    }

    $ zig build-exe wasi.zig -target wasm32-wasi

    $ wasmer run wasi.wasm 123 hello
    0: wasi.wasm
    1: 123
    2: hello

# Targets §

Zig supports generating code for all targets that LLVM supports. Here is what it looks like to execute `zig targets` on a Linux x86_64 computer:

    $ zig targets
    Architectures:
      arm
        v8_4a
        v8_3a
        v8_2a
        v8_1a
        v8
        v8r
        v8m_baseline
        v8m_mainline
        v7
        v7em
        v7m
        v7s
        v7k
        v7ve
        v6
        v6m
        v6k
        v6t2
        v5
        v5te
        v4t
      armeb
        v8_4a
        v8_3a
        v8_2a
        v8_1a
        v8
        v8r
        v8m_baseline
        v8m_mainline
        v7
        v7em
        v7m
        v7s
        v7k
        v7ve
        v6
        v6m
        v6k
        v6t2
        v5
        v5te
        v4t
      aarch64
        v8_4a
        v8_3a
        v8_2a
        v8_1a
        v8
        v8r
        v8m_baseline
        v8m_mainline
      aarch64_be
        v8_4a
        v8_3a
        v8_2a
        v8_1a
        v8
        v8r
        v8m_baseline
        v8m_mainline
      avr
      bpfel
      bpfeb
      hexagon
      mips
      mipsel
      mips64
      mips64el
      msp430
      powerpc
      powerpc64
      powerpc64le
      r600
      amdgcn
      riscv32
      riscv64
      sparc
      sparcv9
      sparcel
      s390x
      thumb
        v8_4a
        v8_3a
        v8_2a
        v8_1a
        v8
        v8r
        v8m_baseline
        v8m_mainline
        v7
        v7em
        v7m
        v7s
        v7k
        v7ve
        v6
        v6m
        v6k
        v6t2
        v5
        v5te
        v4t
      thumbeb
        v8_4a
        v8_3a
        v8_2a
        v8_1a
        v8
        v8r
        v8m_baseline
        v8m_mainline
        v7
        v7em
        v7m
        v7s
        v7k
        v7ve
        v6
        v6m
        v6k
        v6t2
        v5
        v5te
        v4t
      i386
      x86_64 (native)
      xcore
      nvptx
      nvptx64
      lanai
      wasm32
      wasm64

    Operating Systems:
      freestanding
      ananas
      cloudabi
      dragonfly
      freebsd
      fuchsia
      ios
      kfreebsd
      linux (native)
      lv2
      macosx
      netbsd
      openbsd
      solaris
      windows
      haiku
      minix
      rtems
      nacl
      cnk
      aix
      cuda
      nvcl
      amdhsa
      ps4
      elfiamcu
      tvos
      watchos
      mesa3d
      contiki
      amdpal
      zen
      uefi

    C ABIs:
      none
      gnu (native)
      gnuabin32
      gnuabi64
      gnueabi
      gnueabihf
      gnux32
      code16
      eabi
      eabihf
      android
      musl
      musleabi
      musleabihf
      msvc
      itanium
      cygnus
      coreclr
      simulator

    Available libcs:
      aarch64_be-linux-gnu
      aarch64_be-linux-musl
      aarch64-linux-gnu
      aarch64-linux-musleabi
      armeb-linux-gnueabi
      armeb-linux-gnueabihf
      armeb-linux-musleabi
      armeb-linux-musleabihf
      arm-linux-gnueabi
      arm-linux-gnueabihf
      arm-linux-musleabi
      arm-linux-musleabihf
      i386-linux-gnu
      i386-linux-musl
      mips64el-linux-gnuabi64
      mips64el-linux-gnuabin32
      mips64el-linux-musl
      mips64-linux-gnuabi64
      mips64-linux-gnuabin32
      mips64-linux-musl
      mipsel-linux-gnu
      mipsel-linux-musl
      mips-linux-gnu
      mips-linux-musl
      nios2-linux-gnu
      powerpc64le-linux-gnu
      powerpc64le-linux-musl
      powerpc64-linux-gnu
      powerpc64-linux-musl
      powerpc-linux-gnu
      powerpc-linux-musl
      riscv32-linux-musl
      riscv64-linux-gnu
      riscv64-linux-musl
      s390x-linux-gnu
      s390x-linux-musl
      sparc-linux-gnu
      sparcv9-linux-gnu
      wasm32-freestanding-musl
      x86_64-linux-gnu
      x86_64-linux-gnux32
      x86_64-linux-musl

The Zig Standard Library (`@import("std")`) has architecture, environment, and operating system abstractions, and thus takes additional work to support more platforms. Not all standard library code requires operating system abstractions, however, so things such as generic data structures work on all above platforms.

