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//! 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.
//!
//! Example:
//! ```
//! var s = Semaphore{};
//!
//! fn consumer() void {
//! s.wait();
//! }
//!
//! fn producer() void {
//! s.post();
//! }
//!
//! const thread = try std.Thread.spawn(.{}, producer, .{});
//! consumer();
//! thread.join();
//! ```
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 timedWait(sem: *Semaphore, timeout_ns: u64) error{Timeout}!void {
var timeout_timer = std.time.Timer.start() catch unreachable;
sem.mutex.lock();
defer sem.mutex.unlock();
while (sem.permits == 0) {
const elapsed = timeout_timer.read();
if (elapsed > timeout_ns)
return error.Timeout;
const local_timeout_ns = timeout_ns - elapsed;
try sem.cond.timedWait(&sem.mutex, local_timeout_ns);
}
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 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);
}
test timedWait {
var sem = Semaphore{};
try testing.expectEqual(0, sem.permits);
try testing.expectError(error.Timeout, sem.timedWait(1));
sem.post();
try testing.expectEqual(1, sem.permits);
try sem.timedWait(1);
try testing.expectEqual(0, sem.permits);
}
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