    //! This file contains thin wrappers around OS-specific APIs, with these
    //! specific goals in mind:
    //! * Convert "errno"-style error codes into Zig errors.
    //! * When null-terminated byte buffers are required, provide APIs which accept
    //!   slices as well as APIs which accept null-terminated byte buffers. Same goes
    //!   for UTF-16LE encoding.
    //! * Where operating systems share APIs, e.g. POSIX, these thin wrappers provide
    //!   cross platform abstracting.
    //! * When there exists a corresponding libc function and linking libc, the libc
    //!   implementation is used. Exceptions are made for known buggy areas of libc.
    //!   On Linux libc can be side-stepped by using `std.os.linux` directly.
    //! * For Windows, this file represents the API that libc would provide for
    //!   Windows. For thin wrappers around Windows-specific APIs, see `std.os.windows`.
    //! Note: The Zig standard library does not support POSIX thread cancellation, and
    //! in general EINTR is handled by trying again.

    const root = @import("root");
    const std = @import("std.zig");
    const builtin = @import("builtin");
    const assert = std.debug.assert;
    const math = std.math;
    const mem = std.mem;
    const elf = std.elf;
    const fs = std.fs;
    const dl = @import("dynamic_library.zig");
    const MAX_PATH_BYTES = std.fs.MAX_PATH_BYTES;
    const is_windows = builtin.os.tag == .windows;
    const Allocator = std.mem.Allocator;
    const Preopen = std.fs.wasi.Preopen;
    const PreopenList = std.fs.wasi.PreopenList;

    pub const darwin = @import("os/darwin.zig");
    pub const dragonfly = std.c;
    pub const freebsd = std.c;
    pub const haiku = std.c;
    pub const netbsd = std.c;
    pub const openbsd = std.c;
    pub const solaris = std.c;
    pub const linux = @import("os/linux.zig");
    pub const plan9 = @import("os/plan9.zig");
    pub const uefi = @import("os/uefi.zig");
    pub const wasi = @import("os/wasi.zig");
    pub const windows = @import("os/windows.zig");
    pub const posix_spawn = @import("os/posix_spawn.zig");

    comptime {
        assert(@import("std") == std); // std lib tests require --zig-lib-dir

    }

    test {
        _ = darwin;
        _ = linux;
        if (builtin.os.tag == .uefi) {
            _ = uefi;
        }
        _ = wasi;
        _ = windows;
        _ = posix_spawn;

        _ = @import("os/test.zig");
    }

    /// Applications can override the `system` API layer in their root source file.
    /// Otherwise, when linking libc, this is the C API.
    /// When not linking libc, it is the OS-specific system interface.
    pub const system = if (@hasDecl(root, "os") and root.os != @This())
        root.os.system
    else if (builtin.link_libc or is_windows)
        std.c
    else switch (builtin.os.tag) {
        .linux => linux,
        .wasi => wasi,
        .uefi => uefi,
        else => struct {},
    };

    pub const AF = system.AF;
    pub const AF_SUN = system.AF_SUN;
    pub const ARCH = system.ARCH;
    pub const AT = system.AT;
    pub const AT_SUN = system.AT_SUN;
    pub const CLOCK = system.CLOCK;
    pub const CPU_COUNT = system.CPU_COUNT;
    pub const CTL = system.CTL;
    pub const DT = system.DT;
    pub const E = system.E;
    pub const Elf_Symndx = system.Elf_Symndx;
    pub const F = system.F;
    pub const FD_CLOEXEC = system.FD_CLOEXEC;
    pub const Flock = system.Flock;
    pub const HOST_NAME_MAX = system.HOST_NAME_MAX;
    pub const IFNAMESIZE = system.IFNAMESIZE;
    pub const IOV_MAX = system.IOV_MAX;
    pub const IPPROTO = system.IPPROTO;
    pub const KERN = system.KERN;
    pub const Kevent = system.Kevent;
    pub const LOCK = system.LOCK;
    pub const MADV = system.MADV;
    pub const MAP = system.MAP;
    pub const MSF = system.MSF;
    pub const MAX_ADDR_LEN = system.MAX_ADDR_LEN;
    pub const MFD = system.MFD;
    pub const MMAP2_UNIT = system.MMAP2_UNIT;
    pub const MSG = system.MSG;
    pub const NAME_MAX = system.NAME_MAX;
    pub const O = switch (builtin.os.tag) {
        // We want to expose the POSIX-like OFLAGS, so we use std.c.wasi.O instead

        // of std.os.wasi.O, which is for non-POSIX-like `wasi.path_open`, etc.

        .wasi => std.c.O,
        else => system.O,
    };
    pub const PATH_MAX = system.PATH_MAX;
    pub const POLL = system.POLL;
    pub const POSIX_FADV = system.POSIX_FADV;
    pub const PR = system.PR;
    pub const PROT = system.PROT;
    pub const REG = system.REG;
    pub const RIGHT = system.RIGHT;
    pub const RLIM = system.RLIM;
    pub const RR = system.RR;
    pub const S = system.S;
    pub const SA = system.SA;
    pub const SC = system.SC;
    pub const _SC = system._SC;
    pub const SEEK = system.SEEK;
    pub const SHUT = system.SHUT;
    pub const SIG = system.SIG;
    pub const SIOCGIFINDEX = system.SIOCGIFINDEX;
    pub const SO = system.SO;
    pub const SOCK = system.SOCK;
    pub const SOL = system.SOL;
    pub const STDERR_FILENO = system.STDERR_FILENO;
    pub const STDIN_FILENO = system.STDIN_FILENO;
    pub const STDOUT_FILENO = system.STDOUT_FILENO;
    pub const SYS = system.SYS;
    pub const Sigaction = system.Sigaction;
    pub const Stat = system.Stat;
    pub const TCSA = system.TCSA;
    pub const TCP = system.TCP;
    pub const VDSO = system.VDSO;
    pub const W = system.W;
    pub const addrinfo = system.addrinfo;
    pub const blkcnt_t = system.blkcnt_t;
    pub const blksize_t = system.blksize_t;
    pub const clock_t = system.clock_t;
    pub const cpu_set_t = system.cpu_set_t;
    pub const dev_t = system.dev_t;
    pub const dl_phdr_info = system.dl_phdr_info;
    pub const empty_sigset = system.empty_sigset;
    pub const fd_t = system.fd_t;
    pub const fdflags_t = system.fdflags_t;
    pub const fdstat_t = system.fdstat_t;
    pub const gid_t = system.gid_t;
    pub const ifreq = system.ifreq;
    pub const ino_t = system.ino_t;
    pub const lookupflags_t = system.lookupflags_t;
    pub const mcontext_t = system.mcontext_t;
    pub const mode_t = system.mode_t;
    pub const msghdr = system.msghdr;
    pub const msghdr_const = system.msghdr_const;
    pub const nfds_t = system.nfds_t;
    pub const nlink_t = system.nlink_t;
    pub const off_t = system.off_t;
    pub const oflags_t = system.oflags_t;
    pub const pid_t = system.pid_t;
    pub const pollfd = system.pollfd;
    pub const port_t = system.port_t;
    pub const port_event = system.port_event;
    pub const port_notify = system.port_notify;
    pub const file_obj = system.file_obj;
    pub const rights_t = system.rights_t;
    pub const rlim_t = system.rlim_t;
    pub const rlimit = system.rlimit;
    pub const rlimit_resource = system.rlimit_resource;
    pub const rusage = system.rusage;
    pub const sa_family_t = system.sa_family_t;
    pub const siginfo_t = system.siginfo_t;
    pub const sigset_t = system.sigset_t;
    pub const sockaddr = system.sockaddr;
    pub const socklen_t = system.socklen_t;
    pub const stack_t = system.stack_t;
    pub const tcflag_t = system.tcflag_t;
    pub const termios = system.termios;
    pub const time_t = system.time_t;
    pub const timespec = system.timespec;
    pub const timestamp_t = system.timestamp_t;
    pub const timeval = system.timeval;
    pub const timezone = system.timezone;
    pub const ucontext_t = system.ucontext_t;
    pub const uid_t = system.uid_t;
    pub const user_desc = system.user_desc;
    pub const utsname = system.utsname;

    pub const F_OK = system.F_OK;
    pub const R_OK = system.R_OK;
    pub const W_OK = system.W_OK;
    pub const X_OK = system.X_OK;

    pub const iovec = extern struct {
        iov_base: [*]u8,
        iov_len: usize,
    };

    pub const iovec_const = extern struct {
        iov_base: [*]const u8,
        iov_len: usize,
    };

    pub const LOG = struct {
        /// system is unusable
        pub const EMERG = 0;
        /// action must be taken immediately
        pub const ALERT = 1;
        /// critical conditions
        pub const CRIT = 2;
        /// error conditions
        pub const ERR = 3;
        /// warning conditions
        pub const WARNING = 4;
        /// normal but significant condition
        pub const NOTICE = 5;
        /// informational
        pub const INFO = 6;
        /// debug-level messages
        pub const DEBUG = 7;
    };

    /// An fd-relative file path
    ///
    /// This is currently only used for WASI-specific functionality, but the concept
    /// is the same as the dirfd/pathname pairs in the `*at(...)` POSIX functions.
    pub const RelativePathWasi = struct {
        /// Handle to directory
        dir_fd: fd_t,
        /// Path to resource within `dir_fd`.
        relative_path: []const u8,
    };

    pub const socket_t = if (builtin.os.tag == .windows) windows.ws2_32.SOCKET else fd_t;

    /// See also `getenv`. Populated by startup code before main().
    /// TODO this is a footgun because the value will be undefined when using `zig build-lib`.
    /// https://github.com/ziglang/zig/issues/4524
    pub var environ: [][*:0]u8 = undefined;

    /// Populated by startup code before main().
    /// Not available on WASI or Windows without libc. See `std.process.argsAlloc`
    /// or `std.process.argsWithAllocator` for a cross-platform alternative.
    pub var argv: [][*:0]u8 = if (builtin.link_libc) undefined else switch (builtin.os.tag) {
        .windows => @compileError("argv isn't supported on Windows: use std.process.argsAlloc instead"),
        .wasi => @compileError("argv isn't supported on WASI: use std.process.argsAlloc instead"),
        else => undefined,
    };

    /// To obtain errno, call this function with the return value of the
    /// system function call. For some systems this will obtain the value directly
    /// from the return code; for others it will use a thread-local errno variable.
    /// Therefore, this function only returns a well-defined value when it is called
    /// directly after the system function call which one wants to learn the errno
    /// value of.
    pub const errno = system.getErrno;

    /// Closes the file descriptor.
    /// This function is not capable of returning any indication of failure. An
    /// application which wants to ensure writes have succeeded before closing
    /// must call `fsync` before `close`.
    /// Note: The Zig standard library does not support POSIX thread cancellation.
    pub fn close(fd: fd_t) void {
        if (builtin.os.tag == .windows) {
            return windows.CloseHandle(fd);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            _ = wasi.fd_close(fd);
            return;
        }
        if (comptime builtin.target.isDarwin()) {
            // This avoids the EINTR problem.

            switch (darwin.getErrno(darwin.@"close$NOCANCEL"(fd))) {
                .BADF => unreachable, // Always a race condition.

                else => return,
            }
        }
        switch (errno(system.close(fd))) {
            .BADF => unreachable, // Always a race condition.

            .INTR => return, // This is still a success. See https://github.com/ziglang/zig/issues/2425

            else => return,
        }
    }

    pub const FChmodError = error{
        AccessDenied,
        InputOutput,
        SymLinkLoop,
        FileNotFound,
        SystemResources,
        ReadOnlyFileSystem,
    } || UnexpectedError;

    /// Changes the mode of the file referred to by the file descriptor.
    /// The process must have the correct privileges in order to do this
    /// successfully, or must have the effective user ID matching the owner
    /// of the file.
    pub fn fchmod(fd: fd_t, mode: mode_t) FChmodError!void {
        if (builtin.os.tag == .windows or builtin.os.tag == .wasi)
            @compileError("Unsupported OS");

        while (true) {
            const res = system.fchmod(fd, mode);

            switch (system.getErrno(res)) {
                .SUCCESS => return,
                .INTR => continue,
                .BADF => unreachable, // Can be reached if the fd refers to a non-iterable directory.

                .FAULT => unreachable,
                .INVAL => unreachable,
                .ACCES => return error.AccessDenied,
                .IO => return error.InputOutput,
                .LOOP => return error.SymLinkLoop,
                .NOENT => return error.FileNotFound,
                .NOMEM => return error.SystemResources,
                .NOTDIR => return error.FileNotFound,
                .PERM => return error.AccessDenied,
                .ROFS => return error.ReadOnlyFileSystem,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const FChownError = error{
        AccessDenied,
        InputOutput,
        SymLinkLoop,
        FileNotFound,
        SystemResources,
        ReadOnlyFileSystem,
    } || UnexpectedError;

    /// Changes the owner and group of the file referred to by the file descriptor.
    /// The process must have the correct privileges in order to do this
    /// successfully. The group may be changed by the owner of the directory to
    /// any group of which the owner is a member. If the owner or group is
    /// specified as `null`, the ID is not changed.
    pub fn fchown(fd: fd_t, owner: ?uid_t, group: ?gid_t) FChownError!void {
        if (builtin.os.tag == .windows or builtin.os.tag == .wasi)
            @compileError("Unsupported OS");

        while (true) {
            const res = system.fchown(fd, owner orelse @as(u32, 0) -% 1, group orelse @as(u32, 0) -% 1);

            switch (system.getErrno(res)) {
                .SUCCESS => return,
                .INTR => continue,
                .BADF => unreachable, // Can be reached if the fd refers to a non-iterable directory.

                .FAULT => unreachable,
                .INVAL => unreachable,
                .ACCES => return error.AccessDenied,
                .IO => return error.InputOutput,
                .LOOP => return error.SymLinkLoop,
                .NOENT => return error.FileNotFound,
                .NOMEM => return error.SystemResources,
                .NOTDIR => return error.FileNotFound,
                .PERM => return error.AccessDenied,
                .ROFS => return error.ReadOnlyFileSystem,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const RebootError = error{
        PermissionDenied,
    } || UnexpectedError;

    pub const RebootCommand = switch (builtin.os.tag) {
        .linux => union(linux.LINUX_REBOOT.CMD) {
            RESTART: void,
            HALT: void,
            CAD_ON: void,
            CAD_OFF: void,
            POWER_OFF: void,
            RESTART2: [*:0]const u8,
            SW_SUSPEND: void,
            KEXEC: void,
        },
        else => @compileError("Unsupported OS"),
    };

    pub fn reboot(cmd: RebootCommand) RebootError!void {
        switch (builtin.os.tag) {
            .linux => {
                switch (system.getErrno(linux.reboot(
                    .MAGIC1,
                    .MAGIC2,
                    @as(linux.LINUX_REBOOT.CMD, cmd),
                    switch (cmd) {
                        .RESTART2 => |s| s,
                        else => null,
                    },
                ))) {
                    .SUCCESS => {},
                    .PERM => return error.PermissionDenied,
                    else => |err| return std.os.unexpectedErrno(err),
                }
                switch (cmd) {
                    .CAD_OFF => {},
                    .CAD_ON => {},
                    .SW_SUSPEND => {},

                    .HALT => unreachable,
                    .KEXEC => unreachable,
                    .POWER_OFF => unreachable,
                    .RESTART => unreachable,
                    .RESTART2 => unreachable,
                }
            },
            else => @compileError("Unsupported OS"),
        }
    }

    pub const GetRandomError = OpenError;

    /// Obtain a series of random bytes. These bytes can be used to seed user-space
    /// random number generators or for cryptographic purposes.
    /// When linking against libc, this calls the
    /// appropriate OS-specific library call. Otherwise it uses the zig standard
    /// library implementation.
    pub fn getrandom(buffer: []u8) GetRandomError!void {
        if (builtin.os.tag == .windows) {
            return windows.RtlGenRandom(buffer);
        }
        if (builtin.os.tag == .linux or builtin.os.tag == .freebsd) {
            var buf = buffer;
            const use_c = builtin.os.tag != .linux or
                std.c.versionCheck(std.builtin.Version{ .major = 2, .minor = 25, .patch = 0 }).ok;

            while (buf.len != 0) {
                const res = if (use_c) blk: {
                    const rc = std.c.getrandom(buf.ptr, buf.len, 0);
                    break :blk .{
                        .num_read = @bitCast(usize, rc),
                        .err = std.c.getErrno(rc),
                    };
                } else blk: {
                    const rc = linux.getrandom(buf.ptr, buf.len, 0);
                    break :blk .{
                        .num_read = rc,
                        .err = linux.getErrno(rc),
                    };
                };

                switch (res.err) {
                    .SUCCESS => buf = buf[res.num_read..],
                    .INVAL => unreachable,
                    .FAULT => unreachable,
                    .INTR => continue,
                    .NOSYS => return getRandomBytesDevURandom(buf),
                    else => return unexpectedErrno(res.err),
                }
            }
            return;
        }
        switch (builtin.os.tag) {
            .netbsd, .openbsd, .macos, .ios, .tvos, .watchos => {
                system.arc4random_buf(buffer.ptr, buffer.len);
                return;
            },
            .wasi => switch (wasi.random_get(buffer.ptr, buffer.len)) {
                .SUCCESS => return,
                else => |err| return unexpectedErrno(err),
            },
            else => return getRandomBytesDevURandom(buffer),
        }
    }

    fn getRandomBytesDevURandom(buf: []u8) !void {
        const fd = try openZ("/dev/urandom", O.RDONLY | O.CLOEXEC, 0);
        defer close(fd);

        const st = try fstat(fd);
        if (!S.ISCHR(st.mode)) {
            return error.NoDevice;
        }

        const file = std.fs.File{
            .handle = fd,
            .capable_io_mode = .blocking,
            .intended_io_mode = .blocking,
        };
        const stream = file.reader();
        stream.readNoEof(buf) catch return error.Unexpected;
    }

    /// Causes abnormal process termination.
    /// If linking against libc, this calls the abort() libc function. Otherwise
    /// it raises SIGABRT followed by SIGKILL and finally lo
    /// Invokes the current signal handler for SIGABRT, if any.
    pub fn abort() noreturn {
        @setCold(true);
        // MSVCRT abort() sometimes opens a popup window which is undesirable, so

        // even when linking libc on Windows we use our own abort implementation.

        // See https://github.com/ziglang/zig/issues/2071 for more details.

        if (builtin.os.tag == .windows) {
            if (builtin.mode == .Debug) {
                @breakpoint();
            }
            windows.kernel32.ExitProcess(3);
        }
        if (!builtin.link_libc and builtin.os.tag == .linux) {
            // The Linux man page says that the libc abort() function

            // "first unblocks the SIGABRT signal", but this is a footgun

            // for user-defined signal handlers that want to restore some state in

            // some program sections and crash in others.

            // So, the user-installed SIGABRT handler is run, if present.

            raise(SIG.ABRT) catch {};

            // Disable all signal handlers.

            sigprocmask(SIG.BLOCK, &linux.all_mask, null);

            // Only one thread may proceed to the rest of abort().

            if (!builtin.single_threaded) {
                const global = struct {
                    var abort_entered: bool = false;
                };
                while (@cmpxchgWeak(bool, &global.abort_entered, false, true, .SeqCst, .SeqCst)) |_| {}
            }

            // Install default handler so that the tkill below will terminate.

            const sigact = Sigaction{
                .handler = .{ .handler = SIG.DFL },
                .mask = empty_sigset,
                .flags = 0,
            };
            sigaction(SIG.ABRT, &sigact, null) catch |err| switch (err) {
                error.OperationNotSupported => unreachable,
            };

            _ = linux.tkill(linux.gettid(), SIG.ABRT);

            const sigabrtmask: linux.sigset_t = [_]u32{0} ** 31 ++ [_]u32{1 << (SIG.ABRT - 1)};
            sigprocmask(SIG.UNBLOCK, &sigabrtmask, null);

            // Beyond this point should be unreachable.

            @intToPtr(*allowzero volatile u8, 0).* = 0;
            raise(SIG.KILL) catch {};
            exit(127); // Pid 1 might not be signalled in some containers.

        }
        if (builtin.os.tag == .uefi) {
            exit(0); // TODO choose appropriate exit code

        }
        if (builtin.os.tag == .wasi) {
            @breakpoint();
            exit(1);
        }
        if (builtin.os.tag == .cuda) {
            // TODO: introduce `@trap` instead of abusing https://github.com/ziglang/zig/issues/2291

            @"llvm.trap"();
        }

        system.abort();
    }

    extern fn @"llvm.trap"() noreturn;

    pub const RaiseError = UnexpectedError;

    pub fn raise(sig: u8) RaiseError!void {
        if (builtin.link_libc) {
            switch (errno(system.raise(sig))) {
                .SUCCESS => return,
                else => |err| return unexpectedErrno(err),
            }
        }

        if (builtin.os.tag == .linux) {
            var set: sigset_t = undefined;
            // block application signals

            sigprocmask(SIG.BLOCK, &linux.app_mask, &set);

            const tid = linux.gettid();
            const rc = linux.tkill(tid, sig);

            // restore signal mask

            sigprocmask(SIG.SETMASK, &set, null);

            switch (errno(rc)) {
                .SUCCESS => return,
                else => |err| return unexpectedErrno(err),
            }
        }

        @compileError("std.os.raise unimplemented for this target");
    }

    pub const KillError = error{PermissionDenied} || UnexpectedError;

    pub fn kill(pid: pid_t, sig: u8) KillError!void {
        switch (errno(system.kill(pid, sig))) {
            .SUCCESS => return,
            .INVAL => unreachable, // invalid signal

            .PERM => return error.PermissionDenied,
            .SRCH => unreachable, // always a race condition

            else => |err| return unexpectedErrno(err),
        }
    }

    /// Exits the program cleanly with the specified status code.
    pub fn exit(status: u8) noreturn {
        if (builtin.link_libc) {
            system.exit(status);
        }
        if (builtin.os.tag == .windows) {
            windows.kernel32.ExitProcess(status);
        }
        if (builtin.os.tag == .wasi) {
            wasi.proc_exit(status);
        }
        if (builtin.os.tag == .linux and !builtin.single_threaded) {
            linux.exit_group(status);
        }
        if (builtin.os.tag == .uefi) {
            // exit() is only avaliable if exitBootServices() has not been called yet.

            // This call to exit should not fail, so we don't care about its return value.

            if (uefi.system_table.boot_services) |bs| {
                _ = bs.exit(uefi.handle, @intToEnum(uefi.Status, status), 0, null);
            }
            // If we can't exit, reboot the system instead.

            uefi.system_table.runtime_services.resetSystem(uefi.tables.ResetType.ResetCold, @intToEnum(uefi.Status, status), 0, null);
        }
        system.exit(status);
    }

    pub const ReadError = error{
        InputOutput,
        SystemResources,
        IsDir,
        OperationAborted,
        BrokenPipe,
        ConnectionResetByPeer,
        ConnectionTimedOut,
        NotOpenForReading,

        /// This error occurs when no global event loop is configured,
        /// and reading from the file descriptor would block.
        WouldBlock,

        /// In WASI, this error occurs when the file descriptor does
        /// not hold the required rights to read from it.
        AccessDenied,
    } || UnexpectedError;

    /// Returns the number of bytes that were read, which can be less than
    /// buf.len. If 0 bytes were read, that means EOF.
    /// If `fd` is opened in non blocking mode, the function will return error.WouldBlock
    /// when EAGAIN is received.
    ///
    /// Linux has a limit on how many bytes may be transferred in one `read` call, which is `0x7ffff000`
    /// on both 64-bit and 32-bit systems. This is due to using a signed C int as the return value, as
    /// well as stuffing the errno codes into the last `4096` values. This is noted on the `read` man page.
    /// The limit on Darwin is `0x7fffffff`, trying to read more than that returns EINVAL.
    /// The corresponding POSIX limit is `math.maxInt(isize)`.
    pub fn read(fd: fd_t, buf: []u8) ReadError!usize {
        if (buf.len == 0) return 0;
        if (builtin.os.tag == .windows) {
            return windows.ReadFile(fd, buf, null, std.io.default_mode);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            const iovs = [1]iovec{iovec{
                .iov_base = buf.ptr,
                .iov_len = buf.len,
            }};

            var nread: usize = undefined;
            switch (wasi.fd_read(fd, &iovs, iovs.len, &nread)) {
                .SUCCESS => return nread,
                .INTR => unreachable,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => unreachable,
                .BADF => return error.NotOpenForReading, // Can be a race condition.

                .IO => return error.InputOutput,
                .ISDIR => return error.IsDir,
                .NOBUFS => return error.SystemResources,
                .NOMEM => return error.SystemResources,
                .CONNRESET => return error.ConnectionResetByPeer,
                .TIMEDOUT => return error.ConnectionTimedOut,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }

        // Prevents EINVAL.

        const max_count = switch (builtin.os.tag) {
            .linux => 0x7ffff000,
            .macos, .ios, .watchos, .tvos => math.maxInt(i32),
            else => math.maxInt(isize),
        };
        const adjusted_len = @min(max_count, buf.len);

        while (true) {
            const rc = system.read(fd, buf.ptr, adjusted_len);
            switch (errno(rc)) {
                .SUCCESS => return @intCast(usize, rc),
                .INTR => continue,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => return error.WouldBlock,
                .BADF => return error.NotOpenForReading, // Can be a race condition.

                .IO => return error.InputOutput,
                .ISDIR => return error.IsDir,
                .NOBUFS => return error.SystemResources,
                .NOMEM => return error.SystemResources,
                .CONNRESET => return error.ConnectionResetByPeer,
                .TIMEDOUT => return error.ConnectionTimedOut,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    /// Number of bytes read is returned. Upon reading end-of-file, zero is returned.
    ///
    /// For POSIX systems, if `fd` is opened in non blocking mode, the function will
    /// return error.WouldBlock when EAGAIN is received.
    /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
    /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
    ///
    /// This operation is non-atomic on the following systems:
    /// * Windows
    /// On these systems, the read races with concurrent writes to the same file descriptor.
    pub fn readv(fd: fd_t, iov: []const iovec) ReadError!usize {
        if (builtin.os.tag == .windows) {
            // TODO improve this to use ReadFileScatter

            if (iov.len == 0) return @as(usize, 0);
            const first = iov[0];
            return read(fd, first.iov_base[0..first.iov_len]);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var nread: usize = undefined;
            switch (wasi.fd_read(fd, iov.ptr, iov.len, &nread)) {
                .SUCCESS => return nread,
                .INTR => unreachable,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => unreachable, // currently not support in WASI

                .BADF => return error.NotOpenForReading, // can be a race condition

                .IO => return error.InputOutput,
                .ISDIR => return error.IsDir,
                .NOBUFS => return error.SystemResources,
                .NOMEM => return error.SystemResources,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }
        const iov_count = math.cast(u31, iov.len) orelse math.maxInt(u31);
        while (true) {
            // TODO handle the case when iov_len is too large and get rid of this @intCast

            const rc = system.readv(fd, iov.ptr, iov_count);
            switch (errno(rc)) {
                .SUCCESS => return @intCast(usize, rc),
                .INTR => continue,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => return error.WouldBlock,
                .BADF => return error.NotOpenForReading, // can be a race condition

                .IO => return error.InputOutput,
                .ISDIR => return error.IsDir,
                .NOBUFS => return error.SystemResources,
                .NOMEM => return error.SystemResources,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const PReadError = ReadError || error{Unseekable};

    /// Number of bytes read is returned. Upon reading end-of-file, zero is returned.
    ///
    /// Retries when interrupted by a signal.
    ///
    /// For POSIX systems, if `fd` is opened in non blocking mode, the function will
    /// return error.WouldBlock when EAGAIN is received.
    /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
    /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
    ///
    /// Linux has a limit on how many bytes may be transferred in one `pread` call, which is `0x7ffff000`
    /// on both 64-bit and 32-bit systems. This is due to using a signed C int as the return value, as
    /// well as stuffing the errno codes into the last `4096` values. This is noted on the `read` man page.
    /// The limit on Darwin is `0x7fffffff`, trying to read more than that returns EINVAL.
    /// The corresponding POSIX limit is `math.maxInt(isize)`.
    pub fn pread(fd: fd_t, buf: []u8, offset: u64) PReadError!usize {
        if (buf.len == 0) return 0;
        if (builtin.os.tag == .windows) {
            return windows.ReadFile(fd, buf, offset, std.io.default_mode);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            const iovs = [1]iovec{iovec{
                .iov_base = buf.ptr,
                .iov_len = buf.len,
            }};

            var nread: usize = undefined;
            switch (wasi.fd_pread(fd, &iovs, iovs.len, offset, &nread)) {
                .SUCCESS => return nread,
                .INTR => unreachable,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => unreachable,
                .BADF => return error.NotOpenForReading, // Can be a race condition.

                .IO => return error.InputOutput,
                .ISDIR => return error.IsDir,
                .NOBUFS => return error.SystemResources,
                .NOMEM => return error.SystemResources,
                .CONNRESET => return error.ConnectionResetByPeer,
                .NXIO => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }

        // Prevent EINVAL.

        const max_count = switch (builtin.os.tag) {
            .linux => 0x7ffff000,
            .macos, .ios, .watchos, .tvos => math.maxInt(i32),
            else => math.maxInt(isize),
        };
        const adjusted_len = @min(max_count, buf.len);

        const pread_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.pread64
        else
            system.pread;

        const ioffset = @bitCast(i64, offset); // the OS treats this as unsigned

        while (true) {
            const rc = pread_sym(fd, buf.ptr, adjusted_len, ioffset);
            switch (errno(rc)) {
                .SUCCESS => return @intCast(usize, rc),
                .INTR => continue,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => return error.WouldBlock,
                .BADF => return error.NotOpenForReading, // Can be a race condition.

                .IO => return error.InputOutput,
                .ISDIR => return error.IsDir,
                .NOBUFS => return error.SystemResources,
                .NOMEM => return error.SystemResources,
                .CONNRESET => return error.ConnectionResetByPeer,
                .NXIO => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const TruncateError = error{
        FileTooBig,
        InputOutput,
        FileBusy,

        /// In WASI, this error occurs when the file descriptor does
        /// not hold the required rights to call `ftruncate` on it.
        AccessDenied,
    } || UnexpectedError;

    pub fn ftruncate(fd: fd_t, length: u64) TruncateError!void {
        if (builtin.os.tag == .windows) {
            var io_status_block: windows.IO_STATUS_BLOCK = undefined;
            var eof_info = windows.FILE_END_OF_FILE_INFORMATION{
                .EndOfFile = @bitCast(windows.LARGE_INTEGER, length),
            };

            const rc = windows.ntdll.NtSetInformationFile(
                fd,
                &io_status_block,
                &eof_info,
                @sizeOf(windows.FILE_END_OF_FILE_INFORMATION),
                .FileEndOfFileInformation,
            );

            switch (rc) {
                .SUCCESS => return,
                .INVALID_HANDLE => unreachable, // Handle not open for writing

                .ACCESS_DENIED => return error.AccessDenied,
                else => return windows.unexpectedStatus(rc),
            }
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            switch (wasi.fd_filestat_set_size(fd, length)) {
                .SUCCESS => return,
                .INTR => unreachable,
                .FBIG => return error.FileTooBig,
                .IO => return error.InputOutput,
                .PERM => return error.AccessDenied,
                .TXTBSY => return error.FileBusy,
                .BADF => unreachable, // Handle not open for writing

                .INVAL => unreachable, // Handle not open for writing

                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }

        while (true) {
            const ftruncate_sym = if (builtin.os.tag == .linux and builtin.link_libc)
                system.ftruncate64
            else
                system.ftruncate;

            const ilen = @bitCast(i64, length); // the OS treats this as unsigned

            switch (errno(ftruncate_sym(fd, ilen))) {
                .SUCCESS => return,
                .INTR => continue,
                .FBIG => return error.FileTooBig,
                .IO => return error.InputOutput,
                .PERM => return error.AccessDenied,
                .TXTBSY => return error.FileBusy,
                .BADF => unreachable, // Handle not open for writing

                .INVAL => unreachable, // Handle not open for writing

                else => |err| return unexpectedErrno(err),
            }
        }
    }

    /// Number of bytes read is returned. Upon reading end-of-file, zero is returned.
    ///
    /// Retries when interrupted by a signal.
    ///
    /// For POSIX systems, if `fd` is opened in non blocking mode, the function will
    /// return error.WouldBlock when EAGAIN is received.
    /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
    /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
    ///
    /// This operation is non-atomic on the following systems:
    /// * Darwin
    /// * Windows
    /// On these systems, the read races with concurrent writes to the same file descriptor.
    pub fn preadv(fd: fd_t, iov: []const iovec, offset: u64) PReadError!usize {
        const have_pread_but_not_preadv = switch (builtin.os.tag) {
            .windows, .macos, .ios, .watchos, .tvos, .haiku => true,
            else => false,
        };
        if (have_pread_but_not_preadv) {
            // We could loop here; but proper usage of `preadv` must handle partial reads anyway.