The current list of targets supported by the Zig Standard Library is:

- Linux x86_64
- Windows x86_64
- macOS x86_64

# Style Guide §

These coding conventions are not enforced by the compiler, but they are shipped in this documentation along with the compiler in order to provide a point of reference, should anyone wish to point to an authority on agreed upon Zig coding style.

## Whitespace §

- 4 space indentation
- Open braces on same line, unless you need to wrap.
- If a list of things is longer than 2, put each item on its own line and exercise the ability to put an extra comma at the end.
- Line length: aim for 100; use common sense.

## Names §

Roughly speaking: `camelCaseFunctionName`, `TitleCaseTypeName`, `snake_case_variable_name`. More precisely:

- If `x` is a `type` then `x` should be `TitleCase`, unless it is a `struct` with 0 fields and is never meant to be instantiated, in which case it is considered to be a "namespace" and uses `snake_case`.
- If `x` is callable, and `x`'s return type is `type`, then `x` should be `TitleCase`.
- If `x` is otherwise callable, then `x` should be `camelCase`.
- Otherwise, `x` should be `snake_case`.

Acronyms, initialisms, proper nouns, or any other word that has capitalization rules in written English are subject to naming conventions just like any other word. Even acronyms that are only 2 letters long are subject to these conventions.

These are general rules of thumb; if it makes sense to do something different, do what makes sense. For example, if there is an established convention such as `ENOENT`, follow the established convention.

## Examples §

    const namespace_name = @import("dir_name/file_name.zig");
    var global_var: i32 = undefined;
    const const_name = 42;
    const primitive_type_alias = f32;
    const string_alias = []u8;

    const StructName = struct {
        field: i32,
    };
    const StructAlias = StructName;

    fn functionName(param_name: TypeName) void {
        var functionPointer = functionName;
        functionPointer();
        functionPointer = otherFunction;
        functionPointer();
    }
    const functionAlias = functionName;

    fn ListTemplateFunction(comptime ChildType: type, comptime fixed_size: usize) type {
        return List(ChildType, fixed_size);
    }

    fn ShortList(comptime T: type, comptime n: usize) type {
        return struct {
            field_name: [n]T,
            fn methodName() void {}
        };
    }

    // The word XML loses its casing when used in Zig identifiers.
    const xml_document =
        \\<?xml version="1.0" encoding="UTF-8"?>
        \\<document>
        \\</document>
    ;
    const XmlParser = struct {
        field: i32,
    };

    // The initials BE (Big Endian) are just another word in Zig identifier names.
    fn readU32Be() u32 {}

See the Zig Standard Library for more examples.

# Source Encoding §

Zig source code is encoded in UTF-8. An invalid UTF-8 byte sequence results in a compile error.

Throughout all zig source code (including in comments), some codepoints are never allowed:

- Ascii control characters, except for U+000a (LF): U+0000 - U+0009, U+000b - U+0001f, U+007f. (Note that Windows line endings (CRLF) are not allowed, and hard tabs are not allowed.)
- Non-Ascii Unicode line endings: U+0085 (NEL), U+2028 (LS), U+2029 (PS).

The codepoint U+000a (LF) (which is encoded as the single-byte value 0x0a) is the line terminator character. This character always terminates a line of zig source code (except possibly the last line of the file).

For some discussion on the rationale behind these design decisions, see issue #663

# Keyword Reference §

TODO the rest of the keywords. Most of these can just be links to the relevant section.

## Keyword: pub §

The `pub` in front of a top level declaration makes the declaration available to reference from a different file than the one it is declared in.

