[libc-commits] [libc] [libc] Mutex implementation for single-threaded baremetal (PR #145358)

[William Huynh via libc-commits]

https://github.com/saturn691 created https://github.com/llvm/llvm-project/pull/145358

In order to add, https://github.com/llvm/llvm-project/issues/145349, we need a Mutex, to allow `atexit` to work.

>From d47e596dbdf02e624f0ed3a80eb0ee1cf9507e8b Mon Sep 17 00:00:00 2001
From: William Huynh <William.Huynh at arm.com>
Date: Mon, 23 Jun 2025 17:30:31 +0100
Subject: [PATCH] [libc] Mutex implementation for single-threaded baremetal

---
 .../threads/baremetal/CMakeLists.txt          |  5 +++
 libc/src/__support/threads/baremetal/mutex.h  | 32 +++++++++++++++++++
 libc/src/__support/threads/mutex.h            |  2 ++
 3 files changed, 39 insertions(+)
 create mode 100644 libc/src/__support/threads/baremetal/CMakeLists.txt
 create mode 100644 libc/src/__support/threads/baremetal/mutex.h

diff --git a/libc/src/__support/threads/baremetal/CMakeLists.txt b/libc/src/__support/threads/baremetal/CMakeLists.txt
new file mode 100644
index 0000000000000..ea89feb0c5c68
--- /dev/null
+++ b/libc/src/__support/threads/baremetal/CMakeLists.txt
@@ -0,0 +1,5 @@
+add_header_library(
+  mutex
+  HDRS
+    mutex.h
+)
diff --git a/libc/src/__support/threads/baremetal/mutex.h b/libc/src/__support/threads/baremetal/mutex.h
new file mode 100644
index 0000000000000..77a0b61ea9f5b
--- /dev/null
+++ b/libc/src/__support/threads/baremetal/mutex.h
@@ -0,0 +1,32 @@
+//===--- Implementation of a mutex class for baremetal ----------*- C++ -*-===//
+//
+// 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___SUPPORT_THREADS_BAREMETAL_MUTEX_H
+#define LLVM_LIBC_SRC___SUPPORT_THREADS_BAREMETAL_MUTEX_H
+
+#include "src/__support/macros/attributes.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/threads/mutex_common.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+/// Implementation of a simple passthrough mutex which guards nothing. For
+/// single threaded processors, this is the implementation.
+struct Mutex {
+  LIBC_INLINE constexpr Mutex(bool, bool, bool, bool) {}
+
+  LIBC_INLINE MutexError lock() { return MutexError::NONE; }
+  LIBC_INLINE MutexError unlock() { return MutexError::NONE; }
+  LIBC_INLINE MutexError reset() { return MutexError::NONE; }
+};
+
+// TODO: add multithreading support here
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_THREADS_BAREMETAL_MUTEX_H
diff --git a/libc/src/__support/threads/mutex.h b/libc/src/__support/threads/mutex.h
index 392b38984dc0a..a600e2ff017f5 100644
--- a/libc/src/__support/threads/mutex.h
+++ b/libc/src/__support/threads/mutex.h
@@ -41,6 +41,8 @@
 #include "src/__support/threads/linux/mutex.h"
 #elif defined(LIBC_TARGET_ARCH_IS_GPU)
 #include "src/__support/threads/gpu/mutex.h"
+#elif defined(__ELF__)
+#include "src/__support/threads/baremetal/mutex.h"
 #endif // __linux__
 
 #endif // LLVM_LIBC_SRC___SUPPORT_THREADS_MUTEX_H