            // So we simply read into the first vector only.

            if (iov.len == 0) return @as(usize, 0);
            const first = iov[0];
            return pread(fd, first.iov_base[0..first.iov_len], offset);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var nread: usize = undefined;
            switch (wasi.fd_pread(fd, iov.ptr, iov.len, offset, &nread)) {
                .SUCCESS => return nread,
                .INTR => unreachable,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => unreachable,
                .BADF => return error.NotOpenForReading, // can be a race condition

                .IO => return error.InputOutput,
                .ISDIR => return error.IsDir,
                .NOBUFS => return error.SystemResources,
                .NOMEM => return error.SystemResources,
                .NXIO => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }

        const iov_count = math.cast(u31, iov.len) orelse math.maxInt(u31);

        const preadv_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.preadv64
        else
            system.preadv;

        const ioffset = @bitCast(i64, offset); // the OS treats this as unsigned

        while (true) {
            const rc = preadv_sym(fd, iov.ptr, iov_count, ioffset);
            switch (errno(rc)) {
                .SUCCESS => return @bitCast(usize, rc),
                .INTR => continue,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => return error.WouldBlock,
                .BADF => return error.NotOpenForReading, // can be a race condition

                .IO => return error.InputOutput,
                .ISDIR => return error.IsDir,
                .NOBUFS => return error.SystemResources,
                .NOMEM => return error.SystemResources,
                .NXIO => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const WriteError = error{
        DiskQuota,
        FileTooBig,
        InputOutput,
        NoSpaceLeft,

        /// In WASI, this error may occur when the file descriptor does
        /// not hold the required rights to write to it.
        AccessDenied,
        BrokenPipe,
        SystemResources,
        OperationAborted,
        NotOpenForWriting,

        /// The process cannot access the file because another process has locked
        /// a portion of the file. Windows-only.
        LockViolation,

        /// This error occurs when no global event loop is configured,
        /// and reading from the file descriptor would block.
        WouldBlock,

        /// Connection reset by peer.
        ConnectionResetByPeer,
    } || UnexpectedError;

    /// Write to a file descriptor.
    /// Retries when interrupted by a signal.
    /// Returns the number of bytes written. If nonzero bytes were supplied, this will be nonzero.
    ///
    /// Note that a successful write() may transfer fewer than count bytes.  Such partial  writes  can
    /// occur  for  various reasons; for example, because there was insufficient space on the disk
    /// device to write all of the requested bytes, or because a blocked write() to a socket,  pipe,  or
    /// similar  was  interrupted by a signal handler after it had transferred some, but before it had
    /// transferred all of the requested bytes.  In the event of a partial write, the caller can  make
    /// another  write() call to transfer the remaining bytes.  The subsequent call will either
    /// transfer further bytes or may result in an error (e.g., if the disk is now full).
    ///
    /// For POSIX systems, if `fd` is opened in non blocking mode, the function will
    /// return error.WouldBlock when EAGAIN is received.
    /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
    /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
    ///
    /// Linux has a limit on how many bytes may be transferred in one `write` call, which is `0x7ffff000`
    /// on both 64-bit and 32-bit systems. This is due to using a signed C int as the return value, as
    /// well as stuffing the errno codes into the last `4096` values. This is noted on the `write` man page.
    /// The limit on Darwin is `0x7fffffff`, trying to read more than that returns EINVAL.
    /// The corresponding POSIX limit is `math.maxInt(isize)`.
    pub fn write(fd: fd_t, bytes: []const u8) WriteError!usize {
        if (bytes.len == 0) return 0;
        if (builtin.os.tag == .windows) {
            return windows.WriteFile(fd, bytes, null, std.io.default_mode);
        }

        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            const ciovs = [_]iovec_const{iovec_const{
                .iov_base = bytes.ptr,
                .iov_len = bytes.len,
            }};
            var nwritten: usize = undefined;
            switch (wasi.fd_write(fd, &ciovs, ciovs.len, &nwritten)) {
                .SUCCESS => return nwritten,
                .INTR => unreachable,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => unreachable,
                .BADF => return error.NotOpenForWriting, // can be a race condition.

                .DESTADDRREQ => unreachable, // `connect` was never called.

                .DQUOT => return error.DiskQuota,
                .FBIG => return error.FileTooBig,
                .IO => return error.InputOutput,
                .NOSPC => return error.NoSpaceLeft,
                .PERM => return error.AccessDenied,
                .PIPE => return error.BrokenPipe,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }

        const max_count = switch (builtin.os.tag) {
            .linux => 0x7ffff000,
            .macos, .ios, .watchos, .tvos => math.maxInt(i32),
            else => math.maxInt(isize),
        };
        const adjusted_len = @min(max_count, bytes.len);

        while (true) {
            const rc = system.write(fd, bytes.ptr, adjusted_len);
            switch (errno(rc)) {
                .SUCCESS => return @intCast(usize, rc),
                .INTR => continue,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => return error.WouldBlock,
                .BADF => return error.NotOpenForWriting, // can be a race condition.

                .DESTADDRREQ => unreachable, // `connect` was never called.

                .DQUOT => return error.DiskQuota,
                .FBIG => return error.FileTooBig,
                .IO => return error.InputOutput,
                .NOSPC => return error.NoSpaceLeft,
                .PERM => return error.AccessDenied,
                .PIPE => return error.BrokenPipe,
                .CONNRESET => return error.ConnectionResetByPeer,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    /// Write multiple buffers to a file descriptor.
    /// Retries when interrupted by a signal.
    /// Returns the number of bytes written. If nonzero bytes were supplied, this will be nonzero.
    ///
    /// Note that a successful write() may transfer fewer bytes than supplied.  Such partial  writes  can
    /// occur  for  various reasons; for example, because there was insufficient space on the disk
    /// device to write all of the requested bytes, or because a blocked write() to a socket,  pipe,  or
    /// similar  was  interrupted by a signal handler after it had transferred some, but before it had
    /// transferred all of the requested bytes.  In the event of a partial write, the caller can  make
    /// another  write() call to transfer the remaining bytes.  The subsequent call will either
    /// transfer further bytes or may result in an error (e.g., if the disk is now full).
    ///
    /// For POSIX systems, if `fd` is opened in non blocking mode, the function will
    /// return error.WouldBlock when EAGAIN is received.
    /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
    /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
    ///
    /// If `iov.len` is larger than `IOV_MAX`, a partial write will occur.
    pub fn writev(fd: fd_t, iov: []const iovec_const) WriteError!usize {
        if (builtin.os.tag == .windows) {
            // TODO improve this to use WriteFileScatter

            if (iov.len == 0) return @as(usize, 0);
            const first = iov[0];
            return write(fd, first.iov_base[0..first.iov_len]);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var nwritten: usize = undefined;
            switch (wasi.fd_write(fd, iov.ptr, iov.len, &nwritten)) {
                .SUCCESS => return nwritten,
                .INTR => unreachable,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => unreachable,
                .BADF => return error.NotOpenForWriting, // can be a race condition.

                .DESTADDRREQ => unreachable, // `connect` was never called.

                .DQUOT => return error.DiskQuota,
                .FBIG => return error.FileTooBig,
                .IO => return error.InputOutput,
                .NOSPC => return error.NoSpaceLeft,
                .PERM => return error.AccessDenied,
                .PIPE => return error.BrokenPipe,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }

        const iov_count = if (iov.len > IOV_MAX) IOV_MAX else @intCast(u31, iov.len);
        while (true) {
            const rc = system.writev(fd, iov.ptr, iov_count);
            switch (errno(rc)) {
                .SUCCESS => return @intCast(usize, rc),
                .INTR => continue,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => return error.WouldBlock,
                .BADF => return error.NotOpenForWriting, // Can be a race condition.

                .DESTADDRREQ => unreachable, // `connect` was never called.

                .DQUOT => return error.DiskQuota,
                .FBIG => return error.FileTooBig,
                .IO => return error.InputOutput,
                .NOSPC => return error.NoSpaceLeft,
                .PERM => return error.AccessDenied,
                .PIPE => return error.BrokenPipe,
                .CONNRESET => return error.ConnectionResetByPeer,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const PWriteError = WriteError || error{Unseekable};

    /// Write to a file descriptor, with a position offset.
    /// Retries when interrupted by a signal.
    /// Returns the number of bytes written. If nonzero bytes were supplied, this will be nonzero.
    ///
    /// Note that a successful write() may transfer fewer bytes than supplied.  Such partial  writes  can
    /// occur  for  various reasons; for example, because there was insufficient space on the disk
    /// device to write all of the requested bytes, or because a blocked write() to a socket,  pipe,  or
    /// similar  was  interrupted by a signal handler after it had transferred some, but before it had
    /// transferred all of the requested bytes.  In the event of a partial write, the caller can  make
    /// another  write() call to transfer the remaining bytes.  The subsequent call will either
    /// transfer further bytes or may result in an error (e.g., if the disk is now full).
    ///
    /// For POSIX systems, if `fd` is opened in non blocking mode, the function will
    /// return error.WouldBlock when EAGAIN is received.
    /// On Windows, if the application has a global event loop enabled, I/O Completion Ports are
    /// used to perform the I/O. `error.WouldBlock` is not possible on Windows.
    ///
    /// Linux has a limit on how many bytes may be transferred in one `pwrite` call, which is `0x7ffff000`
    /// on both 64-bit and 32-bit systems. This is due to using a signed C int as the return value, as
    /// well as stuffing the errno codes into the last `4096` values. This is noted on the `write` man page.
    /// The limit on Darwin is `0x7fffffff`, trying to write more than that returns EINVAL.
    /// The corresponding POSIX limit is `math.maxInt(isize)`.
    pub fn pwrite(fd: fd_t, bytes: []const u8, offset: u64) PWriteError!usize {
        if (bytes.len == 0) return 0;
        if (builtin.os.tag == .windows) {
            return windows.WriteFile(fd, bytes, offset, std.io.default_mode);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            const ciovs = [1]iovec_const{iovec_const{
                .iov_base = bytes.ptr,
                .iov_len = bytes.len,
            }};

            var nwritten: usize = undefined;
            switch (wasi.fd_pwrite(fd, &ciovs, ciovs.len, offset, &nwritten)) {
                .SUCCESS => return nwritten,
                .INTR => unreachable,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => unreachable,
                .BADF => return error.NotOpenForWriting, // can be a race condition.

                .DESTADDRREQ => unreachable, // `connect` was never called.

                .DQUOT => return error.DiskQuota,
                .FBIG => return error.FileTooBig,
                .IO => return error.InputOutput,
                .NOSPC => return error.NoSpaceLeft,
                .PERM => return error.AccessDenied,
                .PIPE => return error.BrokenPipe,
                .NXIO => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }

        // Prevent EINVAL.

        const max_count = switch (builtin.os.tag) {
            .linux => 0x7ffff000,
            .macos, .ios, .watchos, .tvos => math.maxInt(i32),
            else => math.maxInt(isize),
        };
        const adjusted_len = @min(max_count, bytes.len);

        const pwrite_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.pwrite64
        else
            system.pwrite;

        const ioffset = @bitCast(i64, offset); // the OS treats this as unsigned

        while (true) {
            const rc = pwrite_sym(fd, bytes.ptr, adjusted_len, ioffset);
            switch (errno(rc)) {
                .SUCCESS => return @intCast(usize, rc),
                .INTR => continue,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => return error.WouldBlock,
                .BADF => return error.NotOpenForWriting, // Can be a race condition.

                .DESTADDRREQ => unreachable, // `connect` was never called.

                .DQUOT => return error.DiskQuota,
                .FBIG => return error.FileTooBig,
                .IO => return error.InputOutput,
                .NOSPC => return error.NoSpaceLeft,
                .PERM => return error.AccessDenied,
                .PIPE => return error.BrokenPipe,
                .NXIO => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    /// Write multiple buffers to a file descriptor, with a position offset.
    /// Retries when interrupted by a signal.
    /// Returns the number of bytes written. If nonzero bytes were supplied, this will be nonzero.
    ///
    /// Note that a successful write() may transfer fewer than count bytes.  Such partial  writes  can
    /// occur  for  various reasons; for example, because there was insufficient space on the disk
    /// device to write all of the requested bytes, or because a blocked write() to a socket,  pipe,  or
    /// similar  was  interrupted by a signal handler after it had transferred some, but before it had
    /// transferred all of the requested bytes.  In the event of a partial write, the caller can  make
    /// another  write() call to transfer the remaining bytes.  The subsequent call will either
    /// transfer further bytes or may result in an error (e.g., if the disk is now full).
    ///
    /// If `fd` is opened in non blocking mode, the function will
    /// return error.WouldBlock when EAGAIN is received.
    ///
    /// The following systems do not have this syscall, and will return partial writes if more than one
    /// vector is provided:
    /// * Darwin
    /// * Windows
    ///
    /// If `iov.len` is larger than `IOV_MAX`, a partial write will occur.
    pub fn pwritev(fd: fd_t, iov: []const iovec_const, offset: u64) PWriteError!usize {
        const have_pwrite_but_not_pwritev = switch (builtin.os.tag) {
            .windows, .macos, .ios, .watchos, .tvos, .haiku => true,
            else => false,
        };

        if (have_pwrite_but_not_pwritev) {
            // We could loop here; but proper usage of `pwritev` must handle partial writes anyway.

            // So we simply write the first vector only.

            if (iov.len == 0) return @as(usize, 0);
            const first = iov[0];
            return pwrite(fd, first.iov_base[0..first.iov_len], offset);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var nwritten: usize = undefined;
            switch (wasi.fd_pwrite(fd, iov.ptr, iov.len, offset, &nwritten)) {
                .SUCCESS => return nwritten,
                .INTR => unreachable,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => unreachable,
                .BADF => return error.NotOpenForWriting, // Can be a race condition.

                .DESTADDRREQ => unreachable, // `connect` was never called.

                .DQUOT => return error.DiskQuota,
                .FBIG => return error.FileTooBig,
                .IO => return error.InputOutput,
                .NOSPC => return error.NoSpaceLeft,
                .PERM => return error.AccessDenied,
                .PIPE => return error.BrokenPipe,
                .NXIO => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }

        const pwritev_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.pwritev64
        else
            system.pwritev;

        const iov_count = if (iov.len > IOV_MAX) IOV_MAX else @intCast(u31, iov.len);
        const ioffset = @bitCast(i64, offset); // the OS treats this as unsigned

        while (true) {
            const rc = pwritev_sym(fd, iov.ptr, iov_count, ioffset);
            switch (errno(rc)) {
                .SUCCESS => return @intCast(usize, rc),
                .INTR => continue,
                .INVAL => unreachable,
                .FAULT => unreachable,
                .AGAIN => return error.WouldBlock,
                .BADF => return error.NotOpenForWriting, // Can be a race condition.

                .DESTADDRREQ => unreachable, // `connect` was never called.

                .DQUOT => return error.DiskQuota,
                .FBIG => return error.FileTooBig,
                .IO => return error.InputOutput,
                .NOSPC => return error.NoSpaceLeft,
                .PERM => return error.AccessDenied,
                .PIPE => return error.BrokenPipe,
                .NXIO => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const OpenError = error{
        /// In WASI, this error may occur when the provided file handle is invalid.
        InvalidHandle,

        /// In WASI, this error may occur when the file descriptor does
        /// not hold the required rights to open a new resource relative to it.
        AccessDenied,
        SymLinkLoop,
        ProcessFdQuotaExceeded,
        SystemFdQuotaExceeded,
        NoDevice,
        FileNotFound,

        /// The path exceeded `MAX_PATH_BYTES` bytes.
        NameTooLong,

        /// Insufficient kernel memory was available, or
        /// the named file is a FIFO and per-user hard limit on
        /// memory allocation for pipes has been reached.
        SystemResources,

        /// The file is too large to be opened. This error is unreachable
        /// for 64-bit targets, as well as when opening directories.
        FileTooBig,

        /// The path refers to directory but the `O.DIRECTORY` flag was not provided.
        IsDir,

        /// A new path cannot be created because the device has no room for the new file.
        /// This error is only reachable when the `O.CREAT` flag is provided.
        NoSpaceLeft,

        /// A component used as a directory in the path was not, in fact, a directory, or
        /// `O.DIRECTORY` was specified and the path was not a directory.
        NotDir,

        /// The path already exists and the `O.CREAT` and `O.EXCL` flags were provided.
        PathAlreadyExists,
        DeviceBusy,

        /// The underlying filesystem does not support file locks
        FileLocksNotSupported,

        BadPathName,
        InvalidUtf8,

        /// One of these three things:
        /// * pathname  refers to an executable image which is currently being
        ///   executed and write access was requested.
        /// * pathname refers to a file that is currently in  use  as  a  swap
        ///   file, and the O_TRUNC flag was specified.
        /// * pathname  refers  to  a file that is currently being read by the
        ///   kernel (e.g., for module/firmware loading), and write access was
        ///   requested.
        FileBusy,

        WouldBlock,
    } || UnexpectedError;

    /// Open and possibly create a file. Keeps trying if it gets interrupted.
    /// See also `openZ`.
    pub fn open(file_path: []const u8, flags: u32, perm: mode_t) OpenError!fd_t {
        if (builtin.os.tag == .windows) {
            const file_path_w = try windows.sliceToPrefixedFileW(file_path);
            return openW(file_path_w.span(), flags, perm);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return openat(wasi.AT.FDCWD, file_path, flags, perm);
        }
        const file_path_c = try toPosixPath(file_path);
        return openZ(&file_path_c, flags, perm);
    }

    /// Open and possibly create a file. Keeps trying if it gets interrupted.
    /// See also `open`.
    pub fn openZ(file_path: [*:0]const u8, flags: u32, perm: mode_t) OpenError!fd_t {
        if (builtin.os.tag == .windows) {
            const file_path_w = try windows.cStrToPrefixedFileW(file_path);
            return openW(file_path_w.span(), flags, perm);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return open(mem.sliceTo(file_path, 0), flags, perm);
        }

        const open_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.open64
        else
            system.open;

        while (true) {
            const rc = open_sym(file_path, flags, perm);
            switch (errno(rc)) {
                .SUCCESS => return @intCast(fd_t, rc),
                .INTR => continue,

                .FAULT => unreachable,
                .INVAL => unreachable,
                .ACCES => return error.AccessDenied,
                .FBIG => return error.FileTooBig,
                .OVERFLOW => return error.FileTooBig,
                .ISDIR => return error.IsDir,
                .LOOP => return error.SymLinkLoop,
                .MFILE => return error.ProcessFdQuotaExceeded,
                .NAMETOOLONG => return error.NameTooLong,
                .NFILE => return error.SystemFdQuotaExceeded,
                .NODEV => return error.NoDevice,
                .NOENT => return error.FileNotFound,
                .NOMEM => return error.SystemResources,
                .NOSPC => return error.NoSpaceLeft,
                .NOTDIR => return error.NotDir,
                .PERM => return error.AccessDenied,
                .EXIST => return error.PathAlreadyExists,
                .BUSY => return error.DeviceBusy,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    fn openOptionsFromFlagsWindows(flags: u32) windows.OpenFileOptions {
        const w = windows;

        var access_mask: w.ULONG = w.READ_CONTROL | w.FILE_WRITE_ATTRIBUTES | w.SYNCHRONIZE;
        if (flags & O.RDWR != 0) {
            access_mask |= w.GENERIC_READ | w.GENERIC_WRITE;
        } else if (flags & O.WRONLY != 0) {
            access_mask |= w.GENERIC_WRITE;
        } else {
            access_mask |= w.GENERIC_READ | w.GENERIC_WRITE;
        }

        const filter: windows.OpenFileOptions.Filter = if (flags & O.DIRECTORY != 0) .dir_only else .file_only;
        const follow_symlinks: bool = flags & O.NOFOLLOW == 0;

        const creation: w.ULONG = blk: {
            if (flags & O.CREAT != 0) {
                if (flags & O.EXCL != 0) {
                    break :blk w.FILE_CREATE;
                }
            }
            break :blk w.FILE_OPEN;
        };

        return .{
            .access_mask = access_mask,
            .io_mode = .blocking,
            .creation = creation,
            .filter = filter,
            .follow_symlinks = follow_symlinks,
        };
    }

    /// Windows-only. The path parameter is
    /// [WTF-16](https://simonsapin.github.io/wtf-8/#potentially-ill-formed-utf-16) encoded.
    /// Translates the POSIX open API call to a Windows API call.
    /// TODO currently, this function does not handle all flag combinations
    /// or makes use of perm argument.
    pub fn openW(file_path_w: []const u16, flags: u32, perm: mode_t) OpenError!fd_t {
        _ = perm;
        var options = openOptionsFromFlagsWindows(flags);
        options.dir = std.fs.cwd().fd;
        return windows.OpenFile(file_path_w, options) catch |err| switch (err) {
            error.WouldBlock => unreachable,
            error.PipeBusy => unreachable,
            else => |e| return e,
        };
    }

    var wasi_cwd = if (builtin.os.tag == .wasi and !builtin.link_libc) struct {
        // List of available Preopens

        preopens: ?PreopenList = null,
        // Memory buffer for storing the relative portion of the CWD

        path_buffer: [MAX_PATH_BYTES]u8 = undefined,
        // The absolute path associated with the current working directory

        cwd: []const u8 = "/",
    }{} else undefined;

    /// Initialize the available Preopen list on WASI and set the CWD to `cwd_init`.
    /// Note that `cwd_init` corresponds to a Preopen directory, not necessarily
    /// a POSIX path. For example, "." matches a Preopen provided with `--dir=.`
    ///
    /// This must be called before using any relative or absolute paths with `std.os`
    /// functions, if you are on WASI without linking libc.
    ///
    /// The current working directory is initialized to `cwd_root`, and `cwd_root`
    /// is inserted as a prefix for any Preopens whose dir begins with "."
    ///   For example:
    ///      "./foo/bar" - canonicalizes to -> "{cwd_root}/foo/bar"
    ///      "foo/bar"   - canonicalizes to -> "/foo/bar"
    ///      "/foo/bar"  - canonicalizes to -> "/foo/bar"
    ///
    /// `cwd_root` must be an absolute path. For initialization behavior similar to
    /// wasi-libc, use "/" as the `cwd_root`
    ///
    /// `alloc` must not be a temporary or leak-detecting allocator, since `std.os`
    /// retains ownership of allocations internally and may never call free().
    pub fn initPreopensWasi(alloc: Allocator, cwd_root: []const u8) !void {
        if (builtin.os.tag == .wasi) {
            if (!builtin.link_libc) {
                var preopen_list = PreopenList.init(alloc);
                errdefer preopen_list.deinit();
                try preopen_list.populate(cwd_root);

                var path_alloc = std.heap.FixedBufferAllocator.init(&wasi_cwd.path_buffer);
                wasi_cwd.cwd = try path_alloc.allocator().dupe(u8, cwd_root);

                if (wasi_cwd.preopens) |preopens| preopens.deinit();
                wasi_cwd.preopens = preopen_list;
            } else {
                // wasi-libc defaults to an effective CWD root of "/"

                if (!mem.eql(u8, cwd_root, "/")) return error.UnsupportedDirectory;
            }
        }
    }

    /// Resolve a relative or absolute path to an handle (`fd_t`) and a relative subpath.
    ///
    /// For absolute paths, this automatically searches among available Preopens to find
    /// a match. For relative paths, it uses the "emulated" CWD.
    /// Automatically looks up the correct Preopen corresponding to the provided path.
    pub fn resolvePathWasi(path: []const u8, out_buffer: *[MAX_PATH_BYTES]u8) !RelativePathWasi {
        var allocator = std.heap.FixedBufferAllocator.init(out_buffer);
        var alloc = allocator.allocator();

        const abs_path = fs.path.resolve(alloc, &.{ wasi_cwd.cwd, path }) catch return error.NameTooLong;
        const preopen_uri = wasi_cwd.preopens.?.findContaining(.{ .Dir = abs_path });

        if (preopen_uri) |po| {
            return RelativePathWasi{
                .dir_fd = po.base.fd,
                .relative_path = po.relative_path,
            };
        } else {
            // No matching preopen found

            return error.AccessDenied;
        }
    }

    /// Open and possibly create a file. Keeps trying if it gets interrupted.
    /// `file_path` is relative to the open directory handle `dir_fd`.
    /// See also `openatZ`.
    pub fn openat(dir_fd: fd_t, file_path: []const u8, flags: u32, mode: mode_t) OpenError!fd_t {
        if (builtin.os.tag == .windows) {
            const file_path_w = try windows.sliceToPrefixedFileW(file_path);
            return openatW(dir_fd, file_path_w.span(), flags, mode);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            // `mode` is ignored on WASI, which does not support unix-style file permissions

            const fd = if (dir_fd == wasi.AT.FDCWD or fs.path.isAbsolute(file_path)) blk: {
                // Resolve absolute or CWD-relative paths to a path within a Preopen

                var path_buf: [MAX_PATH_BYTES]u8 = undefined;
                const path = try resolvePathWasi(file_path, &path_buf);

                const opts = try openOptionsFromFlagsWasi(path.dir_fd, flags);
                break :blk try openatWasi(path.dir_fd, path.relative_path, opts.lookup_flags, opts.oflags, opts.fs_flags, opts.fs_rights_base, opts.fs_rights_inheriting);
            } else blk: {
                const opts = try openOptionsFromFlagsWasi(dir_fd, flags);
                break :blk try openatWasi(dir_fd, file_path, opts.lookup_flags, opts.oflags, opts.fs_flags, opts.fs_rights_base, opts.fs_rights_inheriting);
            };
            errdefer close(fd);

            const info = try fstat(fd);
            if (flags & O.WRONLY != 0 and info.filetype == .DIRECTORY)
                return error.IsDir;

            return fd;
        }
        const file_path_c = try toPosixPath(file_path);
        return openatZ(dir_fd, &file_path_c, flags, mode);
    }

    /// A struct to contain all lookup/rights flags accepted by `wasi.path_open`
    const WasiOpenOptions = struct {
        oflags: wasi.oflags_t,
        lookup_flags: wasi.lookupflags_t,
        fs_rights_base: wasi.rights_t,
        fs_rights_inheriting: wasi.rights_t,
        fs_flags: wasi.fdflags_t,
    };

    /// Compute rights + flags corresponding to the provided POSIX access mode.
    fn openOptionsFromFlagsWasi(fd: fd_t, oflag: u32) OpenError!WasiOpenOptions {
        const w = std.os.wasi;

        // First, discover the rights that we can derive from `fd`

        var fsb_cur: wasi.fdstat_t = undefined;
        _ = switch (w.fd_fdstat_get(fd, &fsb_cur)) {
            .SUCCESS => .{},
            .BADF => return error.InvalidHandle,
            else => |err| return unexpectedErrno(err),
        };

        // Next, calculate the read/write rights to request, depending on the

        // provided POSIX access mode

        var rights: w.rights_t = 0;
        if (oflag & O.RDONLY != 0) {
            rights |= w.RIGHT.FD_READ | w.RIGHT.FD_READDIR;
        }
        if (oflag & O.WRONLY != 0) {
            rights |= w.RIGHT.FD_DATASYNC | w.RIGHT.FD_WRITE |
                w.RIGHT.FD_ALLOCATE | w.RIGHT.FD_FILESTAT_SET_SIZE;
        }

        // Request all other rights unconditionally

        rights |= ~(w.RIGHT.FD_DATASYNC | w.RIGHT.FD_READ |
            w.RIGHT.FD_WRITE | w.RIGHT.FD_ALLOCATE |
            w.RIGHT.FD_READDIR | w.RIGHT.FD_FILESTAT_SET_SIZE);

        // But only take rights that we can actually inherit

        rights &= fsb_cur.fs_rights_inheriting;

        return WasiOpenOptions{
            .oflags = @truncate(w.oflags_t, (oflag >> 12)) & 0xfff,
            .lookup_flags = if (oflag & O.NOFOLLOW == 0) w.LOOKUP_SYMLINK_FOLLOW else 0,
            .fs_rights_base = rights,
            .fs_rights_inheriting = fsb_cur.fs_rights_inheriting,
            .fs_flags = @truncate(w.fdflags_t, oflag & 0xfff),
        };
    }

    /// Open and possibly create a file in WASI.
    pub fn openatWasi(dir_fd: fd_t, file_path: []const u8, lookup_flags: lookupflags_t, oflags: oflags_t, fdflags: fdflags_t, base: rights_t, inheriting: rights_t) OpenError!fd_t {
        while (true) {
            var fd: fd_t = undefined;
            switch (wasi.path_open(dir_fd, lookup_flags, file_path.ptr, file_path.len, oflags, base, inheriting, fdflags, &fd)) {
                .SUCCESS => return fd,
                .INTR => continue,

                .FAULT => unreachable,
                .INVAL => unreachable,
                .ACCES => return error.AccessDenied,
                .FBIG => return error.FileTooBig,
                .OVERFLOW => return error.FileTooBig,
                .ISDIR => return error.IsDir,
                .LOOP => return error.SymLinkLoop,
                .MFILE => return error.ProcessFdQuotaExceeded,
                .NAMETOOLONG => return error.NameTooLong,
                .NFILE => return error.SystemFdQuotaExceeded,
                .NODEV => return error.NoDevice,
                .NOENT => return error.FileNotFound,
                .NOMEM => return error.SystemResources,
                .NOSPC => return error.NoSpaceLeft,
                .NOTDIR => return error.NotDir,
                .PERM => return error.AccessDenied,
                .EXIST => return error.PathAlreadyExists,
                .BUSY => return error.DeviceBusy,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    /// Open and possibly create a file. Keeps trying if it gets interrupted.
    /// `file_path` is relative to the open directory handle `dir_fd`.
    /// See also `openat`.
    pub fn openatZ(dir_fd: fd_t, file_path: [*:0]const u8, flags: u32, mode: mode_t) OpenError!fd_t {
        if (builtin.os.tag == .windows) {
            const file_path_w = try windows.cStrToPrefixedFileW(file_path);
            return openatW(dir_fd, file_path_w.span(), flags, mode);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return openat(dir_fd, mem.sliceTo(file_path, 0), flags, mode);
        }

        const openat_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.openat64
        else
            system.openat;

        while (true) {
            const rc = openat_sym(dir_fd, file_path, flags, mode);
            switch (errno(rc)) {
                .SUCCESS => return @intCast(fd_t, rc),
                .INTR => continue,

                .FAULT => unreachable,
                .INVAL => unreachable,
                .BADF => unreachable,
                .ACCES => return error.AccessDenied,
                .FBIG => return error.FileTooBig,
                .OVERFLOW => return error.FileTooBig,
                .ISDIR => return error.IsDir,
                .LOOP => return error.SymLinkLoop,
                .MFILE => return error.ProcessFdQuotaExceeded,
                .NAMETOOLONG => return error.NameTooLong,
                .NFILE => return error.SystemFdQuotaExceeded,
                .NODEV => return error.NoDevice,
                .NOENT => return error.FileNotFound,
                .NOMEM => return error.SystemResources,
                .NOSPC => return error.NoSpaceLeft,
                .NOTDIR => return error.NotDir,
                .PERM => return error.AccessDenied,
                .EXIST => return error.PathAlreadyExists,
                .BUSY => return error.DeviceBusy,
                .OPNOTSUPP => return error.FileLocksNotSupported,
                .AGAIN => return error.WouldBlock,
                .TXTBSY => return error.FileBusy,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    /// Windows-only. Similar to `openat` but with pathname argument null-terminated
    /// WTF16 encoded.
    /// TODO currently, this function does not handle all flag combinations
    /// or makes use of perm argument.
    pub fn openatW(dir_fd: fd_t, file_path_w: []const u16, flags: u32, mode: mode_t) OpenError!fd_t {
        _ = mode;
        var options = openOptionsFromFlagsWindows(flags);
        options.dir = dir_fd;
        return windows.OpenFile(file_path_w, options) catch |err| switch (err) {
            error.WouldBlock => unreachable,
            error.PipeBusy => unreachable,
            else => |e| return e,
        };
    }

    pub fn dup(old_fd: fd_t) !fd_t {
        const rc = system.dup(old_fd);
        return switch (errno(rc)) {
            .SUCCESS => return @intCast(fd_t, rc),
            .MFILE => error.ProcessFdQuotaExceeded,
            .BADF => unreachable, // invalid file descriptor

            else => |err| return unexpectedErrno(err),
        };
    }

    pub fn dup2(old_fd: fd_t, new_fd: fd_t) !void {
        while (true) {
            switch (errno(system.dup2(old_fd, new_fd))) {
                .SUCCESS => return,
                .BUSY, .INTR => continue,
                .MFILE => return error.ProcessFdQuotaExceeded,
                .INVAL => unreachable, // invalid parameters passed to dup2

                .BADF => unreachable, // invalid file descriptor

                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const ExecveError = error{
        SystemResources,
        AccessDenied,
        InvalidExe,
        FileSystem,
        IsDir,
        FileNotFound,
        NotDir,
        FileBusy,
        ProcessFdQuotaExceeded,
        SystemFdQuotaExceeded,
        NameTooLong,
    } || UnexpectedError;

    /// This function ignores PATH environment variable. See `execvpeZ` for that.
    pub fn execveZ(
        path: [*:0]const u8,
        child_argv: [*:null]const ?[*:0]const u8,
        envp: [*:null]const ?[*:0]const u8,
    ) ExecveError {
        switch (errno(system.execve(path, child_argv, envp))) {
            .SUCCESS => unreachable,
            .FAULT => unreachable,
            .@"2BIG" => return error.SystemResources,
            .MFILE => return error.ProcessFdQuotaExceeded,
            .NAMETOOLONG => return error.NameTooLong,
            .NFILE => return error.SystemFdQuotaExceeded,
            .NOMEM => return error.SystemResources,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .INVAL => return error.InvalidExe,
            .NOEXEC => return error.InvalidExe,
            .IO => return error.FileSystem,
            .LOOP => return error.FileSystem,
            .ISDIR => return error.IsDir,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.NotDir,
            .TXTBSY => return error.FileBusy,
            else => |err| switch (builtin.os.tag) {
                .macos, .ios, .tvos, .watchos => switch (err) {
                    .BADEXEC => return error.InvalidExe,
                    .BADARCH => return error.InvalidExe,
                    else => return unexpectedErrno(err),
                },
                .linux, .solaris => switch (err) {
                    .LIBBAD => return error.InvalidExe,
                    else => return unexpectedErrno(err),
                },
                else => return unexpectedErrno(err),
            },
        }
    }

    pub const Arg0Expand = enum {
        expand,
        no_expand,
    };