See also:

- @import

# Grammar §

    Root <- skip ContainerMembers eof

    # *** Top level ***
    ContainerMembers
        <- TestDecl ContainerMembers
         / TopLevelComptime ContainerMembers
         / KEYWORD_pub? TopLevelDecl ContainerMembers
         / ContainerField COMMA ContainerMembers
         / ContainerField
         /

    TestDecl <- KEYWORD_test STRINGLITERALSINGLE Block

    TopLevelComptime <- KEYWORD_comptime BlockExpr

    TopLevelDecl
        <- (KEYWORD_export / KEYWORD_extern STRINGLITERALSINGLE? / KEYWORD_inline)? FnProto (SEMICOLON / Block)
         / (KEYWORD_export / KEYWORD_extern STRINGLITERALSINGLE?)? KEYWORD_threadlocal? VarDecl
         / KEYWORD_usingnamespace Expr SEMICOLON

    FnProto <- FnCC? KEYWORD_fn IDENTIFIER? LPAREN ParamDeclList RPAREN ByteAlign? LinkSection? EXCLAMATIONMARK? (KEYWORD_var / TypeExpr)

    VarDecl <- (KEYWORD_const / KEYWORD_var) IDENTIFIER (COLON TypeExpr)? ByteAlign? LinkSection? (EQUAL Expr)? SEMICOLON

    ContainerField <- IDENTIFIER (COLON TypeExpr)? (EQUAL Expr)?

    # *** Block Level ***
    Statement
        <- KEYWORD_comptime? VarDecl
         / KEYWORD_comptime BlockExprStatement
         / KEYWORD_suspend (SEMICOLON / BlockExprStatement)
         / KEYWORD_defer BlockExprStatement
         / KEYWORD_errdefer BlockExprStatement
         / IfStatement
         / LabeledStatement
         / SwitchExpr
         / AssignExpr SEMICOLON

    IfStatement
        <- IfPrefix BlockExpr ( KEYWORD_else Payload? Statement )?
         / IfPrefix AssignExpr ( SEMICOLON / KEYWORD_else Payload? Statement )

    LabeledStatement <- BlockLabel? (Block / LoopStatement)

    LoopStatement <- KEYWORD_inline? (ForStatement / WhileStatement)

    ForStatement
        <- ForPrefix BlockExpr ( KEYWORD_else Statement )?
         / ForPrefix AssignExpr ( SEMICOLON / KEYWORD_else Statement )

    WhileStatement
        <- WhilePrefix BlockExpr ( KEYWORD_else Payload? Statement )?
         / WhilePrefix AssignExpr ( SEMICOLON / KEYWORD_else Payload? Statement )

    BlockExprStatement
        <- BlockExpr
         / AssignExpr SEMICOLON

    BlockExpr <- BlockLabel? Block

    # *** Expression Level ***
    AssignExpr <- Expr (AssignOp Expr)?

    Expr <- KEYWORD_try* BoolOrExpr

    BoolOrExpr <- BoolAndExpr (KEYWORD_or BoolAndExpr)*

    BoolAndExpr <- CompareExpr (KEYWORD_and CompareExpr)*

    CompareExpr <- BitwiseExpr (CompareOp BitwiseExpr)?

    BitwiseExpr <- BitShiftExpr (BitwiseOp BitShiftExpr)*

    BitShiftExpr <- AdditionExpr (BitShiftOp AdditionExpr)*

    AdditionExpr <- MultiplyExpr (AdditionOp MultiplyExpr)*

    MultiplyExpr <- PrefixExpr (MultiplyOp PrefixExpr)*

    PrefixExpr <- PrefixOp* PrimaryExpr

    PrimaryExpr
        <- AsmExpr
         / IfExpr
         / KEYWORD_break BreakLabel? Expr?
         / KEYWORD_comptime Expr
         / KEYWORD_continue BreakLabel?
         / KEYWORD_resume Expr
         / KEYWORD_return Expr?
         / BlockLabel? LoopExpr
         / Block
         / CurlySuffixExpr

    IfExpr <- IfPrefix Expr (KEYWORD_else Payload? Expr)?

    Block <- LBRACE Statement* RBRACE

    LoopExpr <- KEYWORD_inline? (ForExpr / WhileExpr)

    ForExpr <- ForPrefix Expr (KEYWORD_else Expr)?

    WhileExpr <- WhilePrefix Expr (KEYWORD_else Payload? Expr)?

    CurlySuffixExpr <- TypeExpr InitList?

