[libc-commits] [libc] eb6075e - [libc][semaphore] Add internal unnamed semaphore implementation (#190851)
via libc-commits
libc-commits at lists.llvm.org
Thu Apr 9 16:47:54 PDT 2026
Author: Pengxiang Huang
Date: 2026-04-09T16:47:49-07:00
New Revision: eb6075e3be0a40301f008b0b3640ed125b66929f
URL: https://github.com/llvm/llvm-project/commit/eb6075e3be0a40301f008b0b3640ed125b66929f
DIFF: https://github.com/llvm/llvm-project/commit/eb6075e3be0a40301f008b0b3640ed125b66929f.diff
LOG: [libc][semaphore] Add internal unnamed semaphore implementation (#190851)
Implements the first part for #190847
Add internal unnamed semaphore lifetime support, particularly:
`sem_init`:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/sem_init.html#
`sem_destroy`:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/sem_destroy.html#
`sem_getvalue`:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/sem_getvalue.html#
Added:
libc/src/semaphore/CMakeLists.txt
libc/src/semaphore/posix_semaphore.h
libc/test/src/semaphore/CMakeLists.txt
libc/test/src/semaphore/semaphore_test.cpp
Modified:
libc/src/CMakeLists.txt
libc/test/src/CMakeLists.txt
Removed:
################################################################################
diff --git a/libc/src/CMakeLists.txt b/libc/src/CMakeLists.txt
index 8a0acccaed708..e4b0de3d5c75d 100644
--- a/libc/src/CMakeLists.txt
+++ b/libc/src/CMakeLists.txt
@@ -25,6 +25,7 @@ if(${LIBC_TARGET_OS} STREQUAL "linux")
add_subdirectory(poll)
add_subdirectory(pthread)
add_subdirectory(sched)
+ add_subdirectory(semaphore)
add_subdirectory(sys)
add_subdirectory(termios)
endif()
diff --git a/libc/src/semaphore/CMakeLists.txt b/libc/src/semaphore/CMakeLists.txt
new file mode 100644
index 0000000000000..ac73eb8e87b56
--- /dev/null
+++ b/libc/src/semaphore/CMakeLists.txt
@@ -0,0 +1,8 @@
+add_header_library(
+ posix_semaphore
+ HDRS
+ posix_semaphore.h
+ DEPENDS
+ libc.src.__support.CPP.atomic
+ libc.src.__support.threads.linux.futex_utils
+)
diff --git a/libc/src/semaphore/posix_semaphore.h b/libc/src/semaphore/posix_semaphore.h
new file mode 100644
index 0000000000000..7b14d41165a96
--- /dev/null
+++ b/libc/src/semaphore/posix_semaphore.h
@@ -0,0 +1,59 @@
+//===-- Internal Semaphore implementation 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 "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;
+
+ // A private constant canary used to detect use of uninitialized or
+ // destroyed semaphores. Chose "SEM1" in ASCII (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 constexpr Semaphore(unsigned int value)
+ : value(value), canary(SEM_CANARY) {}
+
+ // Sanity check to detect use of uninitialized or destroyed semaphores.
+ LIBC_INLINE bool is_valid() const { return canary == SEM_CANARY; }
+
+ LIBC_INLINE void destroy() {
+ // Destroying a semaphore while threads are blocked on it is undefined
+ // behavior. Similarly, using a destroyed semaphore is undefined.
+ // Therefore no concurrency safe destruction is required here,
+ // RELAXED memory ordering is sufficient.
+ value.store(0, cpp::MemoryOrder::RELAXED);
+ canary = 0;
+ }
+
+ LIBC_INLINE int getvalue() const {
+ // get value is informational, not a synchronization op.
+ // RELAXED ordering is enough.
+ // TODO: handle the case where the semaphore is locked.
+ return static_cast<int>(
+ const_cast<Futex &>(value).load(cpp::MemoryOrder::RELAXED));
+ }
+};
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_SEMAPHORE_POSIX_SEMAPHORE_H
diff --git a/libc/test/src/CMakeLists.txt b/libc/test/src/CMakeLists.txt
index 5e61c739c066a..236bee337d525 100644
--- a/libc/test/src/CMakeLists.txt
+++ b/libc/test/src/CMakeLists.txt
@@ -106,4 +106,5 @@ add_subdirectory(spawn)
if(${LIBC_TARGET_OS} STREQUAL "linux")
add_subdirectory(pthread)
+ add_subdirectory(semaphore)
endif()
diff --git a/libc/test/src/semaphore/CMakeLists.txt b/libc/test/src/semaphore/CMakeLists.txt
new file mode 100644
index 0000000000000..0c458db5d3669
--- /dev/null
+++ b/libc/test/src/semaphore/CMakeLists.txt
@@ -0,0 +1,11 @@
+add_custom_target(libc_semaphore_unittests)
+
+add_libc_unittest(
+ semaphore_test
+ SUITE
+ libc_semaphore_unittests
+ SRCS
+ semaphore_test.cpp
+ DEPENDS
+ libc.src.semaphore.posix_semaphore
+)
diff --git a/libc/test/src/semaphore/semaphore_test.cpp b/libc/test/src/semaphore/semaphore_test.cpp
new file mode 100644
index 0000000000000..241078ef5a09c
--- /dev/null
+++ b/libc/test/src/semaphore/semaphore_test.cpp
@@ -0,0 +1,25 @@
+//===-- Unittests for the internal Semaphore class ------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/semaphore/posix_semaphore.h"
+#include "test/UnitTest/Test.h"
+
+using LIBC_NAMESPACE::Semaphore;
+
+TEST(LlvmLibcSemaphoreTest, InitAndGetValue) {
+ Semaphore sem(3);
+ ASSERT_TRUE(sem.is_valid());
+ ASSERT_EQ(sem.getvalue(), 3);
+}
+
+TEST(LlvmLibcSemaphoreTest, Destroy) {
+ Semaphore sem(5);
+ ASSERT_TRUE(sem.is_valid());
+ sem.destroy();
+ ASSERT_FALSE(sem.is_valid());
+}
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