    /// Like `execvpeZ` except if `arg0_expand` is `.expand`, then `argv` is mutable,
    /// and `argv[0]` is expanded to be the same absolute path that is passed to the execve syscall.
    /// If this function returns with an error, `argv[0]` will be restored to the value it was when it was passed in.
    pub fn execvpeZ_expandArg0(
        comptime arg0_expand: Arg0Expand,
        file: [*:0]const u8,
        child_argv: switch (arg0_expand) {
            .expand => [*:null]?[*:0]const u8,
            .no_expand => [*:null]const ?[*:0]const u8,
        },
        envp: [*:null]const ?[*:0]const u8,
    ) ExecveError {
        const file_slice = mem.sliceTo(file, 0);
        if (mem.indexOfScalar(u8, file_slice, '/') != null) return execveZ(file, child_argv, envp);

        const PATH = getenvZ("PATH") orelse "/usr/local/bin:/bin/:/usr/bin";
        // Use of MAX_PATH_BYTES here is valid as the path_buf will be passed

        // directly to the operating system in execveZ.

        var path_buf: [MAX_PATH_BYTES]u8 = undefined;
        var it = mem.tokenize(u8, PATH, ":");
        var seen_eacces = false;
        var err: ExecveError = error.FileNotFound;

        // In case of expanding arg0 we must put it back if we return with an error.

        const prev_arg0 = child_argv[0];
        defer switch (arg0_expand) {
            .expand => child_argv[0] = prev_arg0,
            .no_expand => {},
        };

        while (it.next()) |search_path| {
            const path_len = search_path.len + file_slice.len + 1;
            if (path_buf.len < path_len + 1) return error.NameTooLong;
            mem.copy(u8, &path_buf, search_path);
            path_buf[search_path.len] = '/';
            mem.copy(u8, path_buf[search_path.len + 1 ..], file_slice);
            path_buf[path_len] = 0;
            const full_path = path_buf[0..path_len :0].ptr;
            switch (arg0_expand) {
                .expand => child_argv[0] = full_path,
                .no_expand => {},
            }
            err = execveZ(full_path, child_argv, envp);
            switch (err) {
                error.AccessDenied => seen_eacces = true,
                error.FileNotFound, error.NotDir => {},
                else => |e| return e,
            }
        }
        if (seen_eacces) return error.AccessDenied;
        return err;
    }

    /// This function also uses the PATH environment variable to get the full path to the executable.
    /// If `file` is an absolute path, this is the same as `execveZ`.
    pub fn execvpeZ(
        file: [*:0]const u8,
        argv_ptr: [*:null]const ?[*:0]const u8,
        envp: [*:null]const ?[*:0]const u8,
    ) ExecveError {
        return execvpeZ_expandArg0(.no_expand, file, argv_ptr, envp);
    }

    /// Get an environment variable.
    /// See also `getenvZ`.
    pub fn getenv(key: []const u8) ?[]const u8 {
        if (builtin.link_libc) {
            var small_key_buf: [64]u8 = undefined;
            if (key.len < small_key_buf.len) {
                mem.copy(u8, &small_key_buf, key);
                small_key_buf[key.len] = 0;
                const key0 = small_key_buf[0..key.len :0];
                return getenvZ(key0);
            }
            // Search the entire `environ` because we don't have a null terminated pointer.

            var ptr = std.c.environ;
            while (ptr[0]) |line| : (ptr += 1) {
                var line_i: usize = 0;
                while (line[line_i] != 0 and line[line_i] != '=') : (line_i += 1) {}
                const this_key = line[0..line_i];

                if (!mem.eql(u8, this_key, key)) continue;

                var end_i: usize = line_i;
                while (line[end_i] != 0) : (end_i += 1) {}
                const value = line[line_i + 1 .. end_i];

                return value;
            }
            return null;
        }
        if (builtin.os.tag == .windows) {
            @compileError("std.os.getenv is unavailable for Windows because environment string is in WTF-16 format. See std.process.getEnvVarOwned for cross-platform API or std.os.getenvW for Windows-specific API.");
        }
        // TODO see https://github.com/ziglang/zig/issues/4524

        for (environ) |ptr| {
            var line_i: usize = 0;
            while (ptr[line_i] != 0 and ptr[line_i] != '=') : (line_i += 1) {}
            const this_key = ptr[0..line_i];
            if (!mem.eql(u8, key, this_key)) continue;

            var end_i: usize = line_i;
            while (ptr[end_i] != 0) : (end_i += 1) {}
            const this_value = ptr[line_i + 1 .. end_i];

            return this_value;
        }
        return null;
    }

    /// Get an environment variable with a null-terminated name.
    /// See also `getenv`.
    pub fn getenvZ(key: [*:0]const u8) ?[]const u8 {
        if (builtin.link_libc) {
            const value = system.getenv(key) orelse return null;
            return mem.sliceTo(value, 0);
        }
        if (builtin.os.tag == .windows) {
            @compileError("std.os.getenvZ is unavailable for Windows because environment string is in WTF-16 format. See std.process.getEnvVarOwned for cross-platform API or std.os.getenvW for Windows-specific API.");
        }
        return getenv(mem.sliceTo(key, 0));
    }

    /// Windows-only. Get an environment variable with a null-terminated, WTF-16 encoded name.
    /// See also `getenv`.
    /// This function performs a Unicode-aware case-insensitive lookup using RtlEqualUnicodeString.
    pub fn getenvW(key: [*:0]const u16) ?[:0]const u16 {
        if (builtin.os.tag != .windows) {
            @compileError("std.os.getenvW is a Windows-only API");
        }
        const key_slice = mem.sliceTo(key, 0);
        const ptr = windows.peb().ProcessParameters.Environment;
        var i: usize = 0;
        while (ptr[i] != 0) {
            const key_start = i;

            // There are some special environment variables that start with =,

            // so we need a special case to not treat = as a key/value separator

            // if it's the first character.

            // https://devblogs.microsoft.com/oldnewthing/20100506-00/?p=14133

            if (ptr[key_start] == '=') i += 1;

            while (ptr[i] != 0 and ptr[i] != '=') : (i += 1) {}
            const this_key = ptr[key_start..i];

            if (ptr[i] == '=') i += 1;

            const value_start = i;
            while (ptr[i] != 0) : (i += 1) {}
            const this_value = ptr[value_start..i :0];

            const key_string_bytes = @intCast(u16, key_slice.len * 2);
            const key_string = windows.UNICODE_STRING{
                .Length = key_string_bytes,
                .MaximumLength = key_string_bytes,
                .Buffer = @intToPtr([*]u16, @ptrToInt(key)),
            };
            const this_key_string_bytes = @intCast(u16, this_key.len * 2);
            const this_key_string = windows.UNICODE_STRING{
                .Length = this_key_string_bytes,
                .MaximumLength = this_key_string_bytes,
                .Buffer = this_key.ptr,
            };
            if (windows.ntdll.RtlEqualUnicodeString(&key_string, &this_key_string, windows.TRUE) == windows.TRUE) {
                return this_value;
            }

            i += 1; // skip over null byte

        }
        return null;
    }

    pub const GetCwdError = error{
        NameTooLong,
        CurrentWorkingDirectoryUnlinked,
    } || UnexpectedError;

    /// The result is a slice of out_buffer, indexed from 0.
    pub fn getcwd(out_buffer: []u8) GetCwdError![]u8 {
        if (builtin.os.tag == .windows) {
            return windows.GetCurrentDirectory(out_buffer);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            const path = wasi_cwd.cwd;
            if (out_buffer.len < path.len) return error.NameTooLong;
            std.mem.copy(u8, out_buffer, path);
            return out_buffer[0..path.len];
        }

        const err = if (builtin.link_libc) blk: {
            const c_err = if (std.c.getcwd(out_buffer.ptr, out_buffer.len)) |_| 0 else std.c._errno().*;
            break :blk @intToEnum(E, c_err);
        } else blk: {
            break :blk errno(system.getcwd(out_buffer.ptr, out_buffer.len));
        };
        switch (err) {
            .SUCCESS => return mem.sliceTo(std.meta.assumeSentinel(out_buffer.ptr, 0), 0),
            .FAULT => unreachable,
            .INVAL => unreachable,
            .NOENT => return error.CurrentWorkingDirectoryUnlinked,
            .RANGE => return error.NameTooLong,
            else => return unexpectedErrno(err),
        }
    }

    pub const SymLinkError = error{
        /// In WASI, this error may occur when the file descriptor does
        /// not hold the required rights to create a new symbolic link relative to it.
        AccessDenied,
        DiskQuota,
        PathAlreadyExists,
        FileSystem,
        SymLinkLoop,
        FileNotFound,
        SystemResources,
        NoSpaceLeft,
        ReadOnlyFileSystem,
        NotDir,
        NameTooLong,
        InvalidUtf8,
        BadPathName,
    } || UnexpectedError;

    /// Creates a symbolic link named `sym_link_path` which contains the string `target_path`.
    /// A symbolic link (also known as a soft link) may point to an existing file or to a nonexistent
    /// one; the latter case is known as a dangling link.
    /// If `sym_link_path` exists, it will not be overwritten.
    /// See also `symlinkZ.
    pub fn symlink(target_path: []const u8, sym_link_path: []const u8) SymLinkError!void {
        if (builtin.os.tag == .windows) {
            @compileError("symlink is not supported on Windows; use std.os.windows.CreateSymbolicLink instead");
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return symlinkat(target_path, wasi.AT.FDCWD, sym_link_path);
        }
        const target_path_c = try toPosixPath(target_path);
        const sym_link_path_c = try toPosixPath(sym_link_path);
        return symlinkZ(&target_path_c, &sym_link_path_c);
    }

    /// This is the same as `symlink` except the parameters are null-terminated pointers.
    /// See also `symlink`.
    pub fn symlinkZ(target_path: [*:0]const u8, sym_link_path: [*:0]const u8) SymLinkError!void {
        if (builtin.os.tag == .windows) {
            @compileError("symlink is not supported on Windows; use std.os.windows.CreateSymbolicLink instead");
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return symlink(mem.sliceTo(target_path, 0), mem.sliceTo(sym_link_path, 0));
        }
        switch (errno(system.symlink(target_path, sym_link_path))) {
            .SUCCESS => return,
            .FAULT => unreachable,
            .INVAL => unreachable,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .DQUOT => return error.DiskQuota,
            .EXIST => return error.PathAlreadyExists,
            .IO => return error.FileSystem,
            .LOOP => return error.SymLinkLoop,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.NotDir,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .ROFS => return error.ReadOnlyFileSystem,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Similar to `symlink`, however, creates a symbolic link named `sym_link_path` which contains the string
    /// `target_path` **relative** to `newdirfd` directory handle.
    /// A symbolic link (also known as a soft link) may point to an existing file or to a nonexistent
    /// one; the latter case is known as a dangling link.
    /// If `sym_link_path` exists, it will not be overwritten.
    /// See also `symlinkatWasi`, `symlinkatZ` and `symlinkatW`.
    pub fn symlinkat(target_path: []const u8, newdirfd: fd_t, sym_link_path: []const u8) SymLinkError!void {
        if (builtin.os.tag == .windows) {
            @compileError("symlinkat is not supported on Windows; use std.os.windows.CreateSymbolicLink instead");
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            if (newdirfd == wasi.AT.FDCWD or fs.path.isAbsolute(target_path)) {
                // Resolve absolute or CWD-relative paths to a path within a Preopen

                var path_buf: [MAX_PATH_BYTES]u8 = undefined;
                const path = try resolvePathWasi(sym_link_path, &path_buf);
                return symlinkatWasi(target_path, path.dir_fd, path.relative_path);
            }
            return symlinkatWasi(target_path, newdirfd, sym_link_path);
        }
        const target_path_c = try toPosixPath(target_path);
        const sym_link_path_c = try toPosixPath(sym_link_path);
        return symlinkatZ(&target_path_c, newdirfd, &sym_link_path_c);
    }

    /// WASI-only. The same as `symlinkat` but targeting WASI.
    /// See also `symlinkat`.
    pub fn symlinkatWasi(target_path: []const u8, newdirfd: fd_t, sym_link_path: []const u8) SymLinkError!void {
        switch (wasi.path_symlink(target_path.ptr, target_path.len, newdirfd, sym_link_path.ptr, sym_link_path.len)) {
            .SUCCESS => {},
            .FAULT => unreachable,
            .INVAL => unreachable,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .DQUOT => return error.DiskQuota,
            .EXIST => return error.PathAlreadyExists,
            .IO => return error.FileSystem,
            .LOOP => return error.SymLinkLoop,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.NotDir,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .ROFS => return error.ReadOnlyFileSystem,
            .NOTCAPABLE => return error.AccessDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// The same as `symlinkat` except the parameters are null-terminated pointers.
    /// See also `symlinkat`.
    pub fn symlinkatZ(target_path: [*:0]const u8, newdirfd: fd_t, sym_link_path: [*:0]const u8) SymLinkError!void {
        if (builtin.os.tag == .windows) {
            @compileError("symlinkat is not supported on Windows; use std.os.windows.CreateSymbolicLink instead");
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return symlinkat(mem.sliceTo(target_path, 0), newdirfd, mem.sliceTo(sym_link_path, 0));
        }
        switch (errno(system.symlinkat(target_path, newdirfd, sym_link_path))) {
            .SUCCESS => return,
            .FAULT => unreachable,
            .INVAL => unreachable,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .DQUOT => return error.DiskQuota,
            .EXIST => return error.PathAlreadyExists,
            .IO => return error.FileSystem,
            .LOOP => return error.SymLinkLoop,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.NotDir,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .ROFS => return error.ReadOnlyFileSystem,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const LinkError = UnexpectedError || error{
        AccessDenied,
        DiskQuota,
        PathAlreadyExists,
        FileSystem,
        SymLinkLoop,
        LinkQuotaExceeded,
        NameTooLong,
        FileNotFound,
        SystemResources,
        NoSpaceLeft,
        ReadOnlyFileSystem,
        NotSameFileSystem,
    };

    pub fn linkZ(oldpath: [*:0]const u8, newpath: [*:0]const u8, flags: i32) LinkError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return link(mem.sliceTo(oldpath, 0), mem.sliceTo(newpath, 0), flags);
        }
        switch (errno(system.link(oldpath, newpath, flags))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .DQUOT => return error.DiskQuota,
            .EXIST => return error.PathAlreadyExists,
            .FAULT => unreachable,
            .IO => return error.FileSystem,
            .LOOP => return error.SymLinkLoop,
            .MLINK => return error.LinkQuotaExceeded,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .PERM => return error.AccessDenied,
            .ROFS => return error.ReadOnlyFileSystem,
            .XDEV => return error.NotSameFileSystem,
            .INVAL => unreachable,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn link(oldpath: []const u8, newpath: []const u8, flags: i32) LinkError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return linkat(wasi.AT.FDCWD, oldpath, wasi.AT.FDCWD, newpath, flags) catch |err| switch (err) {
                error.NotDir => unreachable, // link() does not support directories

                else => |e| return e,
            };
        }
        const old = try toPosixPath(oldpath);
        const new = try toPosixPath(newpath);
        return try linkZ(&old, &new, flags);
    }

    pub const LinkatError = LinkError || error{NotDir};

    pub fn linkatZ(
        olddir: fd_t,
        oldpath: [*:0]const u8,
        newdir: fd_t,
        newpath: [*:0]const u8,
        flags: i32,
    ) LinkatError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return linkat(olddir, mem.sliceTo(oldpath, 0), newdir, mem.sliceTo(newpath, 0), flags);
        }
        switch (errno(system.linkat(olddir, oldpath, newdir, newpath, flags))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .DQUOT => return error.DiskQuota,
            .EXIST => return error.PathAlreadyExists,
            .FAULT => unreachable,
            .IO => return error.FileSystem,
            .LOOP => return error.SymLinkLoop,
            .MLINK => return error.LinkQuotaExceeded,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .NOTDIR => return error.NotDir,
            .PERM => return error.AccessDenied,
            .ROFS => return error.ReadOnlyFileSystem,
            .XDEV => return error.NotSameFileSystem,
            .INVAL => unreachable,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn linkat(
        olddir: fd_t,
        oldpath: []const u8,
        newdir: fd_t,
        newpath: []const u8,
        flags: i32,
    ) LinkatError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var resolve_olddir: bool = (olddir == wasi.AT.FDCWD or fs.path.isAbsolute(oldpath));
            var resolve_newdir: bool = (newdir == wasi.AT.FDCWD or fs.path.isAbsolute(newpath));

            var old: RelativePathWasi = .{ .dir_fd = olddir, .relative_path = oldpath };
            var new: RelativePathWasi = .{ .dir_fd = newdir, .relative_path = newpath };

            // Resolve absolute or CWD-relative paths to a path within a Preopen

            if (resolve_olddir or resolve_newdir) {
                var buf_old: [MAX_PATH_BYTES]u8 = undefined;
                var buf_new: [MAX_PATH_BYTES]u8 = undefined;

                if (resolve_olddir)
                    old = try resolvePathWasi(oldpath, &buf_old);

                if (resolve_newdir)
                    new = try resolvePathWasi(newpath, &buf_new);

                return linkatWasi(old, new, flags);
            }
            return linkatWasi(old, new, flags);
        }
        const old = try toPosixPath(oldpath);
        const new = try toPosixPath(newpath);
        return try linkatZ(olddir, &old, newdir, &new, flags);
    }

    /// WASI-only. The same as `linkat` but targeting WASI.
    /// See also `linkat`.
    pub fn linkatWasi(old: RelativePathWasi, new: RelativePathWasi, flags: i32) LinkatError!void {
        var old_flags: wasi.lookupflags_t = 0;
        // TODO: Why is this not defined in wasi-libc?

        if (flags & linux.AT.SYMLINK_FOLLOW != 0) old_flags |= wasi.LOOKUP_SYMLINK_FOLLOW;

        switch (wasi.path_link(old.dir_fd, old_flags, old.relative_path.ptr, old.relative_path.len, new.dir_fd, new.relative_path.ptr, new.relative_path.len)) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .DQUOT => return error.DiskQuota,
            .EXIST => return error.PathAlreadyExists,
            .FAULT => unreachable,
            .IO => return error.FileSystem,
            .LOOP => return error.SymLinkLoop,
            .MLINK => return error.LinkQuotaExceeded,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .NOTDIR => return error.NotDir,
            .PERM => return error.AccessDenied,
            .ROFS => return error.ReadOnlyFileSystem,
            .XDEV => return error.NotSameFileSystem,
            .INVAL => unreachable,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const UnlinkError = error{
        FileNotFound,

        /// In WASI, this error may occur when the file descriptor does
        /// not hold the required rights to unlink a resource by path relative to it.
        AccessDenied,
        FileBusy,
        FileSystem,
        IsDir,
        SymLinkLoop,
        NameTooLong,
        NotDir,
        SystemResources,
        ReadOnlyFileSystem,

        /// On Windows, file paths must be valid Unicode.
        InvalidUtf8,

        /// On Windows, file paths cannot contain these characters:
        /// '/', '*', '?', '"', '<', '>', '|'
        BadPathName,
    } || UnexpectedError;

    /// Delete a name and possibly the file it refers to.
    /// See also `unlinkZ`.
    pub fn unlink(file_path: []const u8) UnlinkError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return unlinkat(wasi.AT.FDCWD, file_path, 0) catch |err| switch (err) {
                error.DirNotEmpty => unreachable, // only occurs when targeting directories

                else => |e| return e,
            };
        } else if (builtin.os.tag == .windows) {
            const file_path_w = try windows.sliceToPrefixedFileW(file_path);
            return unlinkW(file_path_w.span());
        } else {
            const file_path_c = try toPosixPath(file_path);
            return unlinkZ(&file_path_c);
        }
    }

    /// Same as `unlink` except the parameter is a null terminated UTF8-encoded string.
    pub fn unlinkZ(file_path: [*:0]const u8) UnlinkError!void {
        if (builtin.os.tag == .windows) {
            const file_path_w = try windows.cStrToPrefixedFileW(file_path);
            return unlinkW(file_path_w.span());
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return unlink(mem.sliceTo(file_path, 0));
        }
        switch (errno(system.unlink(file_path))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .BUSY => return error.FileBusy,
            .FAULT => unreachable,
            .INVAL => unreachable,
            .IO => return error.FileSystem,
            .ISDIR => return error.IsDir,
            .LOOP => return error.SymLinkLoop,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.NotDir,
            .NOMEM => return error.SystemResources,
            .ROFS => return error.ReadOnlyFileSystem,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Windows-only. Same as `unlink` except the parameter is null-terminated, WTF16 encoded.
    pub fn unlinkW(file_path_w: []const u16) UnlinkError!void {
        return windows.DeleteFile(file_path_w, .{ .dir = std.fs.cwd().fd });
    }

    pub const UnlinkatError = UnlinkError || error{
        /// When passing `AT.REMOVEDIR`, this error occurs when the named directory is not empty.
        DirNotEmpty,
    };

    /// Delete a file name and possibly the file it refers to, based on an open directory handle.
    /// Asserts that the path parameter has no null bytes.
    pub fn unlinkat(dirfd: fd_t, file_path: []const u8, flags: u32) UnlinkatError!void {
        if (builtin.os.tag == .windows) {
            const file_path_w = try windows.sliceToPrefixedFileW(file_path);
            return unlinkatW(dirfd, file_path_w.span(), flags);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            if (dirfd == wasi.AT.FDCWD or fs.path.isAbsolute(file_path)) {
                // Resolve absolute or CWD-relative paths to a path within a Preopen

                var path_buf: [MAX_PATH_BYTES]u8 = undefined;
                const path = try resolvePathWasi(file_path, &path_buf);
                return unlinkatWasi(path.dir_fd, path.relative_path, flags);
            }
            return unlinkatWasi(dirfd, file_path, flags);
        } else {
            const file_path_c = try toPosixPath(file_path);
            return unlinkatZ(dirfd, &file_path_c, flags);
        }
    }

    /// WASI-only. Same as `unlinkat` but targeting WASI.
    /// See also `unlinkat`.
    pub fn unlinkatWasi(dirfd: fd_t, file_path: []const u8, flags: u32) UnlinkatError!void {
        const remove_dir = (flags & AT.REMOVEDIR) != 0;
        const res = if (remove_dir)
            wasi.path_remove_directory(dirfd, file_path.ptr, file_path.len)
        else
            wasi.path_unlink_file(dirfd, file_path.ptr, file_path.len);
        switch (res) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .BUSY => return error.FileBusy,
            .FAULT => unreachable,
            .IO => return error.FileSystem,
            .ISDIR => return error.IsDir,
            .LOOP => return error.SymLinkLoop,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.NotDir,
            .NOMEM => return error.SystemResources,
            .ROFS => return error.ReadOnlyFileSystem,
            .NOTEMPTY => return error.DirNotEmpty,
            .NOTCAPABLE => return error.AccessDenied,

            .INVAL => unreachable, // invalid flags, or pathname has . as last component

            .BADF => unreachable, // always a race condition

            else => |err| return unexpectedErrno(err),
        }
    }

    /// Same as `unlinkat` but `file_path` is a null-terminated string.
    pub fn unlinkatZ(dirfd: fd_t, file_path_c: [*:0]const u8, flags: u32) UnlinkatError!void {
        if (builtin.os.tag == .windows) {
            const file_path_w = try windows.cStrToPrefixedFileW(file_path_c);
            return unlinkatW(dirfd, file_path_w.span(), flags);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return unlinkat(dirfd, mem.sliceTo(file_path_c, 0), flags);
        }
        switch (errno(system.unlinkat(dirfd, file_path_c, flags))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .BUSY => return error.FileBusy,
            .FAULT => unreachable,
            .IO => return error.FileSystem,
            .ISDIR => return error.IsDir,
            .LOOP => return error.SymLinkLoop,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.NotDir,
            .NOMEM => return error.SystemResources,
            .ROFS => return error.ReadOnlyFileSystem,
            .EXIST => return error.DirNotEmpty,
            .NOTEMPTY => return error.DirNotEmpty,

            .INVAL => unreachable, // invalid flags, or pathname has . as last component

            .BADF => unreachable, // always a race condition

            else => |err| return unexpectedErrno(err),
        }
    }

    /// Same as `unlinkat` but `sub_path_w` is UTF16LE, NT prefixed. Windows only.
    pub fn unlinkatW(dirfd: fd_t, sub_path_w: []const u16, flags: u32) UnlinkatError!void {
        const remove_dir = (flags & AT.REMOVEDIR) != 0;
        return windows.DeleteFile(sub_path_w, .{ .dir = dirfd, .remove_dir = remove_dir });
    }

    pub const RenameError = error{
        /// In WASI, this error may occur when the file descriptor does
        /// not hold the required rights to rename a resource by path relative to it.
        AccessDenied,
        FileBusy,
        DiskQuota,
        IsDir,
        SymLinkLoop,
        LinkQuotaExceeded,
        NameTooLong,
        FileNotFound,
        NotDir,
        SystemResources,
        NoSpaceLeft,
        PathAlreadyExists,
        ReadOnlyFileSystem,
        RenameAcrossMountPoints,
        InvalidUtf8,
        BadPathName,
        NoDevice,
        SharingViolation,
        PipeBusy,
    } || UnexpectedError;

    /// Change the name or location of a file.
    pub fn rename(old_path: []const u8, new_path: []const u8) RenameError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return renameat(wasi.AT.FDCWD, old_path, wasi.AT.FDCWD, new_path);
        } else if (builtin.os.tag == .windows) {
            const old_path_w = try windows.sliceToPrefixedFileW(old_path);
            const new_path_w = try windows.sliceToPrefixedFileW(new_path);
            return renameW(old_path_w.span().ptr, new_path_w.span().ptr);
        } else {
            const old_path_c = try toPosixPath(old_path);
            const new_path_c = try toPosixPath(new_path);
            return renameZ(&old_path_c, &new_path_c);
        }
    }

    /// Same as `rename` except the parameters are null-terminated byte arrays.
    pub fn renameZ(old_path: [*:0]const u8, new_path: [*:0]const u8) RenameError!void {
        if (builtin.os.tag == .windows) {
            const old_path_w = try windows.cStrToPrefixedFileW(old_path);
            const new_path_w = try windows.cStrToPrefixedFileW(new_path);
            return renameW(old_path_w.span().ptr, new_path_w.span().ptr);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return rename(mem.sliceTo(old_path, 0), mem.sliceTo(new_path, 0));
        }
        switch (errno(system.rename(old_path, new_path))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .BUSY => return error.FileBusy,
            .DQUOT => return error.DiskQuota,
            .FAULT => unreachable,
            .INVAL => unreachable,
            .ISDIR => return error.IsDir,
            .LOOP => return error.SymLinkLoop,
            .MLINK => return error.LinkQuotaExceeded,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.NotDir,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .EXIST => return error.PathAlreadyExists,
            .NOTEMPTY => return error.PathAlreadyExists,
            .ROFS => return error.ReadOnlyFileSystem,
            .XDEV => return error.RenameAcrossMountPoints,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Same as `rename` except the parameters are null-terminated UTF16LE encoded byte arrays.
    /// Assumes target is Windows.
    pub fn renameW(old_path: [*:0]const u16, new_path: [*:0]const u16) RenameError!void {
        const flags = windows.MOVEFILE_REPLACE_EXISTING | windows.MOVEFILE_WRITE_THROUGH;
        return windows.MoveFileExW(old_path, new_path, flags);
    }

    /// Change the name or location of a file based on an open directory handle.
    pub fn renameat(
        old_dir_fd: fd_t,
        old_path: []const u8,
        new_dir_fd: fd_t,
        new_path: []const u8,
    ) RenameError!void {
        if (builtin.os.tag == .windows) {
            const old_path_w = try windows.sliceToPrefixedFileW(old_path);
            const new_path_w = try windows.sliceToPrefixedFileW(new_path);
            return renameatW(old_dir_fd, old_path_w.span(), new_dir_fd, new_path_w.span(), windows.TRUE);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var resolve_old: bool = (old_dir_fd == wasi.AT.FDCWD or fs.path.isAbsolute(old_path));
            var resolve_new: bool = (new_dir_fd == wasi.AT.FDCWD or fs.path.isAbsolute(new_path));

            var old: RelativePathWasi = .{ .dir_fd = old_dir_fd, .relative_path = old_path };
            var new: RelativePathWasi = .{ .dir_fd = new_dir_fd, .relative_path = new_path };

            // Resolve absolute or CWD-relative paths to a path within a Preopen

            if (resolve_old or resolve_new) {
                var buf_old: [MAX_PATH_BYTES]u8 = undefined;
                var buf_new: [MAX_PATH_BYTES]u8 = undefined;

                if (resolve_old)
                    old = try resolvePathWasi(old_path, &buf_old);
                if (resolve_new)
                    new = try resolvePathWasi(new_path, &buf_new);

                return renameatWasi(old, new);
            }
            return renameatWasi(old, new);
        } else {
            const old_path_c = try toPosixPath(old_path);
            const new_path_c = try toPosixPath(new_path);
            return renameatZ(old_dir_fd, &old_path_c, new_dir_fd, &new_path_c);
        }
    }

    /// WASI-only. Same as `renameat` expect targeting WASI.
    /// See also `renameat`.
    pub fn renameatWasi(old: RelativePathWasi, new: RelativePathWasi) RenameError!void {
        switch (wasi.path_rename(old.dir_fd, old.relative_path.ptr, old.relative_path.len, new.dir_fd, new.relative_path.ptr, new.relative_path.len)) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .BUSY => return error.FileBusy,
            .DQUOT => return error.DiskQuota,
            .FAULT => unreachable,
            .INVAL => unreachable,
            .ISDIR => return error.IsDir,
            .LOOP => return error.SymLinkLoop,
            .MLINK => return error.LinkQuotaExceeded,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.NotDir,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .EXIST => return error.PathAlreadyExists,
            .NOTEMPTY => return error.PathAlreadyExists,
            .ROFS => return error.ReadOnlyFileSystem,
            .XDEV => return error.RenameAcrossMountPoints,
            .NOTCAPABLE => return error.AccessDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Same as `renameat` except the parameters are null-terminated byte arrays.
    pub fn renameatZ(
        old_dir_fd: fd_t,
        old_path: [*:0]const u8,
        new_dir_fd: fd_t,
        new_path: [*:0]const u8,
    ) RenameError!void {
        if (builtin.os.tag == .windows) {
            const old_path_w = try windows.cStrToPrefixedFileW(old_path);
            const new_path_w = try windows.cStrToPrefixedFileW(new_path);
            return renameatW(old_dir_fd, old_path_w.span(), new_dir_fd, new_path_w.span(), windows.TRUE);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return renameat(old_dir_fd, mem.sliceTo(old_path, 0), new_dir_fd, mem.sliceTo(new_path, 0));
        }

        switch (errno(system.renameat(old_dir_fd, old_path, new_dir_fd, new_path))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .BUSY => return error.FileBusy,
            .DQUOT => return error.DiskQuota,
            .FAULT => unreachable,
            .INVAL => unreachable,
            .ISDIR => return error.IsDir,
            .LOOP => return error.SymLinkLoop,
            .MLINK => return error.LinkQuotaExceeded,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.NotDir,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .EXIST => return error.PathAlreadyExists,
            .NOTEMPTY => return error.PathAlreadyExists,
            .ROFS => return error.ReadOnlyFileSystem,
            .XDEV => return error.RenameAcrossMountPoints,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Same as `renameat` but Windows-only and the path parameters are
    /// [WTF-16](https://simonsapin.github.io/wtf-8/#potentially-ill-formed-utf-16) encoded.
    pub fn renameatW(
        old_dir_fd: fd_t,
        old_path_w: []const u16,
        new_dir_fd: fd_t,
        new_path_w: []const u16,
        ReplaceIfExists: windows.BOOLEAN,
    ) RenameError!void {
        const src_fd = windows.OpenFile(old_path_w, .{
            .dir = old_dir_fd,
            .access_mask = windows.SYNCHRONIZE | windows.GENERIC_WRITE | windows.DELETE,
            .creation = windows.FILE_OPEN,
            .io_mode = .blocking,
            .filter = .any, // This function is supposed to rename both files and directories.