    InitList
        <- LBRACE FieldInit (COMMA FieldInit)* COMMA? RBRACE
         / LBRACE Expr (COMMA Expr)* COMMA? RBRACE
         / LBRACE RBRACE

    TypeExpr <- PrefixTypeOp* ErrorUnionExpr

    ErrorUnionExpr <- SuffixExpr (EXCLAMATIONMARK TypeExpr)?

    SuffixExpr
        <- KEYWORD_async PrimaryTypeExpr SuffixOp* FnCallArguments
         / PrimaryTypeExpr (SuffixOp / FnCallArguments)*

    PrimaryTypeExpr
        <- BUILTINIDENTIFIER FnCallArguments
         / CHAR_LITERAL
         / ContainerDecl
         / DOT IDENTIFIER
         / DOT InitList
         / ErrorSetDecl
         / FLOAT
         / FnProto
         / GroupedExpr
         / LabeledTypeExpr
         / IDENTIFIER
         / IfTypeExpr
         / INTEGER
         / KEYWORD_comptime TypeExpr
         / KEYWORD_error DOT IDENTIFIER
         / KEYWORD_false
         / KEYWORD_null
         / KEYWORD_promise
         / KEYWORD_true
         / KEYWORD_undefined
         / KEYWORD_unreachable
         / STRINGLITERAL
         / SwitchExpr

    ContainerDecl <- (KEYWORD_extern / KEYWORD_packed)? ContainerDeclAuto

    ErrorSetDecl <- KEYWORD_error LBRACE IdentifierList RBRACE

    GroupedExpr <- LPAREN Expr RPAREN

    IfTypeExpr <- IfPrefix TypeExpr (KEYWORD_else Payload? TypeExpr)?

    LabeledTypeExpr
        <- BlockLabel Block
         / BlockLabel? LoopTypeExpr

    LoopTypeExpr <- KEYWORD_inline? (ForTypeExpr / WhileTypeExpr)

    ForTypeExpr <- ForPrefix TypeExpr (KEYWORD_else TypeExpr)?

    WhileTypeExpr <- WhilePrefix TypeExpr (KEYWORD_else Payload? TypeExpr)?

    SwitchExpr <- KEYWORD_switch LPAREN Expr RPAREN LBRACE SwitchProngList RBRACE

    # *** Assembly ***
    AsmExpr <- KEYWORD_asm KEYWORD_volatile? LPAREN STRINGLITERAL AsmOutput? RPAREN

    AsmOutput <- COLON AsmOutputList AsmInput?

    AsmOutputItem <- LBRACKET IDENTIFIER RBRACKET STRINGLITERAL LPAREN (MINUSRARROW TypeExpr / IDENTIFIER) RPAREN

    AsmInput <- COLON AsmInputList AsmClobbers?

    AsmInputItem <- LBRACKET IDENTIFIER RBRACKET STRINGLITERAL LPAREN Expr RPAREN

    AsmClobbers <- COLON StringList

    # *** Helper grammar ***
    BreakLabel <- COLON IDENTIFIER

    BlockLabel <- IDENTIFIER COLON

    FieldInit <- DOT IDENTIFIER EQUAL Expr

    WhileContinueExpr <- COLON LPAREN AssignExpr RPAREN

    LinkSection <- KEYWORD_linksection LPAREN Expr RPAREN

    # Fn specific
    FnCC
        <- KEYWORD_extern
         / KEYWORD_async

    ParamDecl <- (KEYWORD_noalias / KEYWORD_comptime)? (IDENTIFIER COLON)? ParamType

    ParamType
        <- KEYWORD_var
         / DOT3
         / TypeExpr

    # Control flow prefixes
    IfPrefix <- KEYWORD_if LPAREN Expr RPAREN PtrPayload?

    WhilePrefix <- KEYWORD_while LPAREN Expr RPAREN PtrPayload? WhileContinueExpr?

    ForPrefix <- KEYWORD_for LPAREN Expr RPAREN PtrIndexPayload

    # Payloads
    Payload <- PIPE IDENTIFIER PIPE

    PtrPayload <- PIPE ASTERISK? IDENTIFIER PIPE

    PtrIndexPayload <- PIPE ASTERISK? IDENTIFIER (COMMA IDENTIFIER)? PIPE

    # Switch specific
    SwitchProng <- SwitchCase EQUALRARROW PtrPayload? AssignExpr

    SwitchCase
        <- SwitchItem (COMMA SwitchItem)* COMMA?
         / KEYWORD_else

    SwitchItem <- Expr (DOT3 Expr)?

