[libc-commits] [libc] [libc][semaphore] Add unnamed semaphore implementation (PR #190851)

[Michael Jones via libc-commits]

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@@ -0,0 +1,67 @@
+//===-- Internal Semaphore types and helpers for POSIX semaphores ---------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SEMAPHORE_POSIX_SEMAPHORE_H
+#define LLVM_LIBC_SRC_SEMAPHORE_POSIX_SEMAPHORE_H
+
+#include "hdr/types/sem_t.h"
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/common.h"
+#include "src/__support/threads/linux/futex_utils.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+class Semaphore {
+  Futex value;
+  unsigned int canary;
+
+  // 0x53 = S, 0x45 = E, 0x4D = M, 0x31 = 1
+  static constexpr unsigned int SEM_CANARY = 0x53454D31U;
+
+public:
+  // TODO:
+  // 1. Add named semaphore support: sem_open, sem_close, sem_unlink
+  // 2. Add the posting and waiting operations: sem_post, sem_wait,
+  //    sem_trywait, sem_timedwait, sem_clockwait.
+
+  LIBC_INLINE void init(unsigned int initial_value) {
+    // init happens before the semaphore is published to any threads
+    // initialize a initialized semaphore is undefined
+    // so RELAXED ordering is enough
+    value.store(initial_value, cpp::MemoryOrder::RELAXED);
+    canary = SEM_CANARY;
+  }
+
+  // sanity check for a given semaphore pointer
+  LIBC_INLINE bool is_valid() const { return canary == SEM_CANARY; }
+
+  LIBC_INLINE void destroy() {
+    // invalidate is safe only when no threads is using
+    // blocked by destroyed semaphore is undefined
+    // use a destroyed semaphore is undefined
+    // RELAXED ordering is enough
----------------
michaelrj-google wrote:

This phrasing is confusing. Could you clarify?

https://github.com/llvm/llvm-project/pull/190851