            .follow_symlinks = false,
        }) catch |err| switch (err) {
            error.WouldBlock => unreachable, // Not possible without `.share_access_nonblocking = true`.

            else => |e| return e,
        };
        defer windows.CloseHandle(src_fd);

        const struct_buf_len = @sizeOf(windows.FILE_RENAME_INFORMATION) + (MAX_PATH_BYTES - 1);
        var rename_info_buf: [struct_buf_len]u8 align(@alignOf(windows.FILE_RENAME_INFORMATION)) = undefined;
        const struct_len = @sizeOf(windows.FILE_RENAME_INFORMATION) - 1 + new_path_w.len * 2;
        if (struct_len > struct_buf_len) return error.NameTooLong;

        const rename_info = @ptrCast(*windows.FILE_RENAME_INFORMATION, &rename_info_buf);

        rename_info.* = .{
            .ReplaceIfExists = ReplaceIfExists,
            .RootDirectory = if (std.fs.path.isAbsoluteWindowsWTF16(new_path_w)) null else new_dir_fd,
            .FileNameLength = @intCast(u32, new_path_w.len * 2), // already checked error.NameTooLong

            .FileName = undefined,
        };
        std.mem.copy(u16, @as([*]u16, &rename_info.FileName)[0..new_path_w.len], new_path_w);

        var io_status_block: windows.IO_STATUS_BLOCK = undefined;

        const rc = windows.ntdll.NtSetInformationFile(
            src_fd,
            &io_status_block,
            rename_info,
            @intCast(u32, struct_len), // already checked for error.NameTooLong

            .FileRenameInformation,
        );

        switch (rc) {
            .SUCCESS => return,
            .INVALID_HANDLE => unreachable,
            .INVALID_PARAMETER => unreachable,
            .OBJECT_PATH_SYNTAX_BAD => unreachable,
            .ACCESS_DENIED => return error.AccessDenied,
            .OBJECT_NAME_NOT_FOUND => return error.FileNotFound,
            .OBJECT_PATH_NOT_FOUND => return error.FileNotFound,
            .NOT_SAME_DEVICE => return error.RenameAcrossMountPoints,
            else => return windows.unexpectedStatus(rc),
        }
    }

    pub fn mkdirat(dir_fd: fd_t, sub_dir_path: []const u8, mode: u32) MakeDirError!void {
        if (builtin.os.tag == .windows) {
            const sub_dir_path_w = try windows.sliceToPrefixedFileW(sub_dir_path);
            return mkdiratW(dir_fd, sub_dir_path_w.span(), mode);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            if (dir_fd == wasi.AT.FDCWD or fs.path.isAbsolute(sub_dir_path)) {
                // Resolve absolute or CWD-relative paths to a path within a Preopen

                var path_buf: [MAX_PATH_BYTES]u8 = undefined;
                const path = try resolvePathWasi(sub_dir_path, &path_buf);
                return mkdiratWasi(path.dir_fd, path.relative_path, mode);
            }
            return mkdiratWasi(dir_fd, sub_dir_path, mode);
        } else {
            const sub_dir_path_c = try toPosixPath(sub_dir_path);
            return mkdiratZ(dir_fd, &sub_dir_path_c, mode);
        }
    }

    pub fn mkdiratWasi(dir_fd: fd_t, sub_dir_path: []const u8, mode: u32) MakeDirError!void {
        _ = mode;
        switch (wasi.path_create_directory(dir_fd, sub_dir_path.ptr, sub_dir_path.len)) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .BADF => unreachable,
            .PERM => return error.AccessDenied,
            .DQUOT => return error.DiskQuota,
            .EXIST => return error.PathAlreadyExists,
            .FAULT => unreachable,
            .LOOP => return error.SymLinkLoop,
            .MLINK => return error.LinkQuotaExceeded,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .NOTDIR => return error.NotDir,
            .ROFS => return error.ReadOnlyFileSystem,
            .NOTCAPABLE => return error.AccessDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn mkdiratZ(dir_fd: fd_t, sub_dir_path: [*:0]const u8, mode: u32) MakeDirError!void {
        if (builtin.os.tag == .windows) {
            const sub_dir_path_w = try windows.cStrToPrefixedFileW(sub_dir_path);
            return mkdiratW(dir_fd, sub_dir_path_w.span().ptr, mode);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return mkdirat(dir_fd, mem.sliceTo(sub_dir_path, 0), mode);
        }
        switch (errno(system.mkdirat(dir_fd, sub_dir_path, mode))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .BADF => unreachable,
            .PERM => return error.AccessDenied,
            .DQUOT => return error.DiskQuota,
            .EXIST => return error.PathAlreadyExists,
            .FAULT => unreachable,
            .LOOP => return error.SymLinkLoop,
            .MLINK => return error.LinkQuotaExceeded,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .NOTDIR => return error.NotDir,
            .ROFS => return error.ReadOnlyFileSystem,
            // dragonfly: when dir_fd is unlinked from filesystem

            .NOTCONN => return error.FileNotFound,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn mkdiratW(dir_fd: fd_t, sub_path_w: []const u16, mode: u32) MakeDirError!void {
        _ = mode;
        const sub_dir_handle = windows.OpenFile(sub_path_w, .{
            .dir = dir_fd,
            .access_mask = windows.GENERIC_READ | windows.SYNCHRONIZE,
            .creation = windows.FILE_CREATE,
            .io_mode = .blocking,
            .filter = .dir_only,
        }) catch |err| switch (err) {
            error.IsDir => unreachable,
            error.PipeBusy => unreachable,
            error.WouldBlock => unreachable,
            else => |e| return e,
        };
        windows.CloseHandle(sub_dir_handle);
    }

    pub const MakeDirError = error{
        /// In WASI, this error may occur when the file descriptor does
        /// not hold the required rights to create a new directory relative to it.
        AccessDenied,
        DiskQuota,
        PathAlreadyExists,
        SymLinkLoop,
        LinkQuotaExceeded,
        NameTooLong,
        FileNotFound,
        SystemResources,
        NoSpaceLeft,
        NotDir,
        ReadOnlyFileSystem,
        InvalidUtf8,
        BadPathName,
        NoDevice,
    } || UnexpectedError;

    /// Create a directory.
    /// `mode` is ignored on Windows and WASI.
    pub fn mkdir(dir_path: []const u8, mode: u32) MakeDirError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return mkdirat(wasi.AT.FDCWD, dir_path, mode);
        } else if (builtin.os.tag == .windows) {
            const dir_path_w = try windows.sliceToPrefixedFileW(dir_path);
            return mkdirW(dir_path_w.span(), mode);
        } else {
            const dir_path_c = try toPosixPath(dir_path);
            return mkdirZ(&dir_path_c, mode);
        }
    }

    /// Same as `mkdir` but the parameter is a null-terminated UTF8-encoded string.
    pub fn mkdirZ(dir_path: [*:0]const u8, mode: u32) MakeDirError!void {
        if (builtin.os.tag == .windows) {
            const dir_path_w = try windows.cStrToPrefixedFileW(dir_path);
            return mkdirW(dir_path_w.span(), mode);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return mkdir(mem.sliceTo(dir_path, 0), mode);
        }
        switch (errno(system.mkdir(dir_path, mode))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .DQUOT => return error.DiskQuota,
            .EXIST => return error.PathAlreadyExists,
            .FAULT => unreachable,
            .LOOP => return error.SymLinkLoop,
            .MLINK => return error.LinkQuotaExceeded,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.NoSpaceLeft,
            .NOTDIR => return error.NotDir,
            .ROFS => return error.ReadOnlyFileSystem,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Windows-only. Same as `mkdir` but the parameters is  WTF16 encoded.
    pub fn mkdirW(dir_path_w: []const u16, mode: u32) MakeDirError!void {
        _ = mode;
        const sub_dir_handle = windows.OpenFile(dir_path_w, .{
            .dir = std.fs.cwd().fd,
            .access_mask = windows.GENERIC_READ | windows.SYNCHRONIZE,
            .creation = windows.FILE_CREATE,
            .io_mode = .blocking,
            .filter = .dir_only,
        }) catch |err| switch (err) {
            error.IsDir => unreachable,
            error.PipeBusy => unreachable,
            error.WouldBlock => unreachable,
            else => |e| return e,
        };
        windows.CloseHandle(sub_dir_handle);
    }

    pub const DeleteDirError = error{
        AccessDenied,
        FileBusy,
        SymLinkLoop,
        NameTooLong,
        FileNotFound,
        SystemResources,
        NotDir,
        DirNotEmpty,
        ReadOnlyFileSystem,
        InvalidUtf8,
        BadPathName,
    } || UnexpectedError;

    /// Deletes an empty directory.
    pub fn rmdir(dir_path: []const u8) DeleteDirError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return unlinkat(wasi.AT.FDCWD, dir_path, AT.REMOVEDIR) catch |err| switch (err) {
                error.FileSystem => unreachable, // only occurs when targeting files

                error.IsDir => unreachable, // only occurs when targeting files

                else => |e| return e,
            };
        } else if (builtin.os.tag == .windows) {
            const dir_path_w = try windows.sliceToPrefixedFileW(dir_path);
            return rmdirW(dir_path_w.span());
        } else {
            const dir_path_c = try toPosixPath(dir_path);
            return rmdirZ(&dir_path_c);
        }
    }

    /// Same as `rmdir` except the parameter is null-terminated.
    pub fn rmdirZ(dir_path: [*:0]const u8) DeleteDirError!void {
        if (builtin.os.tag == .windows) {
            const dir_path_w = try windows.cStrToPrefixedFileW(dir_path);
            return rmdirW(dir_path_w.span());
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return rmdir(mem.sliceTo(dir_path, 0));
        }
        switch (errno(system.rmdir(dir_path))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .BUSY => return error.FileBusy,
            .FAULT => unreachable,
            .INVAL => return error.BadPathName,
            .LOOP => return error.SymLinkLoop,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOTDIR => return error.NotDir,
            .EXIST => return error.DirNotEmpty,
            .NOTEMPTY => return error.DirNotEmpty,
            .ROFS => return error.ReadOnlyFileSystem,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Windows-only. Same as `rmdir` except the parameter is WTF16 encoded.
    pub fn rmdirW(dir_path_w: []const u16) DeleteDirError!void {
        return windows.DeleteFile(dir_path_w, .{ .dir = std.fs.cwd().fd, .remove_dir = true }) catch |err| switch (err) {
            error.IsDir => unreachable,
            else => |e| return e,
        };
    }

    pub const ChangeCurDirError = error{
        AccessDenied,
        FileSystem,
        SymLinkLoop,
        NameTooLong,
        FileNotFound,
        SystemResources,
        NotDir,
        BadPathName,

        /// On Windows, file paths must be valid Unicode.
        InvalidUtf8,
    } || UnexpectedError;

    /// Changes the current working directory of the calling process.
    /// `dir_path` is recommended to be a UTF-8 encoded string.
    pub fn chdir(dir_path: []const u8) ChangeCurDirError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var buf: [MAX_PATH_BYTES]u8 = undefined;
            var alloc = std.heap.FixedBufferAllocator.init(&buf);
            const path = try fs.resolve(alloc.allocator(), &.{ wasi_cwd.cwd, dir_path });

            const dirinfo = try fstatat(AT.FDCWD, path, 0);
            if (dirinfo.filetype != .DIRECTORY) {
                return error.NotDir;
            }

            var cwd_alloc = std.heap.FixedBufferAllocator.init(&wasi_cwd.path_buffer);
            wasi_cwd.cwd = try cwd_alloc.allocator().dupe(u8, path);
            return;
        } else if (builtin.os.tag == .windows) {
            var utf16_dir_path: [windows.PATH_MAX_WIDE]u16 = undefined;
            const len = try std.unicode.utf8ToUtf16Le(utf16_dir_path[0..], dir_path);
            if (len > utf16_dir_path.len) return error.NameTooLong;
            return chdirW(utf16_dir_path[0..len]);
        } else {
            const dir_path_c = try toPosixPath(dir_path);
            return chdirZ(&dir_path_c);
        }
    }

    /// Same as `chdir` except the parameter is null-terminated.
    pub fn chdirZ(dir_path: [*:0]const u8) ChangeCurDirError!void {
        if (builtin.os.tag == .windows) {
            var utf16_dir_path: [windows.PATH_MAX_WIDE]u16 = undefined;
            const len = try std.unicode.utf8ToUtf16Le(utf16_dir_path[0..], dir_path);
            if (len > utf16_dir_path.len) return error.NameTooLong;
            return chdirW(utf16_dir_path[0..len]);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return chdir(mem.sliceTo(dir_path, 0));
        }
        switch (errno(system.chdir(dir_path))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .FAULT => unreachable,
            .IO => return error.FileSystem,
            .LOOP => return error.SymLinkLoop,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOTDIR => return error.NotDir,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Windows-only. Same as `chdir` except the paramter is WTF16 encoded.
    pub fn chdirW(dir_path: []const u16) ChangeCurDirError!void {
        windows.SetCurrentDirectory(dir_path) catch |err| switch (err) {
            error.NoDevice => return error.FileSystem,
            else => |e| return e,
        };
    }

    pub const FchdirError = error{
        AccessDenied,
        NotDir,
        FileSystem,
    } || UnexpectedError;

    pub fn fchdir(dirfd: fd_t) FchdirError!void {
        while (true) {
            switch (errno(system.fchdir(dirfd))) {
                .SUCCESS => return,
                .ACCES => return error.AccessDenied,
                .BADF => unreachable,
                .NOTDIR => return error.NotDir,
                .INTR => continue,
                .IO => return error.FileSystem,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const ReadLinkError = error{
        /// In WASI, this error may occur when the file descriptor does
        /// not hold the required rights to read value of a symbolic link relative to it.
        AccessDenied,
        FileSystem,
        SymLinkLoop,
        NameTooLong,
        FileNotFound,
        SystemResources,
        NotLink,
        NotDir,
        InvalidUtf8,
        BadPathName,
        /// Windows-only. This error may occur if the opened reparse point is
        /// of unsupported type.
        UnsupportedReparsePointType,
    } || UnexpectedError;

    /// Read value of a symbolic link.
    /// The return value is a slice of `out_buffer` from index 0.
    pub fn readlink(file_path: []const u8, out_buffer: []u8) ReadLinkError![]u8 {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return readlinkat(wasi.AT.FDCWD, file_path, out_buffer);
        } else if (builtin.os.tag == .windows) {
            const file_path_w = try windows.sliceToPrefixedFileW(file_path);
            return readlinkW(file_path_w.span(), out_buffer);
        } else {
            const file_path_c = try toPosixPath(file_path);
            return readlinkZ(&file_path_c, out_buffer);
        }
    }

    /// Windows-only. Same as `readlink` except `file_path` is WTF16 encoded.
    /// See also `readlinkZ`.
    pub fn readlinkW(file_path: []const u16, out_buffer: []u8) ReadLinkError![]u8 {
        return windows.ReadLink(std.fs.cwd().fd, file_path, out_buffer);
    }

    /// Same as `readlink` except `file_path` is null-terminated.
    pub fn readlinkZ(file_path: [*:0]const u8, out_buffer: []u8) ReadLinkError![]u8 {
        if (builtin.os.tag == .windows) {
            const file_path_w = try windows.cStrToWin32PrefixedFileW(file_path);
            return readlinkW(file_path_w.span(), out_buffer);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return readlink(mem.sliceTo(file_path, 0), out_buffer);
        }
        const rc = system.readlink(file_path, out_buffer.ptr, out_buffer.len);
        switch (errno(rc)) {
            .SUCCESS => return out_buffer[0..@bitCast(usize, rc)],
            .ACCES => return error.AccessDenied,
            .FAULT => unreachable,
            .INVAL => return error.NotLink,
            .IO => return error.FileSystem,
            .LOOP => return error.SymLinkLoop,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOTDIR => return error.NotDir,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Similar to `readlink` except reads value of a symbolink link **relative** to `dirfd` directory handle.
    /// The return value is a slice of `out_buffer` from index 0.
    /// See also `readlinkatWasi`, `realinkatZ` and `realinkatW`.
    pub fn readlinkat(dirfd: fd_t, file_path: []const u8, out_buffer: []u8) ReadLinkError![]u8 {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            if (dirfd == wasi.AT.FDCWD or fs.path.isAbsolute(file_path)) {
                // Resolve absolute or CWD-relative paths to a path within a Preopen

                var path_buf: [MAX_PATH_BYTES]u8 = undefined;
                var path = try resolvePathWasi(file_path, &path_buf);
                return readlinkatWasi(path.dir_fd, path.relative_path, out_buffer);
            }
            return readlinkatWasi(dirfd, file_path, out_buffer);
        }
        if (builtin.os.tag == .windows) {
            const file_path_w = try windows.sliceToPrefixedFileW(file_path);
            return readlinkatW(dirfd, file_path_w.span(), out_buffer);
        }
        const file_path_c = try toPosixPath(file_path);
        return readlinkatZ(dirfd, &file_path_c, out_buffer);
    }

    /// WASI-only. Same as `readlinkat` but targets WASI.
    /// See also `readlinkat`.
    pub fn readlinkatWasi(dirfd: fd_t, file_path: []const u8, out_buffer: []u8) ReadLinkError![]u8 {
        var bufused: usize = undefined;
        switch (wasi.path_readlink(dirfd, file_path.ptr, file_path.len, out_buffer.ptr, out_buffer.len, &bufused)) {
            .SUCCESS => return out_buffer[0..bufused],
            .ACCES => return error.AccessDenied,
            .FAULT => unreachable,
            .INVAL => return error.NotLink,
            .IO => return error.FileSystem,
            .LOOP => return error.SymLinkLoop,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOTDIR => return error.NotDir,
            .NOTCAPABLE => return error.AccessDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Windows-only. Same as `readlinkat` except `file_path` is null-terminated, WTF16 encoded.
    /// See also `readlinkat`.
    pub fn readlinkatW(dirfd: fd_t, file_path: []const u16, out_buffer: []u8) ReadLinkError![]u8 {
        return windows.ReadLink(dirfd, file_path, out_buffer);
    }

    /// Same as `readlinkat` except `file_path` is null-terminated.
    /// See also `readlinkat`.
    pub fn readlinkatZ(dirfd: fd_t, file_path: [*:0]const u8, out_buffer: []u8) ReadLinkError![]u8 {
        if (builtin.os.tag == .windows) {
            const file_path_w = try windows.cStrToPrefixedFileW(file_path);
            return readlinkatW(dirfd, file_path_w.span(), out_buffer);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return readlinkat(dirfd, mem.sliceTo(file_path, 0), out_buffer);
        }
        const rc = system.readlinkat(dirfd, file_path, out_buffer.ptr, out_buffer.len);
        switch (errno(rc)) {
            .SUCCESS => return out_buffer[0..@bitCast(usize, rc)],
            .ACCES => return error.AccessDenied,
            .FAULT => unreachable,
            .INVAL => return error.NotLink,
            .IO => return error.FileSystem,
            .LOOP => return error.SymLinkLoop,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOTDIR => return error.NotDir,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const SetEidError = error{
        InvalidUserId,
        PermissionDenied,
    } || UnexpectedError;

    pub const SetIdError = error{ResourceLimitReached} || SetEidError;

    pub fn setuid(uid: uid_t) SetIdError!void {
        switch (errno(system.setuid(uid))) {
            .SUCCESS => return,
            .AGAIN => return error.ResourceLimitReached,
            .INVAL => return error.InvalidUserId,
            .PERM => return error.PermissionDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn seteuid(uid: uid_t) SetEidError!void {
        switch (errno(system.seteuid(uid))) {
            .SUCCESS => return,
            .INVAL => return error.InvalidUserId,
            .PERM => return error.PermissionDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn setreuid(ruid: uid_t, euid: uid_t) SetIdError!void {
        switch (errno(system.setreuid(ruid, euid))) {
            .SUCCESS => return,
            .AGAIN => return error.ResourceLimitReached,
            .INVAL => return error.InvalidUserId,
            .PERM => return error.PermissionDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn setgid(gid: gid_t) SetIdError!void {
        switch (errno(system.setgid(gid))) {
            .SUCCESS => return,
            .AGAIN => return error.ResourceLimitReached,
            .INVAL => return error.InvalidUserId,
            .PERM => return error.PermissionDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn setegid(uid: uid_t) SetEidError!void {
        switch (errno(system.setegid(uid))) {
            .SUCCESS => return,
            .INVAL => return error.InvalidUserId,
            .PERM => return error.PermissionDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn setregid(rgid: gid_t, egid: gid_t) SetIdError!void {
        switch (errno(system.setregid(rgid, egid))) {
            .SUCCESS => return,
            .AGAIN => return error.ResourceLimitReached,
            .INVAL => return error.InvalidUserId,
            .PERM => return error.PermissionDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Test whether a file descriptor refers to a terminal.
    pub fn isatty(handle: fd_t) bool {
        if (builtin.os.tag == .windows) {
            if (isCygwinPty(handle))
                return true;

            var out: windows.DWORD = undefined;
            return windows.kernel32.GetConsoleMode(handle, &out) != 0;
        }
        if (builtin.link_libc) {
            return system.isatty(handle) != 0;
        }
        if (builtin.os.tag == .wasi) {
            var statbuf: fdstat_t = undefined;
            const err = system.fd_fdstat_get(handle, &statbuf);
            if (err != .SUCCESS) {
                // errno = err;

                return false;
            }

            // A tty is a character device that we can't seek or tell on.

            if (statbuf.fs_filetype != .CHARACTER_DEVICE or
                (statbuf.fs_rights_base & (RIGHT.FD_SEEK | RIGHT.FD_TELL)) != 0)
            {
                // errno = ENOTTY;

                return false;
            }

            return true;
        }
        if (builtin.os.tag == .linux) {
            while (true) {
                var wsz: linux.winsize = undefined;
                const fd = @bitCast(usize, @as(isize, handle));
                const rc = linux.syscall3(.ioctl, fd, linux.T.IOCGWINSZ, @ptrToInt(&wsz));
                switch (linux.getErrno(rc)) {
                    .SUCCESS => return true,
                    .INTR => continue,
                    else => return false,
                }
            }
        }
        return system.isatty(handle) != 0;
    }

    pub fn isCygwinPty(handle: fd_t) bool {
        if (builtin.os.tag != .windows) return false;

        const size = @sizeOf(windows.FILE_NAME_INFO);
        var name_info_bytes align(@alignOf(windows.FILE_NAME_INFO)) = [_]u8{0} ** (size + windows.MAX_PATH);

        if (windows.kernel32.GetFileInformationByHandleEx(
            handle,
            windows.FileNameInfo,
            @ptrCast(*anyopaque, &name_info_bytes),
            name_info_bytes.len,
        ) == 0) {
            return false;
        }

        const name_info = @ptrCast(*const windows.FILE_NAME_INFO, &name_info_bytes[0]);
        const name_bytes = name_info_bytes[size .. size + @as(usize, name_info.FileNameLength)];
        const name_wide = mem.bytesAsSlice(u16, name_bytes);
        return mem.indexOf(u16, name_wide, &[_]u16{ 'm', 's', 'y', 's', '-' }) != null or
            mem.indexOf(u16, name_wide, &[_]u16{ '-', 'p', 't', 'y' }) != null;
    }

    pub const SocketError = error{
        /// Permission to create a socket of the specified type and/or
        /// pro‐tocol is denied.
        PermissionDenied,

        /// The implementation does not support the specified address family.
        AddressFamilyNotSupported,

        /// Unknown protocol, or protocol family not available.
        ProtocolFamilyNotAvailable,

        /// The per-process limit on the number of open file descriptors has been reached.
        ProcessFdQuotaExceeded,

        /// The system-wide limit on the total number of open files has been reached.
        SystemFdQuotaExceeded,

        /// Insufficient memory is available. The socket cannot be created until sufficient
        /// resources are freed.
        SystemResources,

        /// The protocol type or the specified protocol is not supported within this domain.
        ProtocolNotSupported,

        /// The socket type is not supported by the protocol.
        SocketTypeNotSupported,
    } || UnexpectedError;

    pub fn socket(domain: u32, socket_type: u32, protocol: u32) SocketError!socket_t {
        if (builtin.os.tag == .windows) {
            // NOTE: windows translates the SOCK.NONBLOCK/SOCK.CLOEXEC flags into

            // windows-analagous operations

            const filtered_sock_type = socket_type & ~@as(u32, SOCK.NONBLOCK | SOCK.CLOEXEC);
            const flags: u32 = if ((socket_type & SOCK.CLOEXEC) != 0)
                windows.ws2_32.WSA_FLAG_NO_HANDLE_INHERIT
            else
                0;
            const rc = try windows.WSASocketW(
                @bitCast(i32, domain),
                @bitCast(i32, filtered_sock_type),
                @bitCast(i32, protocol),
                null,
                0,
                flags,
            );
            errdefer windows.closesocket(rc) catch unreachable;
            if ((socket_type & SOCK.NONBLOCK) != 0) {
                var mode: c_ulong = 1; // nonblocking

                if (windows.ws2_32.SOCKET_ERROR == windows.ws2_32.ioctlsocket(rc, windows.ws2_32.FIONBIO, &mode)) {
                    switch (windows.ws2_32.WSAGetLastError()) {
                        // have not identified any error codes that should be handled yet

                        else => unreachable,
                    }
                }
            }
            return rc;
        }

        const have_sock_flags = comptime !builtin.target.isDarwin();
        const filtered_sock_type = if (!have_sock_flags)
            socket_type & ~@as(u32, SOCK.NONBLOCK | SOCK.CLOEXEC)
        else
            socket_type;
        const rc = system.socket(domain, filtered_sock_type, protocol);
        switch (errno(rc)) {
            .SUCCESS => {
                const fd = @intCast(fd_t, rc);
                if (!have_sock_flags) {
                    try setSockFlags(fd, socket_type);
                }
                return fd;
            },
            .ACCES => return error.PermissionDenied,
            .AFNOSUPPORT => return error.AddressFamilyNotSupported,
            .INVAL => return error.ProtocolFamilyNotAvailable,
            .MFILE => return error.ProcessFdQuotaExceeded,
            .NFILE => return error.SystemFdQuotaExceeded,
            .NOBUFS => return error.SystemResources,
            .NOMEM => return error.SystemResources,
            .PROTONOSUPPORT => return error.ProtocolNotSupported,
            .PROTOTYPE => return error.SocketTypeNotSupported,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const ShutdownError = error{
        ConnectionAborted,

        /// Connection was reset by peer, application should close socket as it is no longer usable.
        ConnectionResetByPeer,
        BlockingOperationInProgress,

        /// The network subsystem has failed.
        NetworkSubsystemFailed,

        /// The socket is not connected (connection-oriented sockets only).
        SocketNotConnected,
        SystemResources,
    } || UnexpectedError;

    pub const ShutdownHow = enum { recv, send, both };

    /// Shutdown socket send/receive operations
    pub fn shutdown(sock: socket_t, how: ShutdownHow) ShutdownError!void {
        if (builtin.os.tag == .windows) {
            const result = windows.ws2_32.shutdown(sock, switch (how) {
                .recv => windows.ws2_32.SD_RECEIVE,
                .send => windows.ws2_32.SD_SEND,
                .both => windows.ws2_32.SD_BOTH,
            });
            if (0 != result) switch (windows.ws2_32.WSAGetLastError()) {
                .WSAECONNABORTED => return error.ConnectionAborted,
                .WSAECONNRESET => return error.ConnectionResetByPeer,
                .WSAEINPROGRESS => return error.BlockingOperationInProgress,
                .WSAEINVAL => unreachable,
                .WSAENETDOWN => return error.NetworkSubsystemFailed,
                .WSAENOTCONN => return error.SocketNotConnected,
                .WSAENOTSOCK => unreachable,
                .WSANOTINITIALISED => unreachable,
                else => |err| return windows.unexpectedWSAError(err),
            };
        } else {
            const rc = system.shutdown(sock, switch (how) {
                .recv => SHUT.RD,
                .send => SHUT.WR,
                .both => SHUT.RDWR,
            });
            switch (errno(rc)) {
                .SUCCESS => return,
                .BADF => unreachable,
                .INVAL => unreachable,
                .NOTCONN => return error.SocketNotConnected,
                .NOTSOCK => unreachable,
                .NOBUFS => return error.SystemResources,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub fn closeSocket(sock: socket_t) void {
        if (builtin.os.tag == .windows) {
            windows.closesocket(sock) catch unreachable;
        } else {
            close(sock);
        }
    }

    pub const BindError = error{
        /// The address is protected, and the user is not the superuser.
        /// For UNIX domain sockets: Search permission is denied on  a  component
        /// of  the  path  prefix.
        AccessDenied,

        /// The given address is already in use, or in the case of Internet domain sockets,
        /// The  port number was specified as zero in the socket
        /// address structure, but, upon attempting to bind to  an  ephemeral  port,  it  was
        /// determined  that  all  port  numbers in the ephemeral port range are currently in
        /// use.  See the discussion of /proc/sys/net/ipv4/ip_local_port_range ip(7).
        AddressInUse,

        /// A nonexistent interface was requested or the requested address was not local.
        AddressNotAvailable,

        /// The address is not valid for the address family of socket.
        AddressFamilyNotSupported,

        /// Too many symbolic links were encountered in resolving addr.
        SymLinkLoop,

        /// addr is too long.
        NameTooLong,

        /// A component in the directory prefix of the socket pathname does not exist.
        FileNotFound,

        /// Insufficient kernel memory was available.
        SystemResources,

        /// A component of the path prefix is not a directory.
        NotDir,

        /// The socket inode would reside on a read-only filesystem.
        ReadOnlyFileSystem,

        /// The network subsystem has failed.
        NetworkSubsystemFailed,

        FileDescriptorNotASocket,

        AlreadyBound,
    } || UnexpectedError;

    /// addr is `*const T` where T is one of the sockaddr
    pub fn bind(sock: socket_t, addr: *const sockaddr, len: socklen_t) BindError!void {
        if (builtin.os.tag == .windows) {
            const rc = windows.bind(sock, addr, len);
            if (rc == windows.ws2_32.SOCKET_ERROR) {
                switch (windows.ws2_32.WSAGetLastError()) {
                    .WSANOTINITIALISED => unreachable, // not initialized WSA

                    .WSAEACCES => return error.AccessDenied,
                    .WSAEADDRINUSE => return error.AddressInUse,
                    .WSAEADDRNOTAVAIL => return error.AddressNotAvailable,
                    .WSAENOTSOCK => return error.FileDescriptorNotASocket,
                    .WSAEFAULT => unreachable, // invalid pointers

                    .WSAEINVAL => return error.AlreadyBound,
                    .WSAENOBUFS => return error.SystemResources,
                    .WSAENETDOWN => return error.NetworkSubsystemFailed,
                    else => |err| return windows.unexpectedWSAError(err),
                }
                unreachable;
            }
            return;
        } else {
            const rc = system.bind(sock, addr, len);
            switch (errno(rc)) {
                .SUCCESS => return,
                .ACCES => return error.AccessDenied,
                .ADDRINUSE => return error.AddressInUse,
                .BADF => unreachable, // always a race condition if this error is returned

                .INVAL => unreachable, // invalid parameters

                .NOTSOCK => unreachable, // invalid `sockfd`

                .AFNOSUPPORT => return error.AddressFamilyNotSupported,
                .ADDRNOTAVAIL => return error.AddressNotAvailable,
                .FAULT => unreachable, // invalid `addr` pointer

                .LOOP => return error.SymLinkLoop,
                .NAMETOOLONG => return error.NameTooLong,
                .NOENT => return error.FileNotFound,
                .NOMEM => return error.SystemResources,
                .NOTDIR => return error.NotDir,
                .ROFS => return error.ReadOnlyFileSystem,
                else => |err| return unexpectedErrno(err),
            }
        }
        unreachable;
    }

    pub const ListenError = error{
        /// Another socket is already listening on the same port.
        /// For Internet domain sockets, the  socket referred to by sockfd had not previously
        /// been bound to an address and, upon attempting to bind it to an ephemeral port, it
        /// was determined that all port numbers in the ephemeral port range are currently in
        /// use.  See the discussion of /proc/sys/net/ipv4/ip_local_port_range in ip(7).
        AddressInUse,

        /// The file descriptor sockfd does not refer to a socket.
        FileDescriptorNotASocket,

        /// The socket is not of a type that supports the listen() operation.
        OperationNotSupported,

        /// The network subsystem has failed.
        NetworkSubsystemFailed,

        /// Ran out of system resources
        /// On Windows it can either run out of socket descriptors or buffer space
        SystemResources,

        /// Already connected
        AlreadyConnected,

        /// Socket has not been bound yet
        SocketNotBound,
    } || UnexpectedError;

    pub fn listen(sock: socket_t, backlog: u31) ListenError!void {
        if (builtin.os.tag == .windows) {
            const rc = windows.listen(sock, backlog);
            if (rc == windows.ws2_32.SOCKET_ERROR) {
                switch (windows.ws2_32.WSAGetLastError()) {
                    .WSANOTINITIALISED => unreachable, // not initialized WSA

                    .WSAENETDOWN => return error.NetworkSubsystemFailed,
                    .WSAEADDRINUSE => return error.AddressInUse,
                    .WSAEISCONN => return error.AlreadyConnected,
                    .WSAEINVAL => return error.SocketNotBound,
                    .WSAEMFILE, .WSAENOBUFS => return error.SystemResources,
                    .WSAENOTSOCK => return error.FileDescriptorNotASocket,
                    .WSAEOPNOTSUPP => return error.OperationNotSupported,
                    .WSAEINPROGRESS => unreachable,
                    else => |err| return windows.unexpectedWSAError(err),
                }
            }
            return;
        } else {
            const rc = system.listen(sock, backlog);
            switch (errno(rc)) {
                .SUCCESS => return,
                .ADDRINUSE => return error.AddressInUse,
                .BADF => unreachable,
                .NOTSOCK => return error.FileDescriptorNotASocket,
                .OPNOTSUPP => return error.OperationNotSupported,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const AcceptError = error{
        ConnectionAborted,

        /// The file descriptor sockfd does not refer to a socket.
        FileDescriptorNotASocket,

        /// The per-process limit on the number of open file descriptors has been reached.
        ProcessFdQuotaExceeded,

        /// The system-wide limit on the total number of open files has been reached.
        SystemFdQuotaExceeded,

        /// Not enough free memory.  This often means that the memory allocation  is  limited
        /// by the socket buffer limits, not by the system memory.
        SystemResources,

        /// Socket is not listening for new connections.
        SocketNotListening,

        ProtocolFailure,

        /// Firewall rules forbid connection.
        BlockedByFirewall,

        /// This error occurs when no global event loop is configured,
        /// and accepting from the socket would block.
        WouldBlock,

        /// An incoming connection was indicated, but was subsequently terminated by the
        /// remote peer prior to accepting the call.
        ConnectionResetByPeer,

        /// The network subsystem has failed.
        NetworkSubsystemFailed,

        /// The referenced socket is not a type that supports connection-oriented service.
        OperationNotSupported,
    } || UnexpectedError;