    # Operators
    AssignOp
        <- ASTERISKEQUAL
         / SLASHEQUAL
         / PERCENTEQUAL
         / PLUSEQUAL
         / MINUSEQUAL
         / LARROW2EQUAL
         / RARROW2EQUAL
         / AMPERSANDEQUAL
         / CARETEQUAL
         / PIPEEQUAL
         / ASTERISKPERCENTEQUAL
         / PLUSPERCENTEQUAL
         / MINUSPERCENTEQUAL
         / EQUAL

    CompareOp
        <- EQUALEQUAL
         / EXCLAMATIONMARKEQUAL
         / LARROW
         / RARROW
         / LARROWEQUAL
         / RARROWEQUAL

    BitwiseOp
        <- AMPERSAND
         / CARET
         / PIPE
         / KEYWORD_orelse
         / KEYWORD_catch Payload?

    BitShiftOp
        <- LARROW2
         / RARROW2

    AdditionOp
        <- PLUS
         / MINUS
         / PLUS2
         / PLUSPERCENT
         / MINUSPERCENT

    MultiplyOp
        <- PIPE2
         / ASTERISK
         / SLASH
         / PERCENT
         / ASTERISK2
         / ASTERISKPERCENT

    PrefixOp
        <- EXCLAMATIONMARK
         / MINUS
         / TILDE
         / MINUSPERCENT
         / AMPERSAND
         / KEYWORD_try
         / KEYWORD_await

    PrefixTypeOp
        <- QUESTIONMARK
         / KEYWORD_promise MINUSRARROW
         / ArrayTypeStart (ByteAlign / KEYWORD_const / KEYWORD_volatile / KEYWORD_allowzero)*
         / PtrTypeStart (KEYWORD_align LPAREN Expr (COLON INTEGER COLON INTEGER)? RPAREN / KEYWORD_const / KEYWORD_volatile / KEYWORD_allowzero)*

    SuffixOp
        <- LBRACKET Expr (DOT2 Expr?)? RBRACKET
         / DOT IDENTIFIER
         / DOTASTERISK
         / DOTQUESTIONMARK

    FnCallArguments <- LPAREN ExprList RPAREN

    # Ptr specific
    ArrayTypeStart <- LBRACKET Expr? RBRACKET

    PtrTypeStart
        <- ASTERISK
         / ASTERISK2
         / PTRUNKNOWN
         / PTRC

    # ContainerDecl specific
    ContainerDeclAuto <- ContainerDeclType LBRACE ContainerMembers RBRACE

    ContainerDeclType
        <- (KEYWORD_struct / KEYWORD_enum) (LPAREN Expr RPAREN)?
         / KEYWORD_union (LPAREN (KEYWORD_enum (LPAREN Expr RPAREN)? / Expr) RPAREN)?

    # Alignment
    ByteAlign <- KEYWORD_align LPAREN Expr RPAREN

    # Lists
    IdentifierList <- (IDENTIFIER COMMA)* IDENTIFIER?

    SwitchProngList <- (SwitchProng COMMA)* SwitchProng?

    AsmOutputList <- (AsmOutputItem COMMA)* AsmOutputItem?

    AsmInputList <- (AsmInputItem COMMA)* AsmInputItem?

    StringList <- (STRINGLITERAL COMMA)* STRINGLITERAL?

    ParamDeclList <- (ParamDecl COMMA)* ParamDecl?

    ExprList <- (Expr COMMA)* Expr?

