    //! A semaphore is an unsigned integer that blocks the kernel thread if
    //! the number would become negative.
    //! This API supports static initialization and does not require deinitialization.

    mutex: Mutex = .{},
    cond: Condition = .{},
    /// It is OK to initialize this field to any value.
    permits: usize = 0,

    const Semaphore = @This();
    const std = @import("../std.zig");
    const Mutex = std.Thread.Mutex;
    const Condition = std.Thread.Condition;
    const builtin = @import("builtin");
    const testing = std.testing;

    pub fn wait(sem: *Semaphore) void {
        sem.mutex.lock();
        defer sem.mutex.unlock();

        while (sem.permits == 0)
            sem.cond.wait(&sem.mutex);

        sem.permits -= 1;
        if (sem.permits > 0)
            sem.cond.signal();
    }

    pub fn post(sem: *Semaphore) void {
        sem.mutex.lock();
        defer sem.mutex.unlock();

        sem.permits += 1;
        sem.cond.signal();
    }

    test "Thread.Semaphore" {
        if (builtin.single_threaded) {
            return error.SkipZigTest;
        }

        const TestContext = struct {
            sem: *Semaphore,
            n: *i32,
            fn worker(ctx: *@This()) void {
                ctx.sem.wait();
                ctx.n.* += 1;
                ctx.sem.post();
            }
        };
        const num_threads = 3;
        var sem = Semaphore{ .permits = 1 };
        var threads: [num_threads]std.Thread = undefined;
        var n: i32 = 0;
        var ctx = TestContext{ .sem = &sem, .n = &n };

        for (threads) |*t| t.* = try std.Thread.spawn(.{}, TestContext.worker, .{&ctx});
        for (threads) |t| t.join();
        sem.wait();
        try testing.expect(n == num_threads);
    }