    /// Accept a connection on a socket.
    /// If `sockfd` is opened in non blocking mode, the function will
    /// return error.WouldBlock when EAGAIN is received.
    pub fn accept(
        /// This argument is a socket that has been created with `socket`, bound to a local address
        /// with `bind`, and is listening for connections after a `listen`.
        sock: socket_t,
        /// This argument is a pointer to a sockaddr structure.  This structure is filled in with  the
        /// address  of  the  peer  socket, as known to the communications layer.  The exact format of the
        /// address returned addr is determined by the socket's address  family  (see  `socket`  and  the
        /// respective  protocol  man  pages).
        addr: ?*sockaddr,
        /// This argument is a value-result argument: the caller must initialize it to contain  the
        /// size (in bytes) of the structure pointed to by addr; on return it will contain the actual size
        /// of the peer address.
        ///
        /// The returned address is truncated if the buffer provided is too small; in this  case,  `addr_size`
        /// will return a value greater than was supplied to the call.
        addr_size: ?*socklen_t,
        /// The following values can be bitwise ORed in flags to obtain different behavior:
        /// * `SOCK.NONBLOCK` - Set the `O.NONBLOCK` file status flag on the open file description (see `open`)
        ///   referred  to by the new file descriptor.  Using this flag saves extra calls to `fcntl` to achieve
        ///   the same result.
        /// * `SOCK.CLOEXEC`  - Set the close-on-exec (`FD_CLOEXEC`) flag on the new file descriptor.   See  the
        ///   description  of the `O.CLOEXEC` flag in `open` for reasons why this may be useful.
        flags: u32,
    ) AcceptError!socket_t {
        const have_accept4 = comptime !(builtin.target.isDarwin() or builtin.os.tag == .windows);
        assert(0 == (flags & ~@as(u32, SOCK.NONBLOCK | SOCK.CLOEXEC))); // Unsupported flag(s)

        const accepted_sock = while (true) {
            const rc = if (have_accept4)
                system.accept4(sock, addr, addr_size, flags)
            else if (builtin.os.tag == .windows)
                windows.accept(sock, addr, addr_size)
            else
                system.accept(sock, addr, addr_size);

            if (builtin.os.tag == .windows) {
                if (rc == windows.ws2_32.INVALID_SOCKET) {
                    switch (windows.ws2_32.WSAGetLastError()) {
                        .WSANOTINITIALISED => unreachable, // not initialized WSA

                        .WSAECONNRESET => return error.ConnectionResetByPeer,
                        .WSAEFAULT => unreachable,
                        .WSAEINVAL => return error.SocketNotListening,
                        .WSAEMFILE => return error.ProcessFdQuotaExceeded,
                        .WSAENETDOWN => return error.NetworkSubsystemFailed,
                        .WSAENOBUFS => return error.FileDescriptorNotASocket,
                        .WSAEOPNOTSUPP => return error.OperationNotSupported,
                        .WSAEWOULDBLOCK => return error.WouldBlock,
                        else => |err| return windows.unexpectedWSAError(err),
                    }
                } else {
                    break rc;
                }
            } else {
                switch (errno(rc)) {
                    .SUCCESS => {
                        break @intCast(socket_t, rc);
                    },
                    .INTR => continue,
                    .AGAIN => return error.WouldBlock,
                    .BADF => unreachable, // always a race condition

                    .CONNABORTED => return error.ConnectionAborted,
                    .FAULT => unreachable,
                    .INVAL => return error.SocketNotListening,
                    .NOTSOCK => unreachable,
                    .MFILE => return error.ProcessFdQuotaExceeded,
                    .NFILE => return error.SystemFdQuotaExceeded,
                    .NOBUFS => return error.SystemResources,
                    .NOMEM => return error.SystemResources,
                    .OPNOTSUPP => unreachable,
                    .PROTO => return error.ProtocolFailure,
                    .PERM => return error.BlockedByFirewall,
                    else => |err| return unexpectedErrno(err),
                }
            }
        } else unreachable;

        if (!have_accept4) {
            try setSockFlags(accepted_sock, flags);
        }
        return accepted_sock;
    }

    pub const EpollCreateError = error{
        /// The  per-user   limit   on   the   number   of   epoll   instances   imposed   by
        /// /proc/sys/fs/epoll/max_user_instances  was encountered.  See epoll(7) for further
        /// details.
        /// Or, The per-process limit on the number of open file descriptors has been reached.
        ProcessFdQuotaExceeded,

        /// The system-wide limit on the total number of open files has been reached.
        SystemFdQuotaExceeded,

        /// There was insufficient memory to create the kernel object.
        SystemResources,
    } || UnexpectedError;

    pub fn epoll_create1(flags: u32) EpollCreateError!i32 {
        const rc = system.epoll_create1(flags);
        switch (errno(rc)) {
            .SUCCESS => return @intCast(i32, rc),
            else => |err| return unexpectedErrno(err),

            .INVAL => unreachable,
            .MFILE => return error.ProcessFdQuotaExceeded,
            .NFILE => return error.SystemFdQuotaExceeded,
            .NOMEM => return error.SystemResources,
        }
    }

    pub const EpollCtlError = error{
        /// op was EPOLL_CTL_ADD, and the supplied file descriptor fd is  already  registered
        /// with this epoll instance.
        FileDescriptorAlreadyPresentInSet,

        /// fd refers to an epoll instance and this EPOLL_CTL_ADD operation would result in a
        /// circular loop of epoll instances monitoring one another.
        OperationCausesCircularLoop,

        /// op was EPOLL_CTL_MOD or EPOLL_CTL_DEL, and fd is not registered with  this  epoll
        /// instance.
        FileDescriptorNotRegistered,

        /// There was insufficient memory to handle the requested op control operation.
        SystemResources,

        /// The  limit  imposed  by /proc/sys/fs/epoll/max_user_watches was encountered while
        /// trying to register (EPOLL_CTL_ADD) a new file descriptor on  an  epoll  instance.
        /// See epoll(7) for further details.
        UserResourceLimitReached,

        /// The target file fd does not support epoll.  This error can occur if fd refers to,
        /// for example, a regular file or a directory.
        FileDescriptorIncompatibleWithEpoll,
    } || UnexpectedError;

    pub fn epoll_ctl(epfd: i32, op: u32, fd: i32, event: ?*linux.epoll_event) EpollCtlError!void {
        const rc = system.epoll_ctl(epfd, op, fd, event);
        switch (errno(rc)) {
            .SUCCESS => return,
            else => |err| return unexpectedErrno(err),

            .BADF => unreachable, // always a race condition if this happens

            .EXIST => return error.FileDescriptorAlreadyPresentInSet,
            .INVAL => unreachable,
            .LOOP => return error.OperationCausesCircularLoop,
            .NOENT => return error.FileDescriptorNotRegistered,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.UserResourceLimitReached,
            .PERM => return error.FileDescriptorIncompatibleWithEpoll,
        }
    }

    /// Waits for an I/O event on an epoll file descriptor.
    /// Returns the number of file descriptors ready for the requested I/O,
    /// or zero if no file descriptor became ready during the requested timeout milliseconds.
    pub fn epoll_wait(epfd: i32, events: []linux.epoll_event, timeout: i32) usize {
        while (true) {
            // TODO get rid of the @intCast

            const rc = system.epoll_wait(epfd, events.ptr, @intCast(u32, events.len), timeout);
            switch (errno(rc)) {
                .SUCCESS => return @intCast(usize, rc),
                .INTR => continue,
                .BADF => unreachable,
                .FAULT => unreachable,
                .INVAL => unreachable,
                else => unreachable,
            }
        }
    }

    pub const EventFdError = error{
        SystemResources,
        ProcessFdQuotaExceeded,
        SystemFdQuotaExceeded,
    } || UnexpectedError;

    pub fn eventfd(initval: u32, flags: u32) EventFdError!i32 {
        const rc = system.eventfd(initval, flags);
        switch (errno(rc)) {
            .SUCCESS => return @intCast(i32, rc),
            else => |err| return unexpectedErrno(err),

            .INVAL => unreachable, // invalid parameters

            .MFILE => return error.ProcessFdQuotaExceeded,
            .NFILE => return error.SystemFdQuotaExceeded,
            .NODEV => return error.SystemResources,
            .NOMEM => return error.SystemResources,
        }
    }

    pub const GetSockNameError = error{
        /// Insufficient resources were available in the system to perform the operation.
        SystemResources,

        /// The network subsystem has failed.
        NetworkSubsystemFailed,

        /// Socket hasn't been bound yet
        SocketNotBound,

        FileDescriptorNotASocket,
    } || UnexpectedError;

    pub fn getsockname(sock: socket_t, addr: *sockaddr, addrlen: *socklen_t) GetSockNameError!void {
        if (builtin.os.tag == .windows) {
            const rc = windows.getsockname(sock, addr, addrlen);
            if (rc == windows.ws2_32.SOCKET_ERROR) {
                switch (windows.ws2_32.WSAGetLastError()) {
                    .WSANOTINITIALISED => unreachable,
                    .WSAENETDOWN => return error.NetworkSubsystemFailed,
                    .WSAEFAULT => unreachable, // addr or addrlen have invalid pointers or addrlen points to an incorrect value

                    .WSAENOTSOCK => return error.FileDescriptorNotASocket,
                    .WSAEINVAL => return error.SocketNotBound,
                    else => |err| return windows.unexpectedWSAError(err),
                }
            }
            return;
        } else {
            const rc = system.getsockname(sock, addr, addrlen);
            switch (errno(rc)) {
                .SUCCESS => return,
                else => |err| return unexpectedErrno(err),

                .BADF => unreachable, // always a race condition

                .FAULT => unreachable,
                .INVAL => unreachable, // invalid parameters

                .NOTSOCK => return error.FileDescriptorNotASocket,
                .NOBUFS => return error.SystemResources,
            }
        }
    }

    pub fn getpeername(sock: socket_t, addr: *sockaddr, addrlen: *socklen_t) GetSockNameError!void {
        if (builtin.os.tag == .windows) {
            const rc = windows.getpeername(sock, addr, addrlen);
            if (rc == windows.ws2_32.SOCKET_ERROR) {
                switch (windows.ws2_32.WSAGetLastError()) {
                    .WSANOTINITIALISED => unreachable,
                    .WSAENETDOWN => return error.NetworkSubsystemFailed,
                    .WSAEFAULT => unreachable, // addr or addrlen have invalid pointers or addrlen points to an incorrect value

                    .WSAENOTSOCK => return error.FileDescriptorNotASocket,
                    .WSAEINVAL => return error.SocketNotBound,
                    else => |err| return windows.unexpectedWSAError(err),
                }
            }
            return;
        } else {
            const rc = system.getpeername(sock, addr, addrlen);
            switch (errno(rc)) {
                .SUCCESS => return,
                else => |err| return unexpectedErrno(err),

                .BADF => unreachable, // always a race condition

                .FAULT => unreachable,
                .INVAL => unreachable, // invalid parameters

                .NOTSOCK => return error.FileDescriptorNotASocket,
                .NOBUFS => return error.SystemResources,
            }
        }
    }

    pub const ConnectError = error{
        /// For UNIX domain sockets, which are identified by pathname: Write permission is denied on  the  socket
        /// file,  or  search  permission  is  denied  for  one of the directories in the path prefix.
        /// or
        /// The user tried to connect to a broadcast address without having the socket broadcast flag enabled  or
        /// the connection request failed because of a local firewall rule.
        PermissionDenied,

        /// Local address is already in use.
        AddressInUse,

        /// (Internet  domain  sockets)  The  socket  referred  to  by sockfd had not previously been bound to an
        /// address and, upon attempting to bind it to an ephemeral port, it was determined that all port numbers
        /// in    the    ephemeral    port    range    are   currently   in   use.    See   the   discussion   of
        /// /proc/sys/net/ipv4/ip_local_port_range in ip(7).
        AddressNotAvailable,

        /// The passed address didn't have the correct address family in its sa_family field.
        AddressFamilyNotSupported,

        /// Insufficient entries in the routing cache.
        SystemResources,

        /// A connect() on a stream socket found no one listening on the remote address.
        ConnectionRefused,

        /// Network is unreachable.
        NetworkUnreachable,

        /// Timeout  while  attempting  connection.   The server may be too busy to accept new connections.  Note
        /// that for IP sockets the timeout may be very long when syncookies are enabled on the server.
        ConnectionTimedOut,

        /// This error occurs when no global event loop is configured,
        /// and connecting to the socket would block.
        WouldBlock,

        /// The given path for the unix socket does not exist.
        FileNotFound,

        /// Connection was reset by peer before connect could complete.
        ConnectionResetByPeer,

        /// Socket is non-blocking and already has a pending connection in progress.
        ConnectionPending,
    } || UnexpectedError;

    /// Initiate a connection on a socket.
    /// If `sockfd` is opened in non blocking mode, the function will
    /// return error.WouldBlock when EAGAIN or EINPROGRESS is received.
    pub fn connect(sock: socket_t, sock_addr: *const sockaddr, len: socklen_t) ConnectError!void {
        if (builtin.os.tag == .windows) {
            const rc = windows.ws2_32.connect(sock, sock_addr, @intCast(i32, len));
            if (rc == 0) return;
            switch (windows.ws2_32.WSAGetLastError()) {
                .WSAEADDRINUSE => return error.AddressInUse,
                .WSAEADDRNOTAVAIL => return error.AddressNotAvailable,
                .WSAECONNREFUSED => return error.ConnectionRefused,
                .WSAECONNRESET => return error.ConnectionResetByPeer,
                .WSAETIMEDOUT => return error.ConnectionTimedOut,
                .WSAEHOSTUNREACH, // TODO: should we return NetworkUnreachable in this case as well?

                .WSAENETUNREACH,
                => return error.NetworkUnreachable,
                .WSAEFAULT => unreachable,
                .WSAEINVAL => unreachable,
                .WSAEISCONN => unreachable,
                .WSAENOTSOCK => unreachable,
                .WSAEWOULDBLOCK => unreachable,
                .WSAEACCES => unreachable,
                .WSAENOBUFS => return error.SystemResources,
                .WSAEAFNOSUPPORT => return error.AddressFamilyNotSupported,
                else => |err| return windows.unexpectedWSAError(err),
            }
            return;
        }

        while (true) {
            switch (errno(system.connect(sock, sock_addr, len))) {
                .SUCCESS => return,
                .ACCES => return error.PermissionDenied,
                .PERM => return error.PermissionDenied,
                .ADDRINUSE => return error.AddressInUse,
                .ADDRNOTAVAIL => return error.AddressNotAvailable,
                .AFNOSUPPORT => return error.AddressFamilyNotSupported,
                .AGAIN, .INPROGRESS => return error.WouldBlock,
                .ALREADY => return error.ConnectionPending,
                .BADF => unreachable, // sockfd is not a valid open file descriptor.

                .CONNREFUSED => return error.ConnectionRefused,
                .CONNRESET => return error.ConnectionResetByPeer,
                .FAULT => unreachable, // The socket structure address is outside the user's address space.

                .INTR => continue,
                .ISCONN => unreachable, // The socket is already connected.

                .HOSTUNREACH => return error.NetworkUnreachable,
                .NETUNREACH => return error.NetworkUnreachable,
                .NOTSOCK => unreachable, // The file descriptor sockfd does not refer to a socket.

                .PROTOTYPE => unreachable, // The socket type does not support the requested communications protocol.

                .TIMEDOUT => return error.ConnectionTimedOut,
                .NOENT => return error.FileNotFound, // Returned when socket is AF.UNIX and the given path does not exist.

                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub fn getsockoptError(sockfd: fd_t) ConnectError!void {
        var err_code: i32 = undefined;
        var size: u32 = @sizeOf(u32);
        const rc = system.getsockopt(sockfd, SOL.SOCKET, SO.ERROR, @ptrCast([*]u8, &err_code), &size);
        assert(size == 4);
        switch (errno(rc)) {
            .SUCCESS => switch (@intToEnum(E, err_code)) {
                .SUCCESS => return,
                .ACCES => return error.PermissionDenied,
                .PERM => return error.PermissionDenied,
                .ADDRINUSE => return error.AddressInUse,
                .ADDRNOTAVAIL => return error.AddressNotAvailable,
                .AFNOSUPPORT => return error.AddressFamilyNotSupported,
                .AGAIN => return error.SystemResources,
                .ALREADY => return error.ConnectionPending,
                .BADF => unreachable, // sockfd is not a valid open file descriptor.

                .CONNREFUSED => return error.ConnectionRefused,
                .FAULT => unreachable, // The socket structure address is outside the user's address space.

                .ISCONN => unreachable, // The socket is already connected.

                .HOSTUNREACH => return error.NetworkUnreachable,
                .NETUNREACH => return error.NetworkUnreachable,
                .NOTSOCK => unreachable, // The file descriptor sockfd does not refer to a socket.

                .PROTOTYPE => unreachable, // The socket type does not support the requested communications protocol.

                .TIMEDOUT => return error.ConnectionTimedOut,
                .CONNRESET => return error.ConnectionResetByPeer,
                else => |err| return unexpectedErrno(err),
            },
            .BADF => unreachable, // The argument sockfd is not a valid file descriptor.

            .FAULT => unreachable, // The address pointed to by optval or optlen is not in a valid part of the process address space.

            .INVAL => unreachable,
            .NOPROTOOPT => unreachable, // The option is unknown at the level indicated.

            .NOTSOCK => unreachable, // The file descriptor sockfd does not refer to a socket.

            else => |err| return unexpectedErrno(err),
        }
    }

    pub const WaitPidResult = struct {
        pid: pid_t,
        status: u32,
    };

    /// Use this version of the `waitpid` wrapper if you spawned your child process using explicit
    /// `fork` and `execve` method. If you spawned your child process using `posix_spawn` method,
    /// use `std.os.posix_spawn.waitpid` instead.
    pub fn waitpid(pid: pid_t, flags: u32) WaitPidResult {
        const Status = if (builtin.link_libc) c_int else u32;
        var status: Status = undefined;
        while (true) {
            const rc = system.waitpid(pid, &status, if (builtin.link_libc) @intCast(c_int, flags) else flags);
            switch (errno(rc)) {
                .SUCCESS => return .{
                    .pid = @intCast(pid_t, rc),
                    .status = @bitCast(u32, status),
                },
                .INTR => continue,
                .CHILD => unreachable, // The process specified does not exist. It would be a race condition to handle this error.

                .INVAL => unreachable, // Invalid flags.

                else => unreachable,
            }
        }
    }

    pub const FStatError = error{
        SystemResources,

        /// In WASI, this error may occur when the file descriptor does
        /// not hold the required rights to get its filestat information.
        AccessDenied,
    } || UnexpectedError;

    /// Return information about a file descriptor.
    pub fn fstat(fd: fd_t) FStatError!Stat {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var stat: wasi.filestat_t = undefined;
            switch (wasi.fd_filestat_get(fd, &stat)) {
                .SUCCESS => return Stat.fromFilestat(stat),
                .INVAL => unreachable,
                .BADF => unreachable, // Always a race condition.

                .NOMEM => return error.SystemResources,
                .ACCES => return error.AccessDenied,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }
        if (builtin.os.tag == .windows) {
            @compileError("fstat is not yet implemented on Windows");
        }

        const fstat_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.fstat64
        else
            system.fstat;

        var stat = mem.zeroes(Stat);
        switch (errno(fstat_sym(fd, &stat))) {
            .SUCCESS => return stat,
            .INVAL => unreachable,
            .BADF => unreachable, // Always a race condition.

            .NOMEM => return error.SystemResources,
            .ACCES => return error.AccessDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const FStatAtError = FStatError || error{ NameTooLong, FileNotFound, SymLinkLoop };

    /// Similar to `fstat`, but returns stat of a resource pointed to by `pathname`
    /// which is relative to `dirfd` handle.
    /// See also `fstatatZ` and `fstatatWasi`.
    pub fn fstatat(dirfd: fd_t, pathname: []const u8, flags: u32) FStatAtError!Stat {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            const wasi_flags = if (flags & linux.AT.SYMLINK_NOFOLLOW == 0) wasi.LOOKUP_SYMLINK_FOLLOW else 0;
            if (dirfd == wasi.AT.FDCWD or fs.path.isAbsolute(pathname)) {
                // Resolve absolute or CWD-relative paths to a path within a Preopen

                var path_buf: [MAX_PATH_BYTES]u8 = undefined;
                const path = try resolvePathWasi(pathname, &path_buf);
                return fstatatWasi(path.dir_fd, path.relative_path, wasi_flags);
            }
            return fstatatWasi(dirfd, pathname, wasi_flags);
        } else if (builtin.os.tag == .windows) {
            @compileError("fstatat is not yet implemented on Windows");
        } else {
            const pathname_c = try toPosixPath(pathname);
            return fstatatZ(dirfd, &pathname_c, flags);
        }
    }

    /// WASI-only. Same as `fstatat` but targeting WASI.
    /// See also `fstatat`.
    pub fn fstatatWasi(dirfd: fd_t, pathname: []const u8, flags: u32) FStatAtError!Stat {
        var stat: wasi.filestat_t = undefined;
        switch (wasi.path_filestat_get(dirfd, flags, pathname.ptr, pathname.len, &stat)) {
            .SUCCESS => return Stat.fromFilestat(stat),
            .INVAL => unreachable,
            .BADF => unreachable, // Always a race condition.

            .NOMEM => return error.SystemResources,
            .ACCES => return error.AccessDenied,
            .FAULT => unreachable,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.FileNotFound,
            .NOTCAPABLE => return error.AccessDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Same as `fstatat` but `pathname` is null-terminated.
    /// See also `fstatat`.
    pub fn fstatatZ(dirfd: fd_t, pathname: [*:0]const u8, flags: u32) FStatAtError!Stat {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return fstatatWasi(dirfd, mem.sliceTo(pathname), flags);
        }

        const fstatat_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.fstatat64
        else
            system.fstatat;

        var stat = mem.zeroes(Stat);
        switch (errno(fstatat_sym(dirfd, pathname, &stat, flags))) {
            .SUCCESS => return stat,
            .INVAL => unreachable,
            .BADF => unreachable, // Always a race condition.

            .NOMEM => return error.SystemResources,
            .ACCES => return error.AccessDenied,
            .PERM => return error.AccessDenied,
            .FAULT => unreachable,
            .NAMETOOLONG => return error.NameTooLong,
            .LOOP => return error.SymLinkLoop,
            .NOENT => return error.FileNotFound,
            .NOTDIR => return error.FileNotFound,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const KQueueError = error{
        /// The per-process limit on the number of open file descriptors has been reached.
        ProcessFdQuotaExceeded,

        /// The system-wide limit on the total number of open files has been reached.
        SystemFdQuotaExceeded,
    } || UnexpectedError;

    pub fn kqueue() KQueueError!i32 {
        const rc = system.kqueue();
        switch (errno(rc)) {
            .SUCCESS => return @intCast(i32, rc),
            .MFILE => return error.ProcessFdQuotaExceeded,
            .NFILE => return error.SystemFdQuotaExceeded,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const KEventError = error{
        /// The process does not have permission to register a filter.
        AccessDenied,

        /// The event could not be found to be modified or deleted.
        EventNotFound,

        /// No memory was available to register the event.
        SystemResources,

        /// The specified process to attach to does not exist.
        ProcessNotFound,

        /// changelist or eventlist had too many items on it.
        /// TODO remove this possibility
        Overflow,
    };

    pub fn kevent(
        kq: i32,
        changelist: []const Kevent,
        eventlist: []Kevent,
        timeout: ?*const timespec,
    ) KEventError!usize {
        while (true) {
            const rc = system.kevent(
                kq,
                changelist.ptr,
                math.cast(c_int, changelist.len) orelse return error.Overflow,
                eventlist.ptr,
                math.cast(c_int, eventlist.len) orelse return error.Overflow,
                timeout,
            );
            switch (errno(rc)) {
                .SUCCESS => return @intCast(usize, rc),
                .ACCES => return error.AccessDenied,
                .FAULT => unreachable,
                .BADF => unreachable, // Always a race condition.

                .INTR => continue,
                .INVAL => unreachable,
                .NOENT => return error.EventNotFound,
                .NOMEM => return error.SystemResources,
                .SRCH => return error.ProcessNotFound,
                else => unreachable,
            }
        }
    }

    pub const INotifyInitError = error{
        ProcessFdQuotaExceeded,
        SystemFdQuotaExceeded,
        SystemResources,
    } || UnexpectedError;

    /// initialize an inotify instance
    pub fn inotify_init1(flags: u32) INotifyInitError!i32 {
        const rc = system.inotify_init1(flags);
        switch (errno(rc)) {
            .SUCCESS => return @intCast(i32, rc),
            .INVAL => unreachable,
            .MFILE => return error.ProcessFdQuotaExceeded,
            .NFILE => return error.SystemFdQuotaExceeded,
            .NOMEM => return error.SystemResources,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const INotifyAddWatchError = error{
        AccessDenied,
        NameTooLong,
        FileNotFound,
        SystemResources,
        UserResourceLimitReached,
        NotDir,
        WatchAlreadyExists,
    } || UnexpectedError;

    /// add a watch to an initialized inotify instance
    pub fn inotify_add_watch(inotify_fd: i32, pathname: []const u8, mask: u32) INotifyAddWatchError!i32 {
        const pathname_c = try toPosixPath(pathname);
        return inotify_add_watchZ(inotify_fd, &pathname_c, mask);
    }

    /// Same as `inotify_add_watch` except pathname is null-terminated.
    pub fn inotify_add_watchZ(inotify_fd: i32, pathname: [*:0]const u8, mask: u32) INotifyAddWatchError!i32 {
        const rc = system.inotify_add_watch(inotify_fd, pathname, mask);
        switch (errno(rc)) {
            .SUCCESS => return @intCast(i32, rc),
            .ACCES => return error.AccessDenied,
            .BADF => unreachable,
            .FAULT => unreachable,
            .INVAL => unreachable,
            .NAMETOOLONG => return error.NameTooLong,
            .NOENT => return error.FileNotFound,
            .NOMEM => return error.SystemResources,
            .NOSPC => return error.UserResourceLimitReached,
            .NOTDIR => return error.NotDir,
            .EXIST => return error.WatchAlreadyExists,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// remove an existing watch from an inotify instance
    pub fn inotify_rm_watch(inotify_fd: i32, wd: i32) void {
        switch (errno(system.inotify_rm_watch(inotify_fd, wd))) {
            .SUCCESS => return,
            .BADF => unreachable,
            .INVAL => unreachable,
            else => unreachable,
        }
    }

    pub const MProtectError = error{
        /// The memory cannot be given the specified access.  This can happen, for example, if you
        /// mmap(2)  a  file  to  which  you have read-only access, then ask mprotect() to mark it
        /// PROT_WRITE.
        AccessDenied,

        /// Changing  the  protection  of a memory region would result in the total number of map‐
        /// pings with distinct attributes (e.g., read versus read/write protection) exceeding the
        /// allowed maximum.  (For example, making the protection of a range PROT_READ in the mid‐
        /// dle of a region currently protected as PROT_READ|PROT_WRITE would result in three map‐
        /// pings: two read/write mappings at each end and a read-only mapping in the middle.)
        OutOfMemory,
    } || UnexpectedError;

    /// `memory.len` must be page-aligned.
    pub fn mprotect(memory: []align(mem.page_size) u8, protection: u32) MProtectError!void {
        assert(mem.isAligned(memory.len, mem.page_size));
        switch (errno(system.mprotect(memory.ptr, memory.len, protection))) {
            .SUCCESS => return,
            .INVAL => unreachable,
            .ACCES => return error.AccessDenied,
            .NOMEM => return error.OutOfMemory,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const ForkError = error{SystemResources} || UnexpectedError;

    pub fn fork() ForkError!pid_t {
        const rc = system.fork();
        switch (errno(rc)) {
            .SUCCESS => return @intCast(pid_t, rc),
            .AGAIN => return error.SystemResources,
            .NOMEM => return error.SystemResources,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const MMapError = error{
        /// The underlying filesystem of the specified file does not support memory mapping.
        MemoryMappingNotSupported,

        /// A file descriptor refers to a non-regular file. Or a file mapping was requested,
        /// but the file descriptor is not open for reading. Or `MAP.SHARED` was requested
        /// and `PROT_WRITE` is set, but the file descriptor is not open in `O.RDWR` mode.
        /// Or `PROT_WRITE` is set, but the file is append-only.
        AccessDenied,

        /// The `prot` argument asks for `PROT_EXEC` but the mapped area belongs to a file on
        /// a filesystem that was mounted no-exec.
        PermissionDenied,
        LockedMemoryLimitExceeded,
        OutOfMemory,
    } || UnexpectedError;

    /// Map files or devices into memory.
    /// `length` does not need to be aligned.
    /// Use of a mapped region can result in these signals:
    /// * SIGSEGV - Attempted write into a region mapped as read-only.
    /// * SIGBUS - Attempted  access to a portion of the buffer that does not correspond to the file
    pub fn mmap(
        ptr: ?[*]align(mem.page_size) u8,
        length: usize,
        prot: u32,
        flags: u32,
        fd: fd_t,
        offset: u64,
    ) MMapError![]align(mem.page_size) u8 {
        const mmap_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.mmap64
        else
            system.mmap;

        const ioffset = @bitCast(i64, offset); // the OS treats this as unsigned

        const rc = mmap_sym(ptr, length, prot, flags, fd, ioffset);
        const err = if (builtin.link_libc) blk: {
            if (rc != std.c.MAP.FAILED) return @ptrCast([*]align(mem.page_size) u8, @alignCast(mem.page_size, rc))[0..length];
            break :blk @intToEnum(E, system._errno().*);
        } else blk: {
            const err = errno(rc);
            if (err == .SUCCESS) return @intToPtr([*]align(mem.page_size) u8, rc)[0..length];
            break :blk err;
        };
        switch (err) {
            .SUCCESS => unreachable,
            .TXTBSY => return error.AccessDenied,
            .ACCES => return error.AccessDenied,
            .PERM => return error.PermissionDenied,
            .AGAIN => return error.LockedMemoryLimitExceeded,
            .BADF => unreachable, // Always a race condition.

            .OVERFLOW => unreachable, // The number of pages used for length + offset would overflow.

            .NODEV => return error.MemoryMappingNotSupported,
            .INVAL => unreachable, // Invalid parameters to mmap()

            .NOMEM => return error.OutOfMemory,
            else => return unexpectedErrno(err),
        }
    }

    /// Deletes the mappings for the specified address range, causing
    /// further references to addresses within the range to generate invalid memory references.
    /// Note that while POSIX allows unmapping a region in the middle of an existing mapping,
    /// Zig's munmap function does not, for two reasons:
    /// * It violates the Zig principle that resource deallocation must succeed.
    /// * The Windows function, VirtualFree, has this restriction.
    pub fn munmap(memory: []align(mem.page_size) const u8) void {
        switch (errno(system.munmap(memory.ptr, memory.len))) {
            .SUCCESS => return,
            .INVAL => unreachable, // Invalid parameters.

            .NOMEM => unreachable, // Attempted to unmap a region in the middle of an existing mapping.

            else => unreachable,
        }
    }

    pub const MSyncError = error{
        UnmappedMemory,
    } || UnexpectedError;

    pub fn msync(memory: []align(mem.page_size) u8, flags: i32) MSyncError!void {
        switch (errno(system.msync(memory.ptr, memory.len, flags))) {
            .SUCCESS => return,
            .NOMEM => return error.UnmappedMemory, // Unsuccessful, provided pointer does not point mapped memory

            .INVAL => unreachable, // Invalid parameters.

            else => unreachable,
        }
    }

    pub const AccessError = error{
        PermissionDenied,
        FileNotFound,
        NameTooLong,
        InputOutput,
        SystemResources,
        BadPathName,
        FileBusy,
        SymLinkLoop,
        ReadOnlyFileSystem,

        /// On Windows, file paths must be valid Unicode.
        InvalidUtf8,
    } || UnexpectedError;

    /// check user's permissions for a file
    /// TODO currently this assumes `mode` is `F.OK` on Windows.
    pub fn access(path: []const u8, mode: u32) AccessError!void {
        if (builtin.os.tag == .windows) {
            const path_w = try windows.sliceToPrefixedFileW(path);
            _ = try windows.GetFileAttributesW(path_w.span().ptr);
            return;
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return faccessat(wasi.AT.FDCWD, path, mode, 0);
        }
        const path_c = try toPosixPath(path);
        return accessZ(&path_c, mode);
    }

    /// Same as `access` except `path` is null-terminated.
    pub fn accessZ(path: [*:0]const u8, mode: u32) AccessError!void {
        if (builtin.os.tag == .windows) {
            const path_w = try windows.cStrToPrefixedFileW(path);
            _ = try windows.GetFileAttributesW(path_w.span().ptr);
            return;
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return access(mem.sliceTo(path, 0), mode);
        }
        switch (errno(system.access(path, mode))) {
            .SUCCESS => return,
            .ACCES => return error.PermissionDenied,
            .ROFS => return error.ReadOnlyFileSystem,
            .LOOP => return error.SymLinkLoop,
            .TXTBSY => return error.FileBusy,
            .NOTDIR => return error.FileNotFound,
            .NOENT => return error.FileNotFound,
            .NAMETOOLONG => return error.NameTooLong,
            .INVAL => unreachable,
            .FAULT => unreachable,
            .IO => return error.InputOutput,
            .NOMEM => return error.SystemResources,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Call from Windows-specific code if you already have a UTF-16LE encoded, null terminated string.
    /// Otherwise use `access` or `accessC`.
    /// TODO currently this ignores `mode`.
    pub fn accessW(path: [*:0]const u16, mode: u32) windows.GetFileAttributesError!void {
        _ = mode;
        const ret = try windows.GetFileAttributesW(path);
        if (ret != windows.INVALID_FILE_ATTRIBUTES) {
            return;
        }
        switch (windows.kernel32.GetLastError()) {
            .FILE_NOT_FOUND => return error.FileNotFound,
            .PATH_NOT_FOUND => return error.FileNotFound,
            .ACCESS_DENIED => return error.PermissionDenied,
            else => |err| return windows.unexpectedError(err),
        }
    }

    /// Check user's permissions for a file, based on an open directory handle.
    /// TODO currently this ignores `mode` and `flags` on Windows.
    pub fn faccessat(dirfd: fd_t, path: []const u8, mode: u32, flags: u32) AccessError!void {
        if (builtin.os.tag == .windows) {
            const path_w = try windows.sliceToPrefixedFileW(path);
            return faccessatW(dirfd, path_w.span().ptr, mode, flags);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var resolved = RelativePathWasi{ .dir_fd = dirfd, .relative_path = path };

            const file = blk: {
                if (dirfd == wasi.AT.FDCWD or fs.path.isAbsolute(path)) {
                    // Resolve absolute or CWD-relative paths to a path within a Preopen

                    var path_buf: [MAX_PATH_BYTES]u8 = undefined;
                    resolved = resolvePathWasi(path, &path_buf) catch |err| break :blk @as(FStatAtError!Stat, err);
                    break :blk fstatat(resolved.dir_fd, resolved.relative_path, flags);
                }
                break :blk fstatat(dirfd, path, flags);
            } catch |err| switch (err) {
                error.AccessDenied => return error.PermissionDenied,
                else => |e| return e,
            };

            if (mode != F_OK) {
                var directory: wasi.fdstat_t = undefined;
                if (wasi.fd_fdstat_get(resolved.dir_fd, &directory) != .SUCCESS) {
                    return error.PermissionDenied;
                }

                var rights: wasi.rights_t = 0;
                if (mode & R_OK != 0) {
                    rights |= if (file.filetype == .DIRECTORY)
                        wasi.RIGHT.FD_READDIR
                    else
                        wasi.RIGHT.FD_READ;
                }
                if (mode & W_OK != 0) {
                    rights |= wasi.RIGHT.FD_WRITE;
                }
                // No validation for X_OK

                if ((rights & directory.fs_rights_inheriting) != rights) {
                    return error.PermissionDenied;
                }
            }
            return;
        }
        const path_c = try toPosixPath(path);
        return faccessatZ(dirfd, &path_c, mode, flags);
    }

    /// Same as `faccessat` except the path parameter is null-terminated.
    pub fn faccessatZ(dirfd: fd_t, path: [*:0]const u8, mode: u32, flags: u32) AccessError!void {
        if (builtin.os.tag == .windows) {
            const path_w = try windows.cStrToPrefixedFileW(path);
            return faccessatW(dirfd, path_w.span().ptr, mode, flags);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return faccessat(dirfd, mem.sliceTo(path, 0), mode, flags);
        }
        switch (errno(system.faccessat(dirfd, path, mode, flags))) {
            .SUCCESS => return,
            .ACCES => return error.PermissionDenied,
            .ROFS => return error.ReadOnlyFileSystem,
            .LOOP => return error.SymLinkLoop,
            .TXTBSY => return error.FileBusy,
            .NOTDIR => return error.FileNotFound,
            .NOENT => return error.FileNotFound,
            .NAMETOOLONG => return error.NameTooLong,
            .INVAL => unreachable,
            .FAULT => unreachable,
            .IO => return error.InputOutput,
            .NOMEM => return error.SystemResources,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Same as `faccessat` except asserts the target is Windows and the path parameter
    /// is NtDll-prefixed, null-terminated, WTF-16 encoded.
    /// TODO currently this ignores `mode` and `flags`
    pub fn faccessatW(dirfd: fd_t, sub_path_w: [*:0]const u16, mode: u32, flags: u32) AccessError!void {
        _ = mode;
        _ = flags;
        if (sub_path_w[0] == '.' and sub_path_w[1] == 0) {
            return;
        }
        if (sub_path_w[0] == '.' and sub_path_w[1] == '.' and sub_path_w[2] == 0) {
            return;
        }

        const path_len_bytes = math.cast(u16, mem.sliceTo(sub_path_w, 0).len * 2) orelse return error.NameTooLong;
        var nt_name = windows.UNICODE_STRING{
            .Length = path_len_bytes,
            .MaximumLength = path_len_bytes,
            .Buffer = @intToPtr([*]u16, @ptrToInt(sub_path_w)),
        };
        var attr = windows.OBJECT_ATTRIBUTES{
            .Length = @sizeOf(windows.OBJECT_ATTRIBUTES),
            .RootDirectory = if (std.fs.path.isAbsoluteWindowsW(sub_path_w)) null else dirfd,
            .Attributes = 0, // Note we do not use OBJ_CASE_INSENSITIVE here.

