1 files changed, 39 insertions, 0 deletions
diff --git a/lib/std/Thread/Semaphore.zig b/lib/std/Thread/Semaphore.zig
new file mode 100644
index 0000000000..77a278b355
--- /dev/null
+++ b/lib/std/Thread/Semaphore.zig
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2015-2021 Zig Contributors
+// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
+// The MIT license requires this copyright notice to be included in all copies
+// and substantial portions of the software.
+
+//! 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 RwLock = @This();
+const std = @import("../std.zig");
+const Mutex = std.Thread.Mutex;
+const Condition = std.Thread.Condition;
+
+pub fn wait(sem: *Semaphore) void {
+    const held = sem.mutex.acquire();
+    defer held.release();
+
+    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 {
+    const held = sem.mutex.acquire();
+    defer held.release();
+
+    sem.permits += 1;
+    sem.cond.signal();
+}