    # *** Tokens ***
    eof <- !.
    hex <- [0-9a-fA-F]
    char_escape
        <- "\\x" hex hex
         / "\\u{" hex+ "}"
         / "\\" [nr\\t'"]
    char_char
        <- char_escape
         / [^\\'\n]
    string_char
        <- char_escape
         / [^\\"\n]

    line_comment <- '//'[^\n]*
    line_string <- ("\\\\" [^\n]* [ \n]*)+
    skip <- ([ \n] / line_comment)*

    CHAR_LITERAL <- "'" char_char "'" skip
    FLOAT
        <- "0x" hex+   "." hex+   ([pP] [-+]? hex+)?   skip
         /      [0-9]+ "." [0-9]+ ([eE] [-+]? [0-9]+)? skip
         / "0x" hex+   "."? [pP] [-+]? hex+   skip
         /      [0-9]+ "."? [eE] [-+]? [0-9]+ skip
    INTEGER
        <- "0b" [01]+  skip
         / "0o" [0-7]+ skip
         / "0x" hex+   skip
         /      [0-9]+ skip
    STRINGLITERALSINGLE <- "\"" string_char* "\"" skip
    STRINGLITERAL
        <- STRINGLITERALSINGLE
         / line_string                 skip
    IDENTIFIER
        <- !keyword [A-Za-z_] [A-Za-z0-9_]* skip
         / "@\"" string_char* "\""                            skip
    BUILTINIDENTIFIER <- "@"[A-Za-z_][A-Za-z0-9_]* skip

    AMPERSAND            <- '&'      ![=]      skip
    AMPERSANDEQUAL       <- '&='               skip
    ASTERISK             <- '*'      ![*%=]    skip
    ASTERISK2            <- '**'               skip
    ASTERISKEQUAL        <- '*='               skip
    ASTERISKPERCENT      <- '*%'     ![=]      skip
    ASTERISKPERCENTEQUAL <- '*%='              skip
    CARET                <- '^'      ![=]      skip
    CARETEQUAL           <- '^='               skip
    COLON                <- ':'                skip
    COMMA                <- ','                skip
    DOT                  <- '.'      ![*.?]    skip
    DOT2                 <- '..'     ![.]      skip
    DOT3                 <- '...'              skip
    DOTASTERISK          <- '.*'               skip
    DOTQUESTIONMARK      <- '.?'               skip
    EQUAL                <- '='      ![>=]     skip
    EQUALEQUAL           <- '=='               skip
    EQUALRARROW          <- '=>'               skip
    EXCLAMATIONMARK      <- '!'      ![=]      skip
    EXCLAMATIONMARKEQUAL <- '!='               skip
    LARROW               <- '<'      ![<=]     skip
    LARROW2              <- '<<'     ![=]      skip
    LARROW2EQUAL         <- '<<='              skip
    LARROWEQUAL          <- '<='               skip
    LBRACE               <- '{'                skip
    LBRACKET             <- '['      ![*]      skip
    LPAREN               <- '('                skip
    MINUS                <- '-'      ![%=>]    skip
    MINUSEQUAL           <- '-='               skip
    MINUSPERCENT         <- '-%'     ![=]      skip
    MINUSPERCENTEQUAL    <- '-%='              skip
    MINUSRARROW          <- '->'               skip
    PERCENT              <- '%'      ![=]      skip
    PERCENTEQUAL         <- '%='               skip
    PIPE                 <- '|'      ![|=]     skip
    PIPE2                <- '||'               skip
    PIPEEQUAL            <- '|='               skip
    PLUS                 <- '+'      ![%+=]    skip
    PLUS2                <- '++'               skip
    PLUSEQUAL            <- '+='               skip
    PLUSPERCENT          <- '+%'     ![=]      skip
    PLUSPERCENTEQUAL     <- '+%='              skip
    PTRC                 <- '[*c]'             skip
    PTRUNKNOWN           <- '[*]'              skip
    QUESTIONMARK         <- '?'                skip
    RARROW               <- '>'      ![>=]     skip
    RARROW2              <- '>>'     ![=]      skip
    RARROW2EQUAL         <- '>>='              skip
    RARROWEQUAL          <- '>='               skip
    RBRACE               <- '}'                skip
    RBRACKET             <- ']'                skip
    RPAREN               <- ')'                skip
    SEMICOLON            <- ';'                skip
    SLASH                <- '/'      ![=]      skip
    SLASHEQUAL           <- '/='               skip
    TILDE                <- '~'                skip