            .ObjectName = &nt_name,
            .SecurityDescriptor = null,
            .SecurityQualityOfService = null,
        };
        var basic_info: windows.FILE_BASIC_INFORMATION = undefined;
        switch (windows.ntdll.NtQueryAttributesFile(&attr, &basic_info)) {
            .SUCCESS => return,
            .OBJECT_NAME_NOT_FOUND => return error.FileNotFound,
            .OBJECT_PATH_NOT_FOUND => return error.FileNotFound,
            .OBJECT_NAME_INVALID => unreachable,
            .INVALID_PARAMETER => unreachable,
            .ACCESS_DENIED => return error.PermissionDenied,
            .OBJECT_PATH_SYNTAX_BAD => unreachable,
            else => |rc| return windows.unexpectedStatus(rc),
        }
    }

    pub const PipeError = error{
        SystemFdQuotaExceeded,
        ProcessFdQuotaExceeded,
    } || UnexpectedError;

    /// Creates a unidirectional data channel that can be used for interprocess communication.
    pub fn pipe() PipeError![2]fd_t {
        var fds: [2]fd_t = undefined;
        switch (errno(system.pipe(&fds))) {
            .SUCCESS => return fds,
            .INVAL => unreachable, // Invalid parameters to pipe()

            .FAULT => unreachable, // Invalid fds pointer

            .NFILE => return error.SystemFdQuotaExceeded,
            .MFILE => return error.ProcessFdQuotaExceeded,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn pipe2(flags: u32) PipeError![2]fd_t {
        if (@hasDecl(system, "pipe2")) {
            var fds: [2]fd_t = undefined;
            switch (errno(system.pipe2(&fds, flags))) {
                .SUCCESS => return fds,
                .INVAL => unreachable, // Invalid flags

                .FAULT => unreachable, // Invalid fds pointer

                .NFILE => return error.SystemFdQuotaExceeded,
                .MFILE => return error.ProcessFdQuotaExceeded,
                else => |err| return unexpectedErrno(err),
            }
        }

        var fds: [2]fd_t = try pipe();
        errdefer {
            close(fds[0]);
            close(fds[1]);
        }

        if (flags == 0)
            return fds;

        // O.CLOEXEC is special, it's a file descriptor flag and must be set using

        // F.SETFD.

        if (flags & O.CLOEXEC != 0) {
            for (fds) |fd| {
                switch (errno(system.fcntl(fd, F.SETFD, @as(u32, FD_CLOEXEC)))) {
                    .SUCCESS => {},
                    .INVAL => unreachable, // Invalid flags

                    .BADF => unreachable, // Always a race condition

                    else => |err| return unexpectedErrno(err),
                }
            }
        }

        const new_flags = flags & ~@as(u32, O.CLOEXEC);
        // Set every other flag affecting the file status using F.SETFL.

        if (new_flags != 0) {
            for (fds) |fd| {
                switch (errno(system.fcntl(fd, F.SETFL, new_flags))) {
                    .SUCCESS => {},
                    .INVAL => unreachable, // Invalid flags

                    .BADF => unreachable, // Always a race condition

                    else => |err| return unexpectedErrno(err),
                }
            }
        }

        return fds;
    }

    pub const SysCtlError = error{
        PermissionDenied,
        SystemResources,
        NameTooLong,
        UnknownName,
    } || UnexpectedError;

    pub fn sysctl(
        name: []const c_int,
        oldp: ?*anyopaque,
        oldlenp: ?*usize,
        newp: ?*anyopaque,
        newlen: usize,
    ) SysCtlError!void {
        if (builtin.os.tag == .wasi) {
            @panic("unsupported"); // TODO should be compile error, not panic

        }
        if (builtin.os.tag == .haiku) {
            @panic("unsupported"); // TODO should be compile error, not panic

        }

        const name_len = math.cast(c_uint, name.len) orelse return error.NameTooLong;
        switch (errno(system.sysctl(name.ptr, name_len, oldp, oldlenp, newp, newlen))) {
            .SUCCESS => return,
            .FAULT => unreachable,
            .PERM => return error.PermissionDenied,
            .NOMEM => return error.SystemResources,
            .NOENT => return error.UnknownName,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn sysctlbynameZ(
        name: [*:0]const u8,
        oldp: ?*anyopaque,
        oldlenp: ?*usize,
        newp: ?*anyopaque,
        newlen: usize,
    ) SysCtlError!void {
        if (builtin.os.tag == .wasi) {
            @panic("unsupported"); // TODO should be compile error, not panic

        }
        if (builtin.os.tag == .haiku) {
            @panic("unsupported"); // TODO should be compile error, not panic

        }

        switch (errno(system.sysctlbyname(name, oldp, oldlenp, newp, newlen))) {
            .SUCCESS => return,
            .FAULT => unreachable,
            .PERM => return error.PermissionDenied,
            .NOMEM => return error.SystemResources,
            .NOENT => return error.UnknownName,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn gettimeofday(tv: ?*timeval, tz: ?*timezone) void {
        switch (errno(system.gettimeofday(tv, tz))) {
            .SUCCESS => return,
            .INVAL => unreachable,
            else => unreachable,
        }
    }

    pub const SeekError = error{
        Unseekable,

        /// In WASI, this error may occur when the file descriptor does
        /// not hold the required rights to seek on it.
        AccessDenied,
    } || UnexpectedError;

    /// Repositions read/write file offset relative to the beginning.
    pub fn lseek_SET(fd: fd_t, offset: u64) SeekError!void {
        if (builtin.os.tag == .linux and !builtin.link_libc and @sizeOf(usize) == 4) {
            var result: u64 = undefined;
            switch (errno(system.llseek(fd, offset, &result, SEEK.SET))) {
                .SUCCESS => return,
                .BADF => unreachable, // always a race condition

                .INVAL => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .NXIO => return error.Unseekable,
                else => |err| return unexpectedErrno(err),
            }
        }
        if (builtin.os.tag == .windows) {
            return windows.SetFilePointerEx_BEGIN(fd, offset);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var new_offset: wasi.filesize_t = undefined;
            switch (wasi.fd_seek(fd, @bitCast(wasi.filedelta_t, offset), .SET, &new_offset)) {
                .SUCCESS => return,
                .BADF => unreachable, // always a race condition

                .INVAL => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .NXIO => return error.Unseekable,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }

        const lseek_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.lseek64
        else
            system.lseek;

        const ioffset = @bitCast(i64, offset); // the OS treats this as unsigned

        switch (errno(lseek_sym(fd, ioffset, SEEK.SET))) {
            .SUCCESS => return,
            .BADF => unreachable, // always a race condition

            .INVAL => return error.Unseekable,
            .OVERFLOW => return error.Unseekable,
            .SPIPE => return error.Unseekable,
            .NXIO => return error.Unseekable,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Repositions read/write file offset relative to the current offset.
    pub fn lseek_CUR(fd: fd_t, offset: i64) SeekError!void {
        if (builtin.os.tag == .linux and !builtin.link_libc and @sizeOf(usize) == 4) {
            var result: u64 = undefined;
            switch (errno(system.llseek(fd, @bitCast(u64, offset), &result, SEEK.CUR))) {
                .SUCCESS => return,
                .BADF => unreachable, // always a race condition

                .INVAL => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .NXIO => return error.Unseekable,
                else => |err| return unexpectedErrno(err),
            }
        }
        if (builtin.os.tag == .windows) {
            return windows.SetFilePointerEx_CURRENT(fd, offset);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var new_offset: wasi.filesize_t = undefined;
            switch (wasi.fd_seek(fd, offset, .CUR, &new_offset)) {
                .SUCCESS => return,
                .BADF => unreachable, // always a race condition

                .INVAL => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .NXIO => return error.Unseekable,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }
        const lseek_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.lseek64
        else
            system.lseek;

        const ioffset = @bitCast(i64, offset); // the OS treats this as unsigned

        switch (errno(lseek_sym(fd, ioffset, SEEK.CUR))) {
            .SUCCESS => return,
            .BADF => unreachable, // always a race condition

            .INVAL => return error.Unseekable,
            .OVERFLOW => return error.Unseekable,
            .SPIPE => return error.Unseekable,
            .NXIO => return error.Unseekable,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Repositions read/write file offset relative to the end.
    pub fn lseek_END(fd: fd_t, offset: i64) SeekError!void {
        if (builtin.os.tag == .linux and !builtin.link_libc and @sizeOf(usize) == 4) {
            var result: u64 = undefined;
            switch (errno(system.llseek(fd, @bitCast(u64, offset), &result, SEEK.END))) {
                .SUCCESS => return,
                .BADF => unreachable, // always a race condition

                .INVAL => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .NXIO => return error.Unseekable,
                else => |err| return unexpectedErrno(err),
            }
        }
        if (builtin.os.tag == .windows) {
            return windows.SetFilePointerEx_END(fd, offset);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var new_offset: wasi.filesize_t = undefined;
            switch (wasi.fd_seek(fd, offset, .END, &new_offset)) {
                .SUCCESS => return,
                .BADF => unreachable, // always a race condition

                .INVAL => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .NXIO => return error.Unseekable,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }
        const lseek_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.lseek64
        else
            system.lseek;

        const ioffset = @bitCast(i64, offset); // the OS treats this as unsigned

        switch (errno(lseek_sym(fd, ioffset, SEEK.END))) {
            .SUCCESS => return,
            .BADF => unreachable, // always a race condition

            .INVAL => return error.Unseekable,
            .OVERFLOW => return error.Unseekable,
            .SPIPE => return error.Unseekable,
            .NXIO => return error.Unseekable,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Returns the read/write file offset relative to the beginning.
    pub fn lseek_CUR_get(fd: fd_t) SeekError!u64 {
        if (builtin.os.tag == .linux and !builtin.link_libc and @sizeOf(usize) == 4) {
            var result: u64 = undefined;
            switch (errno(system.llseek(fd, 0, &result, SEEK.CUR))) {
                .SUCCESS => return result,
                .BADF => unreachable, // always a race condition

                .INVAL => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .NXIO => return error.Unseekable,
                else => |err| return unexpectedErrno(err),
            }
        }
        if (builtin.os.tag == .windows) {
            return windows.SetFilePointerEx_CURRENT_get(fd);
        }
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var new_offset: wasi.filesize_t = undefined;
            switch (wasi.fd_seek(fd, 0, .CUR, &new_offset)) {
                .SUCCESS => return new_offset,
                .BADF => unreachable, // always a race condition

                .INVAL => return error.Unseekable,
                .OVERFLOW => return error.Unseekable,
                .SPIPE => return error.Unseekable,
                .NXIO => return error.Unseekable,
                .NOTCAPABLE => return error.AccessDenied,
                else => |err| return unexpectedErrno(err),
            }
        }
        const lseek_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.lseek64
        else
            system.lseek;

        const rc = lseek_sym(fd, 0, SEEK.CUR);
        switch (errno(rc)) {
            .SUCCESS => return @bitCast(u64, rc),
            .BADF => unreachable, // always a race condition

            .INVAL => return error.Unseekable,
            .OVERFLOW => return error.Unseekable,
            .SPIPE => return error.Unseekable,
            .NXIO => return error.Unseekable,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const FcntlError = error{
        PermissionDenied,
        FileBusy,
        ProcessFdQuotaExceeded,
        Locked,
        DeadLock,
        LockedRegionLimitExceeded,
    } || UnexpectedError;

    pub fn fcntl(fd: fd_t, cmd: i32, arg: usize) FcntlError!usize {
        while (true) {
            const rc = system.fcntl(fd, cmd, arg);
            switch (errno(rc)) {
                .SUCCESS => return @intCast(usize, rc),
                .INTR => continue,
                .AGAIN, .ACCES => return error.Locked,
                .BADF => unreachable,
                .BUSY => return error.FileBusy,
                .INVAL => unreachable, // invalid parameters

                .PERM => return error.PermissionDenied,
                .MFILE => return error.ProcessFdQuotaExceeded,
                .NOTDIR => unreachable, // invalid parameter

                .DEADLK => return error.DeadLock,
                .NOLCK => return error.LockedRegionLimitExceeded,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    fn setSockFlags(sock: socket_t, flags: u32) !void {
        if ((flags & SOCK.CLOEXEC) != 0) {
            if (builtin.os.tag == .windows) {
                // TODO: Find out if this is supported for sockets

            } else {
                var fd_flags = fcntl(sock, F.GETFD, 0) catch |err| switch (err) {
                    error.FileBusy => unreachable,
                    error.Locked => unreachable,
                    error.PermissionDenied => unreachable,
                    error.DeadLock => unreachable,
                    error.LockedRegionLimitExceeded => unreachable,
                    else => |e| return e,
                };
                fd_flags |= FD_CLOEXEC;
                _ = fcntl(sock, F.SETFD, fd_flags) catch |err| switch (err) {
                    error.FileBusy => unreachable,
                    error.Locked => unreachable,
                    error.PermissionDenied => unreachable,
                    error.DeadLock => unreachable,
                    error.LockedRegionLimitExceeded => unreachable,
                    else => |e| return e,
                };
            }
        }
        if ((flags & SOCK.NONBLOCK) != 0) {
            if (builtin.os.tag == .windows) {
                var mode: c_ulong = 1;
                if (windows.ws2_32.ioctlsocket(sock, windows.ws2_32.FIONBIO, &mode) == windows.ws2_32.SOCKET_ERROR) {
                    switch (windows.ws2_32.WSAGetLastError()) {
                        .WSANOTINITIALISED => unreachable,
                        .WSAENETDOWN => return error.NetworkSubsystemFailed,
                        .WSAENOTSOCK => return error.FileDescriptorNotASocket,
                        // TODO: handle more errors

                        else => |err| return windows.unexpectedWSAError(err),
                    }
                }
            } else {
                var fl_flags = fcntl(sock, F.GETFL, 0) catch |err| switch (err) {
                    error.FileBusy => unreachable,
                    error.Locked => unreachable,
                    error.PermissionDenied => unreachable,
                    error.DeadLock => unreachable,
                    error.LockedRegionLimitExceeded => unreachable,
                    else => |e| return e,
                };
                fl_flags |= O.NONBLOCK;
                _ = fcntl(sock, F.SETFL, fl_flags) catch |err| switch (err) {
                    error.FileBusy => unreachable,
                    error.Locked => unreachable,
                    error.PermissionDenied => unreachable,
                    error.DeadLock => unreachable,
                    error.LockedRegionLimitExceeded => unreachable,
                    else => |e| return e,
                };
            }
        }
    }

    pub const FlockError = error{
        WouldBlock,

        /// The kernel ran out of memory for allocating file locks
        SystemResources,

        /// The underlying filesystem does not support file locks
        FileLocksNotSupported,
    } || UnexpectedError;

    /// Depending on the operating system `flock` may or may not interact with
    /// `fcntl` locks made by other processes.
    pub fn flock(fd: fd_t, operation: i32) FlockError!void {
        while (true) {
            const rc = system.flock(fd, operation);
            switch (errno(rc)) {
                .SUCCESS => return,
                .BADF => unreachable,
                .INTR => continue,
                .INVAL => unreachable, // invalid parameters

                .NOLCK => return error.SystemResources,
                .AGAIN => return error.WouldBlock, // TODO: integrate with async instead of just returning an error

                .OPNOTSUPP => return error.FileLocksNotSupported,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const RealPathError = error{
        FileNotFound,
        AccessDenied,
        NameTooLong,
        NotSupported,
        NotDir,
        SymLinkLoop,
        InputOutput,
        FileTooBig,
        IsDir,
        ProcessFdQuotaExceeded,
        SystemFdQuotaExceeded,
        NoDevice,
        SystemResources,
        NoSpaceLeft,
        FileSystem,
        BadPathName,
        DeviceBusy,

        SharingViolation,
        PipeBusy,

        /// On WASI, the current CWD may not be associated with an absolute path.
        InvalidHandle,

        /// On Windows, file paths must be valid Unicode.
        InvalidUtf8,

        PathAlreadyExists,
    } || UnexpectedError;

    /// Return the canonicalized absolute pathname.
    /// Expands all symbolic links and resolves references to `.`, `..`, and
    /// extra `/` characters in `pathname`.
    /// The return value is a slice of `out_buffer`, but not necessarily from the beginning.
    /// See also `realpathZ` and `realpathW`.
    pub fn realpath(pathname: []const u8, out_buffer: *[MAX_PATH_BYTES]u8) RealPathError![]u8 {
        if (builtin.os.tag == .windows) {
            const pathname_w = try windows.sliceToPrefixedFileW(pathname);
            return realpathW(pathname_w.span(), out_buffer);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var alloc = std.heap.FixedBufferAllocator.init(out_buffer);

            // NOTE: This emulation is incomplete. Symbolic links are not

            //       currently expanded during path canonicalization.

            const paths = &.{ wasi_cwd.cwd, pathname };
            return fs.path.resolve(alloc.allocator(), paths) catch error.NameTooLong;
        }
        const pathname_c = try toPosixPath(pathname);
        return realpathZ(&pathname_c, out_buffer);
    }

    /// Same as `realpath` except `pathname` is null-terminated.
    pub fn realpathZ(pathname: [*:0]const u8, out_buffer: *[MAX_PATH_BYTES]u8) RealPathError![]u8 {
        if (builtin.os.tag == .windows) {
            const pathname_w = try windows.cStrToPrefixedFileW(pathname);
            return realpathW(pathname_w.span(), out_buffer);
        } else if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return realpath(mem.sliceTo(pathname, 0), out_buffer);
        }
        if (!builtin.link_libc) {
            const flags = if (builtin.os.tag == .linux) O.PATH | O.NONBLOCK | O.CLOEXEC else O.NONBLOCK | O.CLOEXEC;
            const fd = openZ(pathname, flags, 0) catch |err| switch (err) {
                error.FileLocksNotSupported => unreachable,
                error.WouldBlock => unreachable,
                error.FileBusy => unreachable, // not asking for write permissions

                error.InvalidHandle => unreachable, // WASI-only

                else => |e| return e,
            };
            defer close(fd);

            return getFdPath(fd, out_buffer);
        }
        const result_path = std.c.realpath(pathname, out_buffer) orelse switch (@intToEnum(E, std.c._errno().*)) {
            .SUCCESS => unreachable,
            .INVAL => unreachable,
            .BADF => unreachable,
            .FAULT => unreachable,
            .ACCES => return error.AccessDenied,
            .NOENT => return error.FileNotFound,
            .OPNOTSUPP => return error.NotSupported,
            .NOTDIR => return error.NotDir,
            .NAMETOOLONG => return error.NameTooLong,
            .LOOP => return error.SymLinkLoop,
            .IO => return error.InputOutput,
            else => |err| return unexpectedErrno(err),
        };
        return mem.sliceTo(result_path, 0);
    }

    /// Same as `realpath` except `pathname` is UTF16LE-encoded.
    pub fn realpathW(pathname: []const u16, out_buffer: *[MAX_PATH_BYTES]u8) RealPathError![]u8 {
        const w = windows;

        const dir = std.fs.cwd().fd;
        const access_mask = w.GENERIC_READ | w.SYNCHRONIZE;
        const share_access = w.FILE_SHARE_READ;
        const creation = w.FILE_OPEN;
        const h_file = blk: {
            const res = w.OpenFile(pathname, .{
                .dir = dir,
                .access_mask = access_mask,
                .share_access = share_access,
                .creation = creation,
                .io_mode = .blocking,
            }) catch |err| switch (err) {
                error.IsDir => break :blk w.OpenFile(pathname, .{
                    .dir = dir,
                    .access_mask = access_mask,
                    .share_access = share_access,
                    .creation = creation,
                    .io_mode = .blocking,
                    .filter = .dir_only,
                }) catch |er| switch (er) {
                    error.WouldBlock => unreachable,
                    else => |e2| return e2,
                },
                error.WouldBlock => unreachable,
                else => |e| return e,
            };
            break :blk res;
        };
        defer w.CloseHandle(h_file);

        return getFdPath(h_file, out_buffer);
    }

    /// Return canonical path of handle `fd`.
    /// This function is very host-specific and is not universally supported by all hosts.
    /// For example, while it generally works on Linux, macOS, FreeBSD or Windows, it is
    /// unsupported on WASI.
    pub fn getFdPath(fd: fd_t, out_buffer: *[MAX_PATH_BYTES]u8) RealPathError![]u8 {
        switch (builtin.os.tag) {
            .windows => {
                var wide_buf: [windows.PATH_MAX_WIDE]u16 = undefined;
                const wide_slice = try windows.GetFinalPathNameByHandle(fd, .{}, wide_buf[0..]);

                // Trust that Windows gives us valid UTF-16LE.

                const end_index = std.unicode.utf16leToUtf8(out_buffer, wide_slice) catch unreachable;
                return out_buffer[0..end_index];
            },
            .macos, .ios, .watchos, .tvos => {
                // On macOS, we can use F.GETPATH fcntl command to query the OS for

                // the path to the file descriptor.

                @memset(out_buffer, 0, MAX_PATH_BYTES);
                switch (errno(system.fcntl(fd, F.GETPATH, out_buffer))) {
                    .SUCCESS => {},
                    .BADF => return error.FileNotFound,
                    .NOSPC => return error.NameTooLong,
                    // TODO man pages for fcntl on macOS don't really tell you what

                    // errno values to expect when command is F.GETPATH...

                    else => |err| return unexpectedErrno(err),
                }
                const len = mem.indexOfScalar(u8, out_buffer[0..], @as(u8, 0)) orelse MAX_PATH_BYTES;
                return out_buffer[0..len];
            },
            .linux => {
                var procfs_buf: ["/proc/self/fd/-2147483648".len:0]u8 = undefined;
                const proc_path = std.fmt.bufPrint(procfs_buf[0..], "/proc/self/fd/{d}\x00", .{fd}) catch unreachable;

                const target = readlinkZ(std.meta.assumeSentinel(proc_path.ptr, 0), out_buffer) catch |err| {
                    switch (err) {
                        error.UnsupportedReparsePointType => unreachable, // Windows only,

                        error.NotLink => unreachable,
                        else => |e| return e,
                    }
                };
                return target;
            },
            .solaris => {
                var procfs_buf: ["/proc/self/path/-2147483648".len:0]u8 = undefined;
                const proc_path = std.fmt.bufPrintZ(procfs_buf[0..], "/proc/self/path/{d}", .{fd}) catch unreachable;

                const target = readlinkZ(proc_path, out_buffer) catch |err| switch (err) {
                    error.UnsupportedReparsePointType => unreachable,
                    error.NotLink => unreachable,
                    else => |e| return e,
                };
                return target;
            },
            .freebsd => {
                if (comptime builtin.os.version_range.semver.max.order(.{ .major = 13, .minor = 0 }) == .gt) {
                    var kfile: system.kinfo_file = undefined;
                    kfile.structsize = system.KINFO_FILE_SIZE;
                    switch (errno(system.fcntl(fd, system.F.KINFO, @ptrToInt(&kfile)))) {
                        .SUCCESS => {},
                        .BADF => return error.FileNotFound,
                        else => |err| return unexpectedErrno(err),
                    }
                    const len = mem.indexOfScalar(u8, &kfile.path, 0) orelse MAX_PATH_BYTES;
                    if (len == 0) return error.NameTooLong;
                    mem.copy(u8, out_buffer, kfile.path[0..len]);
                    return out_buffer[0..len];
                } else {
                    // This fallback implementation reimplements libutil's `kinfo_getfile()`.

                    // The motivation is to avoid linking -lutil when building zig or general

                    // user executables.

                    var mib = [4]c_int{ CTL.KERN, KERN.PROC, KERN.PROC_FILEDESC, system.getpid() };
                    var len: usize = undefined;
                    sysctl(&mib, null, &len, null, 0) catch |err| switch (err) {
                        error.PermissionDenied => unreachable,
                        error.SystemResources => return error.SystemResources,
                        error.NameTooLong => unreachable,
                        error.UnknownName => unreachable,
                        else => return error.Unexpected,
                    };
                    len = len * 4 / 3;
                    const buf = std.heap.c_allocator.alloc(u8, len) catch return error.SystemResources;
                    defer std.heap.c_allocator.free(buf);
                    len = buf.len;
                    sysctl(&mib, &buf[0], &len, null, 0) catch |err| switch (err) {
                        error.PermissionDenied => unreachable,
                        error.SystemResources => return error.SystemResources,
                        error.NameTooLong => unreachable,
                        error.UnknownName => unreachable,
                        else => return error.Unexpected,
                    };
                    var i: usize = 0;
                    while (i < len) {
                        const kf: *align(1) system.kinfo_file = @ptrCast(*align(1) system.kinfo_file, &buf[i]);
                        if (kf.fd == fd) {
                            len = mem.indexOfScalar(u8, &kf.path, 0) orelse MAX_PATH_BYTES;
                            if (len == 0) return error.NameTooLong;
                            mem.copy(u8, out_buffer, kf.path[0..len]);
                            return out_buffer[0..len];
                        }
                        i += @intCast(usize, kf.structsize);
                    }
                    return error.InvalidHandle;
                }
            },
            .dragonfly => {
                if (comptime builtin.os.version_range.semver.max.order(.{ .major = 6, .minor = 0 }) == .lt) {
                    @compileError("querying for canonical path of a handle is unsupported on this host");
                }
                @memset(out_buffer, 0, MAX_PATH_BYTES);
                switch (errno(system.fcntl(fd, F.GETPATH, out_buffer))) {
                    .SUCCESS => {},
                    .BADF => return error.FileNotFound,
                    .RANGE => return error.NameTooLong,
                    else => |err| return unexpectedErrno(err),
                }
                const len = mem.indexOfScalar(u8, out_buffer[0..], @as(u8, 0)) orelse MAX_PATH_BYTES;
                return out_buffer[0..len];
            },
            .netbsd => {
                if (comptime builtin.os.version_range.semver.max.order(.{ .major = 10, .minor = 0 }) == .lt) {
                    @compileError("querying for canonical path of a handle is unsupported on this host");
                }
                @memset(out_buffer, 0, MAX_PATH_BYTES);
                switch (errno(system.fcntl(fd, F.GETPATH, out_buffer))) {
                    .SUCCESS => {},
                    .ACCES => return error.AccessDenied,
                    .BADF => return error.FileNotFound,
                    .NOENT => return error.FileNotFound,
                    .NOMEM => return error.SystemResources,
                    .RANGE => return error.NameTooLong,
                    else => |err| return unexpectedErrno(err),
                }
                const len = mem.indexOfScalar(u8, out_buffer[0..], @as(u8, 0)) orelse MAX_PATH_BYTES;
                return out_buffer[0..len];
            },
            else => @compileError("querying for canonical path of a handle is unsupported on this host"),
        }
    }

    /// Spurious wakeups are possible and no precision of timing is guaranteed.
    pub fn nanosleep(seconds: u64, nanoseconds: u64) void {
        var req = timespec{
            .tv_sec = math.cast(isize, seconds) orelse math.maxInt(isize),
            .tv_nsec = math.cast(isize, nanoseconds) orelse math.maxInt(isize),
        };
        var rem: timespec = undefined;
        while (true) {
            switch (errno(system.nanosleep(&req, &rem))) {
                .FAULT => unreachable,
                .INVAL => {
                    // Sometimes Darwin returns EINVAL for no reason.

                    // We treat it as a spurious wakeup.

                    return;
                },
                .INTR => {
                    req = rem;
                    continue;
                },
                // This prong handles success as well as unexpected errors.

                else => return,
            }
        }
    }

    pub fn dl_iterate_phdr(
        context: anytype,
        comptime Error: type,
        comptime callback: fn (info: *dl_phdr_info, size: usize, context: @TypeOf(context)) Error!void,
    ) Error!void {
        const Context = @TypeOf(context);

        if (builtin.object_format != .elf)
            @compileError("dl_iterate_phdr is not available for this target");

        if (builtin.link_libc) {
            switch (system.dl_iterate_phdr(struct {
                fn callbackC(info: *dl_phdr_info, size: usize, data: ?*anyopaque) callconv(.C) c_int {
                    const context_ptr = @ptrCast(*const Context, @alignCast(@alignOf(*const Context), data));
                    callback(info, size, context_ptr.*) catch |err| return @errorToInt(err);
                    return 0;
                }
            }.callbackC, @intToPtr(?*anyopaque, @ptrToInt(&context)))) {
                0 => return,
                else => |err| return @errSetCast(Error, @intToError(@intCast(u16, err))), // TODO don't hardcode u16

            }
        }

        const elf_base = std.process.getBaseAddress();
        const ehdr = @intToPtr(*elf.Ehdr, elf_base);
        // Make sure the base address points to an ELF image.

        assert(mem.eql(u8, ehdr.e_ident[0..4], elf.MAGIC));
        const n_phdr = ehdr.e_phnum;
        const phdrs = (@intToPtr([*]elf.Phdr, elf_base + ehdr.e_phoff))[0..n_phdr];

        var it = dl.linkmap_iterator(phdrs) catch unreachable;

        // The executable has no dynamic link segment, create a single entry for

        // the whole ELF image.

        if (it.end()) {
            // Find the base address for the ELF image, if this is a PIE the value

            // is non-zero.

            const base_address = for (phdrs) |*phdr| {
                if (phdr.p_type == elf.PT_PHDR) {
                    break @ptrToInt(phdrs.ptr) - phdr.p_vaddr;
                    // We could try computing the difference between _DYNAMIC and

                    // the p_vaddr of the PT_DYNAMIC section, but using the phdr is

                    // good enough (Is it?).

                }
            } else unreachable;

            var info = dl_phdr_info{
                .dlpi_addr = base_address,
                .dlpi_name = "/proc/self/exe",
                .dlpi_phdr = phdrs.ptr,
                .dlpi_phnum = ehdr.e_phnum,
            };

            return callback(&info, @sizeOf(dl_phdr_info), context);
        }

        // Last return value from the callback function.

        while (it.next()) |entry| {
            var dlpi_phdr: [*]elf.Phdr = undefined;
            var dlpi_phnum: u16 = undefined;

            if (entry.l_addr != 0) {
                const elf_header = @intToPtr(*elf.Ehdr, entry.l_addr);
                dlpi_phdr = @intToPtr([*]elf.Phdr, entry.l_addr + elf_header.e_phoff);
                dlpi_phnum = elf_header.e_phnum;
            } else {
                // This is the running ELF image

                dlpi_phdr = @intToPtr([*]elf.Phdr, elf_base + ehdr.e_phoff);
                dlpi_phnum = ehdr.e_phnum;
            }

            var info = dl_phdr_info{
                .dlpi_addr = entry.l_addr,
                .dlpi_name = entry.l_name,
                .dlpi_phdr = dlpi_phdr,
                .dlpi_phnum = dlpi_phnum,
            };

            try callback(&info, @sizeOf(dl_phdr_info), context);
        }
    }

    pub const ClockGetTimeError = error{UnsupportedClock} || UnexpectedError;

    /// TODO: change this to return the timespec as a return value
    /// TODO: look into making clk_id an enum
    pub fn clock_gettime(clk_id: i32, tp: *timespec) ClockGetTimeError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var ts: timestamp_t = undefined;
            switch (system.clock_time_get(@bitCast(u32, clk_id), 1, &ts)) {
                .SUCCESS => {
                    tp.* = .{
                        .tv_sec = @intCast(i64, ts / std.time.ns_per_s),
                        .tv_nsec = @intCast(isize, ts % std.time.ns_per_s),
                    };
                },
                .INVAL => return error.UnsupportedClock,
                else => |err| return unexpectedErrno(err),
            }
            return;
        }
        if (builtin.os.tag == .windows) {
            if (clk_id == CLOCK.REALTIME) {
                var ft: windows.FILETIME = undefined;
                windows.kernel32.GetSystemTimeAsFileTime(&ft);
                // FileTime has a granularity of 100 nanoseconds and uses the NTFS/Windows epoch.

                const ft64 = (@as(u64, ft.dwHighDateTime) << 32) | ft.dwLowDateTime;
                const ft_per_s = std.time.ns_per_s / 100;
                tp.* = .{
                    .tv_sec = @intCast(i64, ft64 / ft_per_s) + std.time.epoch.windows,
                    .tv_nsec = @intCast(c_long, ft64 % ft_per_s) * 100,
                };
                return;
            } else {
                // TODO POSIX implementation of CLOCK.MONOTONIC on Windows.

                return error.UnsupportedClock;
            }
        }

        switch (errno(system.clock_gettime(clk_id, tp))) {
            .SUCCESS => return,
            .FAULT => unreachable,
            .INVAL => return error.UnsupportedClock,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub fn clock_getres(clk_id: i32, res: *timespec) ClockGetTimeError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var ts: timestamp_t = undefined;
            switch (system.clock_res_get(@bitCast(u32, clk_id), &ts)) {
                .SUCCESS => res.* = .{
                    .tv_sec = @intCast(i64, ts / std.time.ns_per_s),
                    .tv_nsec = @intCast(isize, ts % std.time.ns_per_s),
                },
                .INVAL => return error.UnsupportedClock,
                else => |err| return unexpectedErrno(err),
            }
            return;
        }

        switch (errno(system.clock_getres(clk_id, res))) {
            .SUCCESS => return,
            .FAULT => unreachable,
            .INVAL => return error.UnsupportedClock,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const SchedGetAffinityError = error{PermissionDenied} || UnexpectedError;

    pub fn sched_getaffinity(pid: pid_t) SchedGetAffinityError!cpu_set_t {
        var set: cpu_set_t = undefined;
        switch (errno(system.sched_getaffinity(pid, @sizeOf(cpu_set_t), &set))) {
            .SUCCESS => return set,
            .FAULT => unreachable,
            .INVAL => unreachable,
            .SRCH => unreachable,
            .PERM => return error.PermissionDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Used to convert a slice to a null terminated slice on the stack.
    /// TODO https://github.com/ziglang/zig/issues/287
    pub fn toPosixPath(file_path: []const u8) ![MAX_PATH_BYTES - 1:0]u8 {
        if (std.debug.runtime_safety) assert(std.mem.indexOfScalar(u8, file_path, 0) == null);
        var path_with_null: [MAX_PATH_BYTES - 1:0]u8 = undefined;
        // >= rather than > to make room for the null byte

        if (file_path.len >= MAX_PATH_BYTES) return error.NameTooLong;
        mem.copy(u8, &path_with_null, file_path);
        path_with_null[file_path.len] = 0;
        return path_with_null;
    }

    /// Whether or not error.Unexpected will print its value and a stack trace.
    /// if this happens the fix is to add the error code to the corresponding
    /// switch expression, possibly introduce a new error in the error set, and
    /// send a patch to Zig.
    pub const unexpected_error_tracing = (builtin.zig_backend == .stage1 or builtin.zig_backend == .stage2_llvm) and builtin.mode == .Debug;

    pub const UnexpectedError = error{
        /// The Operating System returned an undocumented error code.
        /// This error is in theory not possible, but it would be better
        /// to handle this error than to invoke undefined behavior.
        Unexpected,
    };

    /// Call this when you made a syscall or something that sets errno
    /// and you get an unexpected error.
    pub fn unexpectedErrno(err: E) UnexpectedError {
        if (unexpected_error_tracing) {
            std.debug.print("unexpected errno: {d}\n", .{@enumToInt(err)});
            std.debug.dumpCurrentStackTrace(null);
        }
        return error.Unexpected;
    }

    pub const SigaltstackError = error{
        /// The supplied stack size was less than MINSIGSTKSZ.
        SizeTooSmall,

        /// Attempted to change the signal stack while it was active.
        PermissionDenied,
    } || UnexpectedError;

    pub fn sigaltstack(ss: ?*stack_t, old_ss: ?*stack_t) SigaltstackError!void {
        switch (errno(system.sigaltstack(ss, old_ss))) {
            .SUCCESS => return,
            .FAULT => unreachable,
            .INVAL => unreachable,
            .NOMEM => return error.SizeTooSmall,
            .PERM => return error.PermissionDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Examine and change a signal action.
    pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) error{OperationNotSupported}!void {
        switch (errno(system.sigaction(sig, act, oact))) {
            .SUCCESS => return,
            .INVAL, .NOSYS => return error.OperationNotSupported,
            else => unreachable,
        }
    }

    /// Sets the thread signal mask.
    pub fn sigprocmask(flags: u32, noalias set: ?*const sigset_t, noalias oldset: ?*sigset_t) void {
        switch (errno(system.sigprocmask(flags, set, oldset))) {
            .SUCCESS => return,
            .FAULT => unreachable,
            .INVAL => unreachable,
            else => unreachable,
        }
    }

    pub const FutimensError = error{
        /// times is NULL, or both tv_nsec values are UTIME_NOW, and either:
        /// *  the effective user ID of the caller does not match the  owner
        ///    of  the  file,  the  caller does not have write access to the
        ///    file, and the caller is not privileged (Linux: does not  have
        ///    either  the  CAP_FOWNER  or the CAP_DAC_OVERRIDE capability);
        ///    or,
        /// *  the file is marked immutable (see chattr(1)).
        AccessDenied,

        /// The caller attempted to change one or both timestamps to a value
        /// other than the current time, or to change one of the  timestamps
        /// to the current time while leaving the other timestamp unchanged,
        /// (i.e., times is not NULL, neither tv_nsec  field  is  UTIME_NOW,
        /// and neither tv_nsec field is UTIME_OMIT) and either:
        /// *  the  caller's  effective  user ID does not match the owner of
        ///    file, and the caller is not privileged (Linux: does not  have
        ///    the CAP_FOWNER capability); or,
        /// *  the file is marked append-only or immutable (see chattr(1)).
        PermissionDenied,

        ReadOnlyFileSystem,
    } || UnexpectedError;

    pub fn futimens(fd: fd_t, times: *const [2]timespec) FutimensError!void {
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            // TODO WASI encodes `wasi.fstflags` to signify magic values

            // similar to UTIME_NOW and UTIME_OMIT. Currently, we ignore

            // this here, but we should really handle it somehow.

            const atim = times[0].toTimestamp();
            const mtim = times[1].toTimestamp();
            switch (wasi.fd_filestat_set_times(fd, atim, mtim, wasi.FILESTAT_SET_ATIM | wasi.FILESTAT_SET_MTIM)) {
                .SUCCESS => return,
                .ACCES => return error.AccessDenied,
                .PERM => return error.PermissionDenied,
                .BADF => unreachable, // always a race condition

                .FAULT => unreachable,
                .INVAL => unreachable,
                .ROFS => return error.ReadOnlyFileSystem,
                else => |err| return unexpectedErrno(err),
            }
        }

        switch (errno(system.futimens(fd, times))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .PERM => return error.PermissionDenied,
            .BADF => unreachable, // always a race condition

            .FAULT => unreachable,
            .INVAL => unreachable,
            .ROFS => return error.ReadOnlyFileSystem,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const GetHostNameError = error{PermissionDenied} || UnexpectedError;

    pub fn gethostname(name_buffer: *[HOST_NAME_MAX]u8) GetHostNameError![]u8 {
        if (builtin.link_libc) {
            switch (errno(system.gethostname(name_buffer, name_buffer.len))) {
                .SUCCESS => return mem.sliceTo(std.meta.assumeSentinel(name_buffer, 0), 0),
                .FAULT => unreachable,
                .NAMETOOLONG => unreachable, // HOST_NAME_MAX prevents this

                .PERM => return error.PermissionDenied,
                else => |err| return unexpectedErrno(err),
            }
        }
        if (builtin.os.tag == .linux) {
            const uts = uname();
            const hostname = mem.sliceTo(std.meta.assumeSentinel(&uts.nodename, 0), 0);
            mem.copy(u8, name_buffer, hostname);
            return name_buffer[0..hostname.len];
        }

        @compileError("TODO implement gethostname for this OS");
    }

    pub fn uname() utsname {
        var uts: utsname = undefined;
        switch (errno(system.uname(&uts))) {
            .SUCCESS => return uts,
            .FAULT => unreachable,
            else => unreachable,
        }
    }

    pub fn res_mkquery(
        op: u4,
        dname: []const u8,
        class: u8,
        ty: u8,
        data: []const u8,
        newrr: ?[*]const u8,
        buf: []u8,
    ) usize {
        _ = data;
        _ = newrr;
        // This implementation is ported from musl libc.

        // A more idiomatic "ziggy" implementation would be welcome.

        var name = dname;
        if (mem.endsWith(u8, name, ".")) name.len -= 1;
        assert(name.len <= 253);
        const n = 17 + name.len + @boolToInt(name.len != 0);

        // Construct query template - ID will be filled later

        var q: [280]u8 = undefined;
        @memset(&q, 0, n);
        q[2] = @as(u8, op) * 8 + 1;
        q[5] = 1;
        mem.copy(u8, q[13..], name);
        var i: usize = 13;
        var j: usize = undefined;
        while (q[i] != 0) : (i = j + 1) {
            j = i;
            while (q[j] != 0 and q[j] != '.') : (j += 1) {}
            // TODO determine the circumstances for this and whether or

            // not this should be an error.

            if (j - i - 1 > 62) unreachable;
            q[i - 1] = @intCast(u8, j - i);
        }
        q[i + 1] = ty;
        q[i + 3] = class;

        // Make a reasonably unpredictable id

        var ts: timespec = undefined;
        clock_gettime(CLOCK.REALTIME, &ts) catch {};
        const UInt = std.meta.Int(.unsigned, @bitSizeOf(@TypeOf(ts.tv_nsec)));
        const unsec = @bitCast(UInt, ts.tv_nsec);
        const id = @truncate(u32, unsec + unsec / 65536);
        q[0] = @truncate(u8, id / 256);
        q[1] = @truncate(u8, id);

        mem.copy(u8, buf, q[0..n]);
        return n;
    }

    pub const SendError = error{
        /// (For UNIX domain sockets, which are identified by pathname) Write permission is  denied
        /// on  the destination socket file, or search permission is denied for one of the
        /// directories the path prefix.  (See path_resolution(7).)
        /// (For UDP sockets) An attempt was made to send to a network/broadcast address as  though
        /// it was a unicast address.
        AccessDenied,

        /// The socket is marked nonblocking and the requested operation would block, and
        /// there is no global event loop configured.
        /// It's also possible to get this error under the following condition:
        /// (Internet  domain datagram sockets) The socket referred to by sockfd had not previously
        /// been bound to an address and, upon attempting to bind it to an ephemeral port,  it  was
        /// determined that all port numbers in the ephemeral port range are currently in use.  See
        /// the discussion of /proc/sys/net/ipv4/ip_local_port_range in ip(7).
        WouldBlock,

        /// Another Fast Open is already in progress.
        FastOpenAlreadyInProgress,

        /// Connection reset by peer.
        ConnectionResetByPeer,

        /// The  socket  type requires that message be sent atomically, and the size of the message
        /// to be sent made this impossible. The message is not transmitted.
        MessageTooBig,

        /// The output queue for a network interface was full.  This generally indicates  that  the
        /// interface  has  stopped sending, but may be caused by transient congestion.  (Normally,
        /// this does not occur in Linux.  Packets are just silently dropped when  a  device  queue
        /// overflows.)
        /// This is also caused when there is not enough kernel memory available.
        SystemResources,

        /// The  local  end  has been shut down on a connection oriented socket.  In this case, the
        /// process will also receive a SIGPIPE unless MSG.NOSIGNAL is set.
        BrokenPipe,

        FileDescriptorNotASocket,

        /// Network is unreachable.
        NetworkUnreachable,

        /// The local network interface used to reach the destination is down.
        NetworkSubsystemFailed,
    } || UnexpectedError;

    pub const SendMsgError = SendError || error{
        /// The passed address didn't have the correct address family in its sa_family field.
        AddressFamilyNotSupported,

        /// Returned when socket is AF.UNIX and the given path has a symlink loop.
        SymLinkLoop,

        /// Returned when socket is AF.UNIX and the given path length exceeds `MAX_PATH_BYTES` bytes.
        NameTooLong,

        /// Returned when socket is AF.UNIX and the given path does not point to an existing file.
        FileNotFound,
        NotDir,

        /// The socket is not connected (connection-oriented sockets only).
        SocketNotConnected,
        AddressNotAvailable,
    };

    pub fn sendmsg(
        /// The file descriptor of the sending socket.
        sockfd: socket_t,
        /// Message header and iovecs
        msg: msghdr_const,
        flags: u32,
    ) SendMsgError!usize {
        while (true) {
            const rc = system.sendmsg(sockfd, @ptrCast(*const std.x.os.Socket.Message, &msg), @intCast(c_int, flags));
            if (builtin.os.tag == .windows) {
                if (rc == windows.ws2_32.SOCKET_ERROR) {
                    switch (windows.ws2_32.WSAGetLastError()) {
                        .WSAEACCES => return error.AccessDenied,
                        .WSAEADDRNOTAVAIL => return error.AddressNotAvailable,
                        .WSAECONNRESET => return error.ConnectionResetByPeer,
                        .WSAEMSGSIZE => return error.MessageTooBig,
                        .WSAENOBUFS => return error.SystemResources,
                        .WSAENOTSOCK => return error.FileDescriptorNotASocket,
                        .WSAEAFNOSUPPORT => return error.AddressFamilyNotSupported,
                        .WSAEDESTADDRREQ => unreachable, // A destination address is required.

                        .WSAEFAULT => unreachable, // The lpBuffers, lpTo, lpOverlapped, lpNumberOfBytesSent, or lpCompletionRoutine parameters are not part of the user address space, or the lpTo parameter is too small.

                        .WSAEHOSTUNREACH => return error.NetworkUnreachable,
                        // TODO: WSAEINPROGRESS, WSAEINTR

                        .WSAEINVAL => unreachable,
                        .WSAENETDOWN => return error.NetworkSubsystemFailed,
                        .WSAENETRESET => return error.ConnectionResetByPeer,
                        .WSAENETUNREACH => return error.NetworkUnreachable,
                        .WSAENOTCONN => return error.SocketNotConnected,
                        .WSAESHUTDOWN => unreachable, // The socket has been shut down; it is not possible to WSASendTo on a socket after shutdown has been invoked with how set to SD_SEND or SD_BOTH.

                        .WSAEWOULDBLOCK => return error.WouldBlock,
                        .WSANOTINITIALISED => unreachable, // A successful WSAStartup call must occur before using this function.

                        else => |err| return windows.unexpectedWSAError(err),
                    }
                } else {
                    return @intCast(usize, rc);
                }
            } else {
                switch (errno(rc)) {
                    .SUCCESS => return @intCast(usize, rc),

                    .ACCES => return error.AccessDenied,
                    .AGAIN => return error.WouldBlock,
                    .ALREADY => return error.FastOpenAlreadyInProgress,
                    .BADF => unreachable, // always a race condition

                    .CONNRESET => return error.ConnectionResetByPeer,
                    .DESTADDRREQ => unreachable, // The socket is not connection-mode, and no peer address is set.

                    .FAULT => unreachable, // An invalid user space address was specified for an argument.

                    .INTR => continue,
                    .INVAL => unreachable, // Invalid argument passed.

                    .ISCONN => unreachable, // connection-mode socket was connected already but a recipient was specified

                    .MSGSIZE => return error.MessageTooBig,
                    .NOBUFS => return error.SystemResources,
                    .NOMEM => return error.SystemResources,
                    .NOTSOCK => unreachable, // The file descriptor sockfd does not refer to a socket.

                    .OPNOTSUPP => unreachable, // Some bit in the flags argument is inappropriate for the socket type.

                    .PIPE => return error.BrokenPipe,
                    .AFNOSUPPORT => return error.AddressFamilyNotSupported,
                    .LOOP => return error.SymLinkLoop,
                    .NAMETOOLONG => return error.NameTooLong,
                    .NOENT => return error.FileNotFound,
                    .NOTDIR => return error.NotDir,
                    .HOSTUNREACH => return error.NetworkUnreachable,
                    .NETUNREACH => return error.NetworkUnreachable,
                    .NOTCONN => return error.SocketNotConnected,
                    .NETDOWN => return error.NetworkSubsystemFailed,
                    else => |err| return unexpectedErrno(err),
                }
            }
        }
    }

    pub const SendToError = SendMsgError || error{
        /// The destination address is not reachable by the bound address.
        UnreachableAddress,
    };

    /// Transmit a message to another socket.
    ///
    /// The `sendto` call may be used only when the socket is in a connected state (so that the intended
    /// recipient  is  known). The  following call
    ///
    ///     send(sockfd, buf, len, flags);
    ///
    /// is equivalent to
    ///
    ///     sendto(sockfd, buf, len, flags, NULL, 0);
    ///
    /// If  sendto()  is used on a connection-mode (`SOCK.STREAM`, `SOCK.SEQPACKET`) socket, the arguments
    /// `dest_addr` and `addrlen` are asserted to be `null` and `0` respectively, and asserted
    /// that the socket was actually connected.
    /// Otherwise, the address of the target is given by `dest_addr` with `addrlen` specifying  its  size.
    ///
    /// If the message is too long to pass atomically through the underlying protocol,
    /// `SendError.MessageTooBig` is returned, and the message is not transmitted.
    ///
    /// There is no  indication  of  failure  to  deliver.
    ///
    /// When the message does not fit into the send buffer of  the  socket,  `sendto`  normally  blocks,
    /// unless  the socket has been placed in nonblocking I/O mode.  In nonblocking mode it would fail
    /// with `SendError.WouldBlock`.  The `select` call may be used  to  determine when it is
    /// possible to send more data.
    pub fn sendto(
        /// The file descriptor of the sending socket.
        sockfd: socket_t,
        /// Message to send.
        buf: []const u8,
        flags: u32,
        dest_addr: ?*const sockaddr,
        addrlen: socklen_t,
    ) SendToError!usize {
        while (true) {
            const rc = system.sendto(sockfd, buf.ptr, buf.len, flags, dest_addr, addrlen);
            if (builtin.os.tag == .windows) {
                if (rc == windows.ws2_32.SOCKET_ERROR) {
                    switch (windows.ws2_32.WSAGetLastError()) {
                        .WSAEACCES => return error.AccessDenied,
                        .WSAEADDRNOTAVAIL => return error.AddressNotAvailable,
                        .WSAECONNRESET => return error.ConnectionResetByPeer,
                        .WSAEMSGSIZE => return error.MessageTooBig,
                        .WSAENOBUFS => return error.SystemResources,
                        .WSAENOTSOCK => return error.FileDescriptorNotASocket,
                        .WSAEAFNOSUPPORT => return error.AddressFamilyNotSupported,
                        .WSAEDESTADDRREQ => unreachable, // A destination address is required.

                        .WSAEFAULT => unreachable, // The lpBuffers, lpTo, lpOverlapped, lpNumberOfBytesSent, or lpCompletionRoutine parameters are not part of the user address space, or the lpTo parameter is too small.

                        .WSAEHOSTUNREACH => return error.NetworkUnreachable,
                        // TODO: WSAEINPROGRESS, WSAEINTR

                        .WSAEINVAL => unreachable,
                        .WSAENETDOWN => return error.NetworkSubsystemFailed,
                        .WSAENETRESET => return error.ConnectionResetByPeer,
                        .WSAENETUNREACH => return error.NetworkUnreachable,
                        .WSAENOTCONN => return error.SocketNotConnected,
                        .WSAESHUTDOWN => unreachable, // The socket has been shut down; it is not possible to WSASendTo on a socket after shutdown has been invoked with how set to SD_SEND or SD_BOTH.

                        .WSAEWOULDBLOCK => return error.WouldBlock,
                        .WSANOTINITIALISED => unreachable, // A successful WSAStartup call must occur before using this function.

                        else => |err| return windows.unexpectedWSAError(err),
                    }
                } else {
                    return @intCast(usize, rc);
                }
            } else {
                switch (errno(rc)) {
                    .SUCCESS => return @intCast(usize, rc),

                    .ACCES => return error.AccessDenied,
                    .AGAIN => return error.WouldBlock,
                    .ALREADY => return error.FastOpenAlreadyInProgress,
                    .BADF => unreachable, // always a race condition

                    .CONNRESET => return error.ConnectionResetByPeer,
                    .DESTADDRREQ => unreachable, // The socket is not connection-mode, and no peer address is set.

                    .FAULT => unreachable, // An invalid user space address was specified for an argument.

                    .INTR => continue,
                    .INVAL => return error.UnreachableAddress,
                    .ISCONN => unreachable, // connection-mode socket was connected already but a recipient was specified

                    .MSGSIZE => return error.MessageTooBig,
                    .NOBUFS => return error.SystemResources,
                    .NOMEM => return error.SystemResources,
                    .NOTSOCK => unreachable, // The file descriptor sockfd does not refer to a socket.

                    .OPNOTSUPP => unreachable, // Some bit in the flags argument is inappropriate for the socket type.

                    .PIPE => return error.BrokenPipe,
                    .AFNOSUPPORT => return error.AddressFamilyNotSupported,
                    .LOOP => return error.SymLinkLoop,
                    .NAMETOOLONG => return error.NameTooLong,
                    .NOENT => return error.FileNotFound,
                    .NOTDIR => return error.NotDir,
                    .HOSTUNREACH => return error.NetworkUnreachable,
                    .NETUNREACH => return error.NetworkUnreachable,
                    .NOTCONN => return error.SocketNotConnected,
                    .NETDOWN => return error.NetworkSubsystemFailed,
                    else => |err| return unexpectedErrno(err),
                }
            }
        }
    }

    /// Transmit a message to another socket.
    ///
    /// The `send` call may be used only when the socket is in a connected state (so that the intended
    /// recipient  is  known).   The  only  difference  between `send` and `write` is the presence of
    /// flags.  With a zero flags argument, `send` is equivalent to  `write`.   Also,  the  following
    /// call
    ///
    ///     send(sockfd, buf, len, flags);
    ///
    /// is equivalent to
    ///
    ///     sendto(sockfd, buf, len, flags, NULL, 0);
    ///
    /// There is no  indication  of  failure  to  deliver.
    ///
    /// When the message does not fit into the send buffer of  the  socket,  `send`  normally  blocks,
    /// unless  the socket has been placed in nonblocking I/O mode.  In nonblocking mode it would fail
    /// with `SendError.WouldBlock`.  The `select` call may be used  to  determine when it is
    /// possible to send more data.
    pub fn send(
        /// The file descriptor of the sending socket.
        sockfd: socket_t,
        buf: []const u8,
        flags: u32,
    ) SendError!usize {
        return sendto(sockfd, buf, flags, null, 0) catch |err| switch (err) {
            error.AddressFamilyNotSupported => unreachable,
            error.SymLinkLoop => unreachable,
            error.NameTooLong => unreachable,
            error.FileNotFound => unreachable,
            error.NotDir => unreachable,
            error.NetworkUnreachable => unreachable,
            error.AddressNotAvailable => unreachable,
            error.SocketNotConnected => unreachable,
            error.UnreachableAddress => unreachable,
            else => |e| return e,
        };
    }

    pub const SendFileError = PReadError || WriteError || SendError;

    fn count_iovec_bytes(iovs: []const iovec_const) usize {
        var count: usize = 0;
        for (iovs) |iov| {
            count += iov.iov_len;
        }
        return count;
    }

    /// Transfer data between file descriptors, with optional headers and trailers.
    /// Returns the number of bytes written, which can be zero.
    ///
    /// The `sendfile` call copies `in_len` bytes from one file descriptor to another. When possible,
    /// this is done within the operating system kernel, which can provide better performance
    /// characteristics than transferring data from kernel to user space and back, such as with
    /// `read` and `write` calls. When `in_len` is `0`, it means to copy until the end of the input file has been
    /// reached. Note, however, that partial writes are still possible in this case.
    ///
    /// `in_fd` must be a file descriptor opened for reading, and `out_fd` must be a file descriptor
    /// opened for writing. They may be any kind of file descriptor; however, if `in_fd` is not a regular
    /// file system file, it may cause this function to fall back to calling `read` and `write`, in which case
    /// atomicity guarantees no longer apply.
    ///
    /// Copying begins reading at `in_offset`. The input file descriptor seek position is ignored and not updated.
    /// If the output file descriptor has a seek position, it is updated as bytes are written. When
    /// `in_offset` is past the end of the input file, it successfully reads 0 bytes.
    ///
    /// `flags` has different meanings per operating system; refer to the respective man pages.
    ///
    /// These systems support atomically sending everything, including headers and trailers:
    /// * macOS
    /// * FreeBSD
    ///
    /// These systems support in-kernel data copying, but headers and trailers are not sent atomically:
    /// * Linux
    ///
    /// Other systems fall back to calling `read` / `write`.
    ///
    /// Linux has a limit on how many bytes may be transferred in one `sendfile` call, which is `0x7ffff000`
    /// on both 64-bit and 32-bit systems. This is due to using a signed C int as the return value, as
    /// well as stuffing the errno codes into the last `4096` values. This is noted on the `sendfile` man page.
    /// The limit on Darwin is `0x7fffffff`, trying to write more than that returns EINVAL.
    /// The corresponding POSIX limit on this is `math.maxInt(isize)`.
    pub fn sendfile(
        out_fd: fd_t,
        in_fd: fd_t,
        in_offset: u64,
        in_len: u64,
        headers: []const iovec_const,
        trailers: []const iovec_const,
        flags: u32,
    ) SendFileError!usize {
        var header_done = false;
        var total_written: usize = 0;

        // Prevents EOVERFLOW.

        const size_t = std.meta.Int(.unsigned, @typeInfo(usize).Int.bits - 1);
        const max_count = switch (builtin.os.tag) {
            .linux => 0x7ffff000,
            .macos, .ios, .watchos, .tvos => math.maxInt(i32),
            else => math.maxInt(size_t),
        };

        switch (builtin.os.tag) {
            .linux => sf: {
                // sendfile() first appeared in Linux 2.2, glibc 2.1.

                const call_sf = comptime if (builtin.link_libc)
                    std.c.versionCheck(.{ .major = 2, .minor = 1 }).ok
                else
                    builtin.os.version_range.linux.range.max.order(.{ .major = 2, .minor = 2 }) != .lt;
                if (!call_sf) break :sf;

                if (headers.len != 0) {
                    const amt = try writev(out_fd, headers);
                    total_written += amt;
                    if (amt < count_iovec_bytes(headers)) return total_written;
                    header_done = true;
                }

                // Here we match BSD behavior, making a zero count value send as many bytes as possible.

                const adjusted_count_tmp = if (in_len == 0) max_count else @min(in_len, @as(size_t, max_count));
                // TODO we should not need this cast; improve return type of @min

                const adjusted_count = @intCast(usize, adjusted_count_tmp);

                const sendfile_sym = if (builtin.link_libc)
                    system.sendfile64
                else
                    system.sendfile;

                while (true) {
                    var offset: off_t = @bitCast(off_t, in_offset);
                    const rc = sendfile_sym(out_fd, in_fd, &offset, adjusted_count);
                    switch (errno(rc)) {
                        .SUCCESS => {
                            const amt = @bitCast(usize, rc);
                            total_written += amt;
                            if (in_len == 0 and amt == 0) {
                                // We have detected EOF from `in_fd`.

                                break;
                            } else if (amt < in_len) {
                                return total_written;
                            } else {
                                break;
                            }
                        },

                        .BADF => unreachable, // Always a race condition.

                        .FAULT => unreachable, // Segmentation fault.

                        .OVERFLOW => unreachable, // We avoid passing too large of a `count`.

                        .NOTCONN => unreachable, // `out_fd` is an unconnected socket.

                        .INVAL, .NOSYS => {
                            // EINVAL could be any of the following situations:

                            // * Descriptor is not valid or locked

                            // * an mmap(2)-like operation is  not  available  for in_fd

                            // * count is negative

                            // * out_fd has the O.APPEND flag set

                            // Because of the "mmap(2)-like operation" possibility, we fall back to doing read/write

                            // manually, the same as ENOSYS.

                            break :sf;
                        },
                        .AGAIN => if (std.event.Loop.instance) |loop| {
                            loop.waitUntilFdWritable(out_fd);
                            continue;
                        } else {
                            return error.WouldBlock;
                        },
                        .IO => return error.InputOutput,
                        .PIPE => return error.BrokenPipe,
                        .NOMEM => return error.SystemResources,
                        .NXIO => return error.Unseekable,
                        .SPIPE => return error.Unseekable,
                        else => |err| {
                            unexpectedErrno(err) catch {};
                            break :sf;
                        },
                    }
                }

                if (trailers.len != 0) {
                    total_written += try writev(out_fd, trailers);
                }

                return total_written;
            },
            .freebsd => sf: {
                var hdtr_data: std.c.sf_hdtr = undefined;
                var hdtr: ?*std.c.sf_hdtr = null;
                if (headers.len != 0 or trailers.len != 0) {
                    // Here we carefully avoid `@intCast` by returning partial writes when

                    // too many io vectors are provided.

                    const hdr_cnt = math.cast(u31, headers.len) orelse math.maxInt(u31);
                    if (headers.len > hdr_cnt) return writev(out_fd, headers);

                    const trl_cnt = math.cast(u31, trailers.len) orelse math.maxInt(u31);

                    hdtr_data = std.c.sf_hdtr{
                        .headers = headers.ptr,
                        .hdr_cnt = hdr_cnt,
                        .trailers = trailers.ptr,
                        .trl_cnt = trl_cnt,
                    };
                    hdtr = &hdtr_data;
                }

                const adjusted_count = @min(in_len, max_count);

                while (true) {
                    var sbytes: off_t = undefined;
                    const offset = @bitCast(off_t, in_offset);
                    const err = errno(system.sendfile(in_fd, out_fd, offset, adjusted_count, hdtr, &sbytes, flags));
                    const amt = @bitCast(usize, sbytes);
                    switch (err) {
                        .SUCCESS => return amt,

                        .BADF => unreachable, // Always a race condition.

                        .FAULT => unreachable, // Segmentation fault.

                        .NOTCONN => unreachable, // `out_fd` is an unconnected socket.

                        .INVAL, .OPNOTSUPP, .NOTSOCK, .NOSYS => {
                            // EINVAL could be any of the following situations:

                            // * The fd argument is not a regular file.

                            // * The s argument is not a SOCK.STREAM type socket.

                            // * The offset argument is negative.

                            // Because of some of these possibilities, we fall back to doing read/write

                            // manually, the same as ENOSYS.

                            break :sf;
                        },

                        .INTR => if (amt != 0) return amt else continue,

                        .AGAIN => if (amt != 0) {
                            return amt;
                        } else if (std.event.Loop.instance) |loop| {
                            loop.waitUntilFdWritable(out_fd);
                            continue;
                        } else {
                            return error.WouldBlock;
                        },

                        .BUSY => if (amt != 0) {
                            return amt;
                        } else if (std.event.Loop.instance) |loop| {
                            loop.waitUntilFdReadable(in_fd);
                            continue;
                        } else {
                            return error.WouldBlock;
                        },

                        .IO => return error.InputOutput,
                        .NOBUFS => return error.SystemResources,
                        .PIPE => return error.BrokenPipe,

                        else => {
                            unexpectedErrno(err) catch {};
                            if (amt != 0) {
                                return amt;
                            } else {
                                break :sf;
                            }
                        },
                    }
                }
            },
            .macos, .ios, .tvos, .watchos => sf: {
                var hdtr_data: std.c.sf_hdtr = undefined;
                var hdtr: ?*std.c.sf_hdtr = null;
                if (headers.len != 0 or trailers.len != 0) {
                    // Here we carefully avoid `@intCast` by returning partial writes when

                    // too many io vectors are provided.

                    const hdr_cnt = math.cast(u31, headers.len) orelse math.maxInt(u31);
                    if (headers.len > hdr_cnt) return writev(out_fd, headers);

                    const trl_cnt = math.cast(u31, trailers.len) orelse math.maxInt(u31);

                    hdtr_data = std.c.sf_hdtr{
                        .headers = headers.ptr,
                        .hdr_cnt = hdr_cnt,
                        .trailers = trailers.ptr,
                        .trl_cnt = trl_cnt,
                    };
                    hdtr = &hdtr_data;
                }

                const adjusted_count_temporary = @min(in_len, @as(u63, max_count));
                // TODO we should not need this int cast; improve the return type of `@min`

                const adjusted_count = @intCast(u63, adjusted_count_temporary);

                while (true) {
                    var sbytes: off_t = adjusted_count;
                    const signed_offset = @bitCast(i64, in_offset);
                    const err = errno(system.sendfile(in_fd, out_fd, signed_offset, &sbytes, hdtr, flags));
                    const amt = @bitCast(usize, sbytes);
                    switch (err) {
                        .SUCCESS => return amt,

                        .BADF => unreachable, // Always a race condition.