    end_of_word <- ![a-zA-Z0-9_] skip
    KEYWORD_align       <- 'align'       end_of_word
    KEYWORD_allowzero   <- 'allowzero'   end_of_word
    KEYWORD_and         <- 'and'         end_of_word
    KEYWORD_asm         <- 'asm'         end_of_word
    KEYWORD_async       <- 'async'       end_of_word
    KEYWORD_await       <- 'await'       end_of_word
    KEYWORD_break       <- 'break'       end_of_word
    KEYWORD_catch       <- 'catch'       end_of_word
    KEYWORD_comptime    <- 'comptime'    end_of_word
    KEYWORD_const       <- 'const'       end_of_word
    KEYWORD_continue    <- 'continue'    end_of_word
    KEYWORD_defer       <- 'defer'       end_of_word
    KEYWORD_else        <- 'else'        end_of_word
    KEYWORD_enum        <- 'enum'        end_of_word
    KEYWORD_errdefer    <- 'errdefer'    end_of_word
    KEYWORD_error       <- 'error'       end_of_word
    KEYWORD_export      <- 'export'      end_of_word
    KEYWORD_extern      <- 'extern'      end_of_word
    KEYWORD_false       <- 'false'       end_of_word
    KEYWORD_fn          <- 'fn'          end_of_word
    KEYWORD_for         <- 'for'         end_of_word
    KEYWORD_if          <- 'if'          end_of_word
    KEYWORD_inline      <- 'inline'      end_of_word
    KEYWORD_noalias     <- 'noalias'     end_of_word
    KEYWORD_null        <- 'null'        end_of_word
    KEYWORD_or          <- 'or'          end_of_word
    KEYWORD_orelse      <- 'orelse'      end_of_word
    KEYWORD_packed      <- 'packed'      end_of_word
    KEYWORD_promise     <- 'promise'     end_of_word
    KEYWORD_pub         <- 'pub'         end_of_word
    KEYWORD_resume      <- 'resume'      end_of_word
    KEYWORD_return      <- 'return'      end_of_word
    KEYWORD_linksection <- 'linksection' end_of_word
    KEYWORD_struct      <- 'struct'      end_of_word
    KEYWORD_suspend     <- 'suspend'     end_of_word
    KEYWORD_switch      <- 'switch'      end_of_word
    KEYWORD_test        <- 'test'        end_of_word
    KEYWORD_threadlocal <- 'threadlocal' end_of_word
    KEYWORD_true        <- 'true'        end_of_word
    KEYWORD_try         <- 'try'         end_of_word
    KEYWORD_undefined   <- 'undefined'   end_of_word
    KEYWORD_union       <- 'union'       end_of_word
    KEYWORD_unreachable <- 'unreachable' end_of_word
    KEYWORD_usingnamespace <- 'usingnamespace' end_of_word
    KEYWORD_var         <- 'var'         end_of_word
    KEYWORD_volatile    <- 'volatile'    end_of_word
    KEYWORD_while       <- 'while'       end_of_word

    keyword <- KEYWORD_align / KEYWORD_and / KEYWORD_allowzero / KEYWORD_asm
             / KEYWORD_async / KEYWORD_await / KEYWORD_break
             / KEYWORD_catch / KEYWORD_comptime / KEYWORD_const / KEYWORD_continue
             / KEYWORD_defer / KEYWORD_else / KEYWORD_enum / KEYWORD_errdefer
             / KEYWORD_error / KEYWORD_export / KEYWORD_extern / KEYWORD_false
             / KEYWORD_fn / KEYWORD_for / KEYWORD_if / KEYWORD_inline
             / KEYWORD_noalias / KEYWORD_null / KEYWORD_or
             / KEYWORD_orelse / KEYWORD_packed / KEYWORD_promise / KEYWORD_pub
             / KEYWORD_resume / KEYWORD_return / KEYWORD_linksection
             / KEYWORD_struct / KEYWORD_suspend
             / KEYWORD_switch / KEYWORD_test / KEYWORD_threadlocal / KEYWORD_true / KEYWORD_try
             / KEYWORD_undefined / KEYWORD_union / KEYWORD_unreachable
             / KEYWORD_usingnamespace / KEYWORD_var / KEYWORD_volatile / KEYWORD_while

# Zen §

- Communicate intent precisely.
- Edge cases matter.
- Favor reading code over writing code.
- Only one obvious way to do things.
- Runtime crashes are better than bugs.
- Compile errors are better than runtime crashes.
- Incremental improvements.
- Avoid local maximums.
- Reduce the amount one must remember.
- Minimize energy spent on coding style.
- Resource deallocation must succeed.
- Together we serve end users.