                        .FAULT => unreachable, // Segmentation fault.

                        .INVAL => unreachable,
                        .NOTCONN => unreachable, // `out_fd` is an unconnected socket.

                        .OPNOTSUPP, .NOTSOCK, .NOSYS => break :sf,

                        .INTR => if (amt != 0) return amt else continue,

                        .AGAIN => if (amt != 0) {
                            return amt;
                        } else if (std.event.Loop.instance) |loop| {
                            loop.waitUntilFdWritable(out_fd);
                            continue;
                        } else {
                            return error.WouldBlock;
                        },

                        .IO => return error.InputOutput,
                        .PIPE => return error.BrokenPipe,

                        else => {
                            unexpectedErrno(err) catch {};
                            if (amt != 0) {
                                return amt;
                            } else {
                                break :sf;
                            }
                        },
                    }
                }
            },
            else => {}, // fall back to read/write

        }

        if (headers.len != 0 and !header_done) {
            const amt = try writev(out_fd, headers);
            total_written += amt;
            if (amt < count_iovec_bytes(headers)) return total_written;
        }

        rw: {
            var buf: [8 * 4096]u8 = undefined;
            // Here we match BSD behavior, making a zero count value send as many bytes as possible.

            const adjusted_count_tmp = if (in_len == 0) buf.len else @min(buf.len, in_len);
            // TODO we should not need this cast; improve return type of @min

            const adjusted_count = @intCast(usize, adjusted_count_tmp);
            const amt_read = try pread(in_fd, buf[0..adjusted_count], in_offset);
            if (amt_read == 0) {
                if (in_len == 0) {
                    // We have detected EOF from `in_fd`.

                    break :rw;
                } else {
                    return total_written;
                }
            }
            const amt_written = try write(out_fd, buf[0..amt_read]);
            total_written += amt_written;
            if (amt_written < in_len or in_len == 0) return total_written;
        }

        if (trailers.len != 0) {
            total_written += try writev(out_fd, trailers);
        }

        return total_written;
    }

    pub const CopyFileRangeError = error{
        FileTooBig,
        InputOutput,
        /// `fd_in` is not open for reading; or `fd_out` is not open  for  writing;
        /// or the  `O.APPEND`  flag  is  set  for `fd_out`.
        FilesOpenedWithWrongFlags,
        IsDir,
        OutOfMemory,
        NoSpaceLeft,
        Unseekable,
        PermissionDenied,
        SwapFile,
    } || PReadError || PWriteError || UnexpectedError;

    var has_copy_file_range_syscall = std.atomic.Atomic(bool).init(true);

    /// Transfer data between file descriptors at specified offsets.
    /// Returns the number of bytes written, which can less than requested.
    ///
    /// The `copy_file_range` call copies `len` bytes from one file descriptor to another. When possible,
    /// this is done within the operating system kernel, which can provide better performance
    /// characteristics than transferring data from kernel to user space and back, such as with
    /// `pread` and `pwrite` calls.
    ///
    /// `fd_in` must be a file descriptor opened for reading, and `fd_out` must be a file descriptor
    /// opened for writing. They may be any kind of file descriptor; however, if `fd_in` is not a regular
    /// file system file, it may cause this function to fall back to calling `pread` and `pwrite`, in which case
    /// atomicity guarantees no longer apply.
    ///
    /// If `fd_in` and `fd_out` are the same, source and target ranges must not overlap.
    /// The file descriptor seek positions are ignored and not updated.
    /// When `off_in` is past the end of the input file, it successfully reads 0 bytes.
    ///
    /// `flags` has different meanings per operating system; refer to the respective man pages.
    ///
    /// These systems support in-kernel data copying:
    /// * Linux 4.5 (cross-filesystem 5.3)
    ///
    /// Other systems fall back to calling `pread` / `pwrite`.
    ///
    /// Maximum offsets on Linux are `math.maxInt(i64)`.
    pub fn copy_file_range(fd_in: fd_t, off_in: u64, fd_out: fd_t, off_out: u64, len: usize, flags: u32) CopyFileRangeError!usize {
        const call_cfr = comptime if (builtin.os.tag == .wasi)
            // WASI-libc doesn't have copy_file_range.

            false
        else if (builtin.link_libc)
            std.c.versionCheck(.{ .major = 2, .minor = 27, .patch = 0 }).ok
        else
            builtin.os.isAtLeast(.linux, .{ .major = 4, .minor = 5 }) orelse true;

        if (call_cfr and has_copy_file_range_syscall.load(.Monotonic)) {
            var off_in_copy = @bitCast(i64, off_in);
            var off_out_copy = @bitCast(i64, off_out);

            const rc = system.copy_file_range(fd_in, &off_in_copy, fd_out, &off_out_copy, len, flags);
            switch (system.getErrno(rc)) {
                .SUCCESS => return @intCast(usize, rc),
                .BADF => return error.FilesOpenedWithWrongFlags,
                .FBIG => return error.FileTooBig,
                .IO => return error.InputOutput,
                .ISDIR => return error.IsDir,
                .NOMEM => return error.OutOfMemory,
                .NOSPC => return error.NoSpaceLeft,
                .OVERFLOW => return error.Unseekable,
                .PERM => return error.PermissionDenied,
                .TXTBSY => return error.SwapFile,
                // these may not be regular files, try fallback

                .INVAL => {},
                // support for cross-filesystem copy added in Linux 5.3, use fallback

                .XDEV => {},
                // syscall added in Linux 4.5, use fallback

                .NOSYS => {
                    has_copy_file_range_syscall.store(false, .Monotonic);
                },
                else => |err| return unexpectedErrno(err),
            }
        }

        var buf: [8 * 4096]u8 = undefined;
        const adjusted_count = @min(buf.len, len);
        const amt_read = try pread(fd_in, buf[0..adjusted_count], off_in);
        // TODO without @as the line below fails to compile for wasm32-wasi:

        // error: integer value 0 cannot be coerced to type 'os.PWriteError!usize'

        if (amt_read == 0) return @as(usize, 0);
        return pwrite(fd_out, buf[0..amt_read], off_out);
    }

    pub const PollError = error{
        /// The network subsystem has failed.
        NetworkSubsystemFailed,

        /// The kernel had no space to allocate file descriptor tables.
        SystemResources,
    } || UnexpectedError;

    pub fn poll(fds: []pollfd, timeout: i32) PollError!usize {
        while (true) {
            const fds_count = math.cast(nfds_t, fds.len) orelse return error.SystemResources;
            const rc = system.poll(fds.ptr, fds_count, timeout);
            if (builtin.os.tag == .windows) {
                if (rc == windows.ws2_32.SOCKET_ERROR) {
                    switch (windows.ws2_32.WSAGetLastError()) {
                        .WSANOTINITIALISED => unreachable,
                        .WSAENETDOWN => return error.NetworkSubsystemFailed,
                        .WSAENOBUFS => return error.SystemResources,
                        // TODO: handle more errors

                        else => |err| return windows.unexpectedWSAError(err),
                    }
                } else {
                    return @intCast(usize, rc);
                }
            } else {
                switch (errno(rc)) {
                    .SUCCESS => return @intCast(usize, rc),
                    .FAULT => unreachable,
                    .INTR => continue,
                    .INVAL => unreachable,
                    .NOMEM => return error.SystemResources,
                    else => |err| return unexpectedErrno(err),
                }
            }
            unreachable;
        }
    }

    pub const PPollError = error{
        /// The operation was interrupted by a delivery of a signal before it could complete.
        SignalInterrupt,

        /// The kernel had no space to allocate file descriptor tables.
        SystemResources,
    } || UnexpectedError;

    pub fn ppoll(fds: []pollfd, timeout: ?*const timespec, mask: ?*const sigset_t) PPollError!usize {
        var ts: timespec = undefined;
        var ts_ptr: ?*timespec = null;
        if (timeout) |timeout_ns| {
            ts_ptr = &ts;
            ts = timeout_ns.*;
        }
        const fds_count = math.cast(nfds_t, fds.len) orelse return error.SystemResources;
        const rc = system.ppoll(fds.ptr, fds_count, ts_ptr, mask);
        switch (errno(rc)) {
            .SUCCESS => return @intCast(usize, rc),
            .FAULT => unreachable,
            .INTR => return error.SignalInterrupt,
            .INVAL => unreachable,
            .NOMEM => return error.SystemResources,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const RecvFromError = error{
        /// The socket is marked nonblocking and the requested operation would block, and
        /// there is no global event loop configured.
        WouldBlock,

        /// A remote host refused to allow the network connection, typically because it is not
        /// running the requested service.
        ConnectionRefused,

        /// Could not allocate kernel memory.
        SystemResources,

        ConnectionResetByPeer,

        /// The socket has not been bound.
        SocketNotBound,

        /// The UDP message was too big for the buffer and part of it has been discarded
        MessageTooBig,

        /// The network subsystem has failed.
        NetworkSubsystemFailed,

        /// The socket is not connected (connection-oriented sockets only).
        SocketNotConnected,
    } || UnexpectedError;

    pub fn recv(sock: socket_t, buf: []u8, flags: u32) RecvFromError!usize {
        return recvfrom(sock, buf, flags, null, null);
    }

    /// If `sockfd` is opened in non blocking mode, the function will
    /// return error.WouldBlock when EAGAIN is received.
    pub fn recvfrom(
        sockfd: socket_t,
        buf: []u8,
        flags: u32,
        src_addr: ?*sockaddr,
        addrlen: ?*socklen_t,
    ) RecvFromError!usize {
        while (true) {
            const rc = system.recvfrom(sockfd, buf.ptr, buf.len, flags, src_addr, addrlen);
            if (builtin.os.tag == .windows) {
                if (rc == windows.ws2_32.SOCKET_ERROR) {
                    switch (windows.ws2_32.WSAGetLastError()) {
                        .WSANOTINITIALISED => unreachable,
                        .WSAECONNRESET => return error.ConnectionResetByPeer,
                        .WSAEINVAL => return error.SocketNotBound,
                        .WSAEMSGSIZE => return error.MessageTooBig,
                        .WSAENETDOWN => return error.NetworkSubsystemFailed,
                        .WSAENOTCONN => return error.SocketNotConnected,
                        .WSAEWOULDBLOCK => return error.WouldBlock,
                        // TODO: handle more errors

                        else => |err| return windows.unexpectedWSAError(err),
                    }
                } else {
                    return @intCast(usize, rc);
                }
            } else {
                switch (errno(rc)) {
                    .SUCCESS => return @intCast(usize, rc),
                    .BADF => unreachable, // always a race condition

                    .FAULT => unreachable,
                    .INVAL => unreachable,
                    .NOTCONN => unreachable,
                    .NOTSOCK => unreachable,
                    .INTR => continue,
                    .AGAIN => return error.WouldBlock,
                    .NOMEM => return error.SystemResources,
                    .CONNREFUSED => return error.ConnectionRefused,
                    .CONNRESET => return error.ConnectionResetByPeer,
                    else => |err| return unexpectedErrno(err),
                }
            }
        }
    }

    pub const DnExpandError = error{InvalidDnsPacket};

    pub fn dn_expand(
        msg: []const u8,
        comp_dn: []const u8,
        exp_dn: []u8,
    ) DnExpandError!usize {
        // This implementation is ported from musl libc.

        // A more idiomatic "ziggy" implementation would be welcome.

        var p = comp_dn.ptr;
        var len: usize = std.math.maxInt(usize);
        const end = msg.ptr + msg.len;
        if (p == end or exp_dn.len == 0) return error.InvalidDnsPacket;
        var dest = exp_dn.ptr;
        const dend = dest + @min(exp_dn.len, 254);
        // detect reference loop using an iteration counter

        var i: usize = 0;
        while (i < msg.len) : (i += 2) {
            // loop invariants: p<end, dest<dend

            if ((p[0] & 0xc0) != 0) {
                if (p + 1 == end) return error.InvalidDnsPacket;
                var j = ((p[0] & @as(usize, 0x3f)) << 8) | p[1];
                if (len == std.math.maxInt(usize)) len = @ptrToInt(p) + 2 - @ptrToInt(comp_dn.ptr);
                if (j >= msg.len) return error.InvalidDnsPacket;
                p = msg.ptr + j;
            } else if (p[0] != 0) {
                if (dest != exp_dn.ptr) {
                    dest[0] = '.';
                    dest += 1;
                }
                var j = p[0];
                p += 1;
                if (j >= @ptrToInt(end) - @ptrToInt(p) or j >= @ptrToInt(dend) - @ptrToInt(dest)) {
                    return error.InvalidDnsPacket;
                }
                while (j != 0) {
                    j -= 1;
                    dest[0] = p[0];
                    dest += 1;
                    p += 1;
                }
            } else {
                dest[0] = 0;
                if (len == std.math.maxInt(usize)) len = @ptrToInt(p) + 1 - @ptrToInt(comp_dn.ptr);
                return len;
            }
        }
        return error.InvalidDnsPacket;
    }

    pub const SetSockOptError = error{
        /// The socket is already connected, and a specified option cannot be set while the socket is connected.
        AlreadyConnected,

        /// The option is not supported by the protocol.
        InvalidProtocolOption,

        /// The send and receive timeout values are too big to fit into the timeout fields in the socket structure.
        TimeoutTooBig,

        /// Insufficient resources are available in the system to complete the call.
        SystemResources,

        // Setting the socket option requires more elevated permissions.

        PermissionDenied,

        NetworkSubsystemFailed,
        FileDescriptorNotASocket,
        SocketNotBound,
        NoDevice,
    } || UnexpectedError;

    /// Set a socket's options.
    pub fn setsockopt(fd: socket_t, level: u32, optname: u32, opt: []const u8) SetSockOptError!void {
        if (builtin.os.tag == .windows) {
            const rc = windows.ws2_32.setsockopt(fd, @intCast(i32, level), @intCast(i32, optname), opt.ptr, @intCast(i32, opt.len));
            if (rc == windows.ws2_32.SOCKET_ERROR) {
                switch (windows.ws2_32.WSAGetLastError()) {
                    .WSANOTINITIALISED => unreachable,
                    .WSAENETDOWN => return error.NetworkSubsystemFailed,
                    .WSAEFAULT => unreachable,
                    .WSAENOTSOCK => return error.FileDescriptorNotASocket,
                    .WSAEINVAL => return error.SocketNotBound,
                    else => |err| return windows.unexpectedWSAError(err),
                }
            }
            return;
        } else {
            switch (errno(system.setsockopt(fd, level, optname, opt.ptr, @intCast(socklen_t, opt.len)))) {
                .SUCCESS => {},
                .BADF => unreachable, // always a race condition

                .NOTSOCK => unreachable, // always a race condition

                .INVAL => unreachable,
                .FAULT => unreachable,
                .DOM => return error.TimeoutTooBig,
                .ISCONN => return error.AlreadyConnected,
                .NOPROTOOPT => return error.InvalidProtocolOption,
                .NOMEM => return error.SystemResources,
                .NOBUFS => return error.SystemResources,
                .PERM => return error.PermissionDenied,
                .NODEV => return error.NoDevice,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const MemFdCreateError = error{
        SystemFdQuotaExceeded,
        ProcessFdQuotaExceeded,
        OutOfMemory,

        /// memfd_create is available in Linux 3.17 and later. This error is returned
        /// for older kernel versions.
        SystemOutdated,
    } || UnexpectedError;

    pub fn memfd_createZ(name: [*:0]const u8, flags: u32) MemFdCreateError!fd_t {
        switch (builtin.os.tag) {
            .linux => {
                // memfd_create is available only in glibc versions starting with 2.27.

                const use_c = std.c.versionCheck(.{ .major = 2, .minor = 27, .patch = 0 }).ok;
                const sys = if (use_c) std.c else linux;
                const getErrno = if (use_c) std.c.getErrno else linux.getErrno;
                const rc = sys.memfd_create(name, flags);
                switch (getErrno(rc)) {
                    .SUCCESS => return @intCast(fd_t, rc),
                    .FAULT => unreachable, // name has invalid memory

                    .INVAL => unreachable, // name/flags are faulty

                    .NFILE => return error.SystemFdQuotaExceeded,
                    .MFILE => return error.ProcessFdQuotaExceeded,
                    .NOMEM => return error.OutOfMemory,
                    .NOSYS => return error.SystemOutdated,
                    else => |err| return unexpectedErrno(err),
                }
            },
            .freebsd => {
                if (comptime builtin.os.version_range.semver.max.order(.{ .major = 13, .minor = 0 }) == .lt)
                    @compileError("memfd_create is unavailable on FreeBSD < 13.0");
                const rc = system.memfd_create(name, flags);
                switch (errno(rc)) {
                    .SUCCESS => return rc,
                    .BADF => unreachable, // name argument NULL

                    .INVAL => unreachable, // name too long or invalid/unsupported flags.

                    .MFILE => return error.ProcessFdQuotaExceeded,
                    .NFILE => return error.SystemFdQuotaExceeded,
                    .NOSYS => return error.SystemOutdated,
                    else => |err| return unexpectedErrno(err),
                }
            },
            else => @compileError("target OS does not support memfd_create()"),
        }
    }

    pub const MFD_NAME_PREFIX = "memfd:";
    pub const MFD_MAX_NAME_LEN = NAME_MAX - MFD_NAME_PREFIX.len;
    fn toMemFdPath(name: []const u8) ![MFD_MAX_NAME_LEN:0]u8 {
        var path_with_null: [MFD_MAX_NAME_LEN:0]u8 = undefined;
        // >= rather than > to make room for the null byte

        if (name.len >= MFD_MAX_NAME_LEN) return error.NameTooLong;
        mem.copy(u8, &path_with_null, name);
        path_with_null[name.len] = 0;
        return path_with_null;
    }

    pub fn memfd_create(name: []const u8, flags: u32) !fd_t {
        const name_t = try toMemFdPath(name);
        return memfd_createZ(&name_t, flags);
    }

    pub fn getrusage(who: i32) rusage {
        var result: rusage = undefined;
        const rc = system.getrusage(who, &result);
        switch (errno(rc)) {
            .SUCCESS => return result,
            .INVAL => unreachable,
            .FAULT => unreachable,
            else => unreachable,
        }
    }

    pub const TermiosGetError = error{NotATerminal} || UnexpectedError;

    pub fn tcgetattr(handle: fd_t) TermiosGetError!termios {
        while (true) {
            var term: termios = undefined;
            switch (errno(system.tcgetattr(handle, &term))) {
                .SUCCESS => return term,
                .INTR => continue,
                .BADF => unreachable,
                .NOTTY => return error.NotATerminal,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const TermiosSetError = TermiosGetError || error{ProcessOrphaned};

    pub fn tcsetattr(handle: fd_t, optional_action: TCSA, termios_p: termios) TermiosSetError!void {
        while (true) {
            switch (errno(system.tcsetattr(handle, optional_action, &termios_p))) {
                .SUCCESS => return,
                .BADF => unreachable,
                .INTR => continue,
                .INVAL => unreachable,
                .NOTTY => return error.NotATerminal,
                .IO => return error.ProcessOrphaned,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub const IoCtl_SIOCGIFINDEX_Error = error{
        FileSystem,
        InterfaceNotFound,
    } || UnexpectedError;

    pub fn ioctl_SIOCGIFINDEX(fd: fd_t, ifr: *ifreq) IoCtl_SIOCGIFINDEX_Error!void {
        while (true) {
            switch (errno(system.ioctl(fd, SIOCGIFINDEX, @ptrToInt(ifr)))) {
                .SUCCESS => return,
                .INVAL => unreachable, // Bad parameters.

                .NOTTY => unreachable,
                .NXIO => unreachable,
                .BADF => unreachable, // Always a race condition.

                .FAULT => unreachable, // Bad pointer parameter.

                .INTR => continue,
                .IO => return error.FileSystem,
                .NODEV => return error.InterfaceNotFound,
                else => |err| return unexpectedErrno(err),
            }
        }
    }

    pub fn signalfd(fd: fd_t, mask: *const sigset_t, flags: u32) !fd_t {
        const rc = system.signalfd(fd, mask, flags);
        switch (errno(rc)) {
            .SUCCESS => return @intCast(fd_t, rc),
            .BADF, .INVAL => unreachable,
            .NFILE => return error.SystemFdQuotaExceeded,
            .NOMEM => return error.SystemResources,
            .MFILE => return error.ProcessResources,
            .NODEV => return error.InodeMountFail,
            .NOSYS => return error.SystemOutdated,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const SyncError = error{
        InputOutput,
        NoSpaceLeft,
        DiskQuota,
        AccessDenied,
    } || UnexpectedError;

    /// Write all pending file contents and metadata modifications to all filesystems.
    pub fn sync() void {
        system.sync();
    }

    /// Write all pending file contents and metadata modifications to the filesystem which contains the specified file.
    pub fn syncfs(fd: fd_t) SyncError!void {
        const rc = system.syncfs(fd);
        switch (errno(rc)) {
            .SUCCESS => return,
            .BADF, .INVAL, .ROFS => unreachable,
            .IO => return error.InputOutput,
            .NOSPC => return error.NoSpaceLeft,
            .DQUOT => return error.DiskQuota,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Write all pending file contents and metadata modifications for the specified file descriptor to the underlying filesystem.
    pub fn fsync(fd: fd_t) SyncError!void {
        if (builtin.os.tag == .windows) {
            if (windows.kernel32.FlushFileBuffers(fd) != 0)
                return;
            switch (windows.kernel32.GetLastError()) {
                .SUCCESS => return,
                .INVALID_HANDLE => unreachable,
                .ACCESS_DENIED => return error.AccessDenied, // a sync was performed but the system couldn't update the access time

                .UNEXP_NET_ERR => return error.InputOutput,
                else => return error.InputOutput,
            }
        }
        const rc = system.fsync(fd);
        switch (errno(rc)) {
            .SUCCESS => return,
            .BADF, .INVAL, .ROFS => unreachable,
            .IO => return error.InputOutput,
            .NOSPC => return error.NoSpaceLeft,
            .DQUOT => return error.DiskQuota,
            else => |err| return unexpectedErrno(err),
        }
    }

    /// Write all pending file contents for the specified file descriptor to the underlying filesystem, but not necessarily the metadata.
    pub fn fdatasync(fd: fd_t) SyncError!void {
        if (builtin.os.tag == .windows) {
            return fsync(fd) catch |err| switch (err) {
                SyncError.AccessDenied => return, // fdatasync doesn't promise that the access time was synced

                else => return err,
            };
        }
        const rc = system.fdatasync(fd);
        switch (errno(rc)) {
            .SUCCESS => return,
            .BADF, .INVAL, .ROFS => unreachable,
            .IO => return error.InputOutput,
            .NOSPC => return error.NoSpaceLeft,
            .DQUOT => return error.DiskQuota,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const PrctlError = error{
        /// Can only occur with PR_SET_SECCOMP/SECCOMP_MODE_FILTER or
        /// PR_SET_MM/PR_SET_MM_EXE_FILE
        AccessDenied,
        /// Can only occur with PR_SET_MM/PR_SET_MM_EXE_FILE
        InvalidFileDescriptor,
        InvalidAddress,
        /// Can only occur with PR_SET_SPECULATION_CTRL, PR_MPX_ENABLE_MANAGEMENT,
        /// or PR_MPX_DISABLE_MANAGEMENT
        UnsupportedFeature,
        /// Can only occur wih PR_SET_FP_MODE
        OperationNotSupported,
        PermissionDenied,
    } || UnexpectedError;

    pub fn prctl(option: PR, args: anytype) PrctlError!u31 {
        if (@typeInfo(@TypeOf(args)) != .Struct)
            @compileError("Expected tuple or struct argument, found " ++ @typeName(@TypeOf(args)));
        if (args.len > 4)
            @compileError("prctl takes a maximum of 4 optional arguments");

        var buf: [4]usize = undefined;
        {
            comptime var i = 0;
            inline while (i < args.len) : (i += 1) buf[i] = args[i];
        }

        const rc = system.prctl(@enumToInt(option), buf[0], buf[1], buf[2], buf[3]);
        switch (errno(rc)) {
            .SUCCESS => return @intCast(u31, rc),
            .ACCES => return error.AccessDenied,
            .BADF => return error.InvalidFileDescriptor,
            .FAULT => return error.InvalidAddress,
            .INVAL => unreachable,
            .NODEV, .NXIO => return error.UnsupportedFeature,
            .OPNOTSUPP => return error.OperationNotSupported,
            .PERM, .BUSY => return error.PermissionDenied,
            .RANGE => unreachable,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const GetrlimitError = UnexpectedError;

    pub fn getrlimit(resource: rlimit_resource) GetrlimitError!rlimit {
        const getrlimit_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.getrlimit64
        else
            system.getrlimit;

        var limits: rlimit = undefined;
        switch (errno(getrlimit_sym(resource, &limits))) {
            .SUCCESS => return limits,
            .FAULT => unreachable, // bogus pointer

            .INVAL => unreachable,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const SetrlimitError = error{ PermissionDenied, LimitTooBig } || UnexpectedError;

    pub fn setrlimit(resource: rlimit_resource, limits: rlimit) SetrlimitError!void {
        const setrlimit_sym = if (builtin.os.tag == .linux and builtin.link_libc)
            system.setrlimit64
        else
            system.setrlimit;

        switch (errno(setrlimit_sym(resource, &limits))) {
            .SUCCESS => return,
            .FAULT => unreachable, // bogus pointer

            .INVAL => return error.LimitTooBig, // this could also mean "invalid resource", but that would be unreachable

            .PERM => return error.PermissionDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const MadviseError = error{
        /// advice is MADV.REMOVE, but the specified address range is not a shared writable mapping.
        AccessDenied,
        /// advice is MADV.HWPOISON, but the caller does not have the CAP_SYS_ADMIN capability.
        PermissionDenied,
        /// A kernel resource was temporarily unavailable.
        SystemResources,
        /// One of the following:
        /// * addr is not page-aligned or length is negative
        /// * advice is not valid
        /// * advice is MADV.DONTNEED or MADV.REMOVE and the specified address range
        ///   includes locked, Huge TLB pages, or VM_PFNMAP pages.
        /// * advice is MADV.MERGEABLE or MADV.UNMERGEABLE, but the kernel was not
        ///   configured with CONFIG_KSM.
        /// * advice is MADV.FREE or MADV.WIPEONFORK but the specified address range
        ///   includes file, Huge TLB, MAP.SHARED, or VM_PFNMAP ranges.
        InvalidSyscall,
        /// (for MADV.WILLNEED) Paging in this area would exceed the process's
        /// maximum resident set size.
        WouldExceedMaximumResidentSetSize,
        /// One of the following:
        /// * (for MADV.WILLNEED) Not enough memory: paging in failed.
        /// * Addresses in the specified range are not currently mapped, or
        ///   are outside the address space of the process.
        OutOfMemory,
        /// The madvise syscall is not available on this version and configuration
        /// of the Linux kernel.
        MadviseUnavailable,
        /// The operating system returned an undocumented error code.
        Unexpected,
    };

    /// Give advice about use of memory.
    /// This syscall is optional and is sometimes configured to be disabled.
    pub fn madvise(ptr: [*]align(mem.page_size) u8, length: usize, advice: u32) MadviseError!void {
        switch (errno(system.madvise(ptr, length, advice))) {
            .SUCCESS => return,
            .ACCES => return error.AccessDenied,
            .AGAIN => return error.SystemResources,
            .BADF => unreachable, // The map exists, but the area maps something that isn't a file.

            .INVAL => return error.InvalidSyscall,
            .IO => return error.WouldExceedMaximumResidentSetSize,
            .NOMEM => return error.OutOfMemory,
            .NOSYS => return error.MadviseUnavailable,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const PerfEventOpenError = error{
        /// Returned if the perf_event_attr size value is too small (smaller
        /// than PERF_ATTR_SIZE_VER0), too big (larger than the page  size),
        /// or  larger  than the kernel supports and the extra bytes are not
        /// zero.  When E2BIG is returned, the perf_event_attr size field is
        /// overwritten by the kernel to be the size of the structure it was
        /// expecting.
        TooBig,
        /// Returned when the requested event requires CAP_SYS_ADMIN permis‐
        /// sions  (or a more permissive perf_event paranoid setting).  Some
        /// common cases where an unprivileged process  may  encounter  this
        /// error:  attaching  to a process owned by a different user; moni‐
        /// toring all processes on a given CPU (i.e.,  specifying  the  pid
        /// argument  as  -1); and not setting exclude_kernel when the para‐
        /// noid setting requires it.
        /// Also:
        /// Returned on many (but not all) architectures when an unsupported
        /// exclude_hv,  exclude_idle,  exclude_user, or exclude_kernel set‐
        /// ting is specified.
        /// It can also happen, as with EACCES, when the requested event re‐
        /// quires   CAP_SYS_ADMIN   permissions   (or   a  more  permissive
        /// perf_event paranoid setting).  This includes  setting  a  break‐
        /// point on a kernel address, and (since Linux 3.13) setting a ker‐
        /// nel function-trace tracepoint.
        PermissionDenied,
        /// Returned if another event already has exclusive  access  to  the
        /// PMU.
        DeviceBusy,
        /// Each  opened  event uses one file descriptor.  If a large number
        /// of events are opened, the per-process limit  on  the  number  of
        /// open file descriptors will be reached, and no more events can be
        /// created.
        ProcessResources,
        EventRequiresUnsupportedCpuFeature,
        /// Returned if  you  try  to  add  more  breakpoint
        /// events than supported by the hardware.
        TooManyBreakpoints,
        /// Returned  if PERF_SAMPLE_STACK_USER is set in sample_type and it
        /// is not supported by hardware.
        SampleStackNotSupported,
        /// Returned if an event requiring a specific  hardware  feature  is
        /// requested  but  there is no hardware support.  This includes re‐
        /// questing low-skid events if not supported, branch tracing if  it
        /// is not available, sampling if no PMU interrupt is available, and
        /// branch stacks for software events.
        EventNotSupported,
        /// Returned  if  PERF_SAMPLE_CALLCHAIN  is   requested   and   sam‐
        /// ple_max_stack   is   larger   than   the  maximum  specified  in
        /// /proc/sys/kernel/perf_event_max_stack.
        SampleMaxStackOverflow,
        /// Returned if attempting to attach to a process that does not  exist.
        ProcessNotFound,
    } || UnexpectedError;

    pub fn perf_event_open(
        attr: *linux.perf_event_attr,
        pid: pid_t,
        cpu: i32,
        group_fd: fd_t,
        flags: usize,
    ) PerfEventOpenError!fd_t {
        const rc = system.perf_event_open(attr, pid, cpu, group_fd, flags);
        switch (errno(rc)) {
            .SUCCESS => return @intCast(fd_t, rc),
            .@"2BIG" => return error.TooBig,
            .ACCES => return error.PermissionDenied,
            .BADF => unreachable, // group_fd file descriptor is not valid.

            .BUSY => return error.DeviceBusy,
            .FAULT => unreachable, // Segmentation fault.

            .INVAL => unreachable, // Bad attr settings.

            .INTR => unreachable, // Mixed perf and ftrace handling for a uprobe.

            .MFILE => return error.ProcessResources,
            .NODEV => return error.EventRequiresUnsupportedCpuFeature,
            .NOENT => unreachable, // Invalid type setting.

            .NOSPC => return error.TooManyBreakpoints,
            .NOSYS => return error.SampleStackNotSupported,
            .OPNOTSUPP => return error.EventNotSupported,
            .OVERFLOW => return error.SampleMaxStackOverflow,
            .PERM => return error.PermissionDenied,
            .SRCH => return error.ProcessNotFound,
            else => |err| return unexpectedErrno(err),
        }
    }

    pub const TimerFdCreateError = error{
        AccessDenied,
        ProcessFdQuotaExceeded,
        SystemFdQuotaExceeded,
        NoDevice,
        SystemResources,
    } || UnexpectedError;

    pub const TimerFdGetError = error{InvalidHandle} || UnexpectedError;
    pub const TimerFdSetError = TimerFdGetError || error{Canceled};

    pub fn timerfd_create(clokid: i32, flags: u32) TimerFdCreateError!fd_t {
        var rc = linux.timerfd_create(clokid, flags);
        return switch (errno(rc)) {
            .SUCCESS => @intCast(fd_t, rc),
            .INVAL => unreachable,
            .MFILE => return error.ProcessFdQuotaExceeded,
            .NFILE => return error.SystemFdQuotaExceeded,
            .NODEV => return error.NoDevice,
            .NOMEM => return error.SystemResources,
            .PERM => return error.AccessDenied,
            else => |err| return unexpectedErrno(err),
        };
    }

    pub fn timerfd_settime(fd: i32, flags: u32, new_value: *const linux.itimerspec, old_value: ?*linux.itimerspec) TimerFdSetError!void {
        var rc = linux.timerfd_settime(fd, flags, new_value, old_value);
        return switch (errno(rc)) {
            .SUCCESS => {},
            .BADF => error.InvalidHandle,
            .FAULT => unreachable,
            .INVAL => unreachable,
            .CANCELED => error.Canceled,
            else => |err| return unexpectedErrno(err),
        };
    }

    pub fn timerfd_gettime(fd: i32) TimerFdGetError!linux.itimerspec {
        var curr_value: linux.itimerspec = undefined;
        var rc = linux.timerfd_gettime(fd, &curr_value);
        return switch (errno(rc)) {
            .SUCCESS => return curr_value,
            .BADF => error.InvalidHandle,
            .FAULT => unreachable,
            .INVAL => unreachable,
            else => |err| return unexpectedErrno(err),
        };
    }