[libcxx-commits] [libcxx] [libc++] `[[nodiscard]]` to concurrency (PR #169463)
Hristo Hristov via libcxx-commits
libcxx-commits at lists.llvm.org
Mon Nov 24 23:10:09 PST 2025
https://github.com/H-G-Hristov created https://github.com/llvm/llvm-project/pull/169463
`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
The following utilities have been anotated in this patch:
- [x] `<barrier>`
- [x] `<condition_variable>`
- [x] `<latch>`
- [x] `<mutex>`
- [x] `<semaphore>`
- [x] `<thread>`
>From 8267f0dc0c76f69eecaa55752f0e3ff8f1590e0a Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sun, 23 Nov 2025 14:21:21 +0200
Subject: [PATCH] [libc++] `[[nodiscard]]` to concurrency
`[[nodiscard]]`` should be applied to functions where discarding the return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
The following utilities have been anotated in this patch:
- [x] `<barrier>`
- [x] `<condition_variable>`
- [x] `<latch>`
- [x] `<mutex>`
- [x] `<semaphore>`
- [x] `<thread>`
---
.../__condition_variable/condition_variable.h | 2 +-
libcxx/include/__mutex/mutex.h | 2 +-
libcxx/include/__thread/thread.h | 8 +-
libcxx/include/barrier | 2 +-
libcxx/include/latch | 2 +-
libcxx/include/mutex | 2 +-
libcxx/include/semaphore | 2 +-
.../test/libcxx/thread/nodiscard.verify.cpp | 94 +++++++++++++++++++
.../thread.jthread/nodiscard.verify.cpp | 29 ------
9 files changed, 104 insertions(+), 39 deletions(-)
create mode 100644 libcxx/test/libcxx/thread/nodiscard.verify.cpp
delete mode 100644 libcxx/test/std/thread/thread.jthread/nodiscard.verify.cpp
diff --git a/libcxx/include/__condition_variable/condition_variable.h b/libcxx/include/__condition_variable/condition_variable.h
index 1e8edd5dcb009..b7151930e9226 100644
--- a/libcxx/include/__condition_variable/condition_variable.h
+++ b/libcxx/include/__condition_variable/condition_variable.h
@@ -170,7 +170,7 @@ class _LIBCPP_EXPORTED_FROM_ABI condition_variable {
wait_for(unique_lock<mutex>& __lk, const chrono::duration<_Rep, _Period>& __d, _Predicate __pred);
typedef __libcpp_condvar_t* native_handle_type;
- _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() { return &__cv_; }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() { return &__cv_; }
private:
void
diff --git a/libcxx/include/__mutex/mutex.h b/libcxx/include/__mutex/mutex.h
index 68c8842b35eda..1fbe09f266832 100644
--- a/libcxx/include/__mutex/mutex.h
+++ b/libcxx/include/__mutex/mutex.h
@@ -41,7 +41,7 @@ class _LIBCPP_EXPORTED_FROM_ABI _LIBCPP_CAPABILITY("mutex") mutex {
_LIBCPP_RELEASE_CAPABILITY void unlock() _NOEXCEPT;
typedef __libcpp_mutex_t* native_handle_type;
- _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() { return &__m_; }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() { return &__m_; }
};
static_assert(is_nothrow_default_constructible<mutex>::value, "the default constructor for std::mutex must be nothrow");
diff --git a/libcxx/include/__thread/thread.h b/libcxx/include/__thread/thread.h
index a3b672bc0f0e7..561f092ddb7c0 100644
--- a/libcxx/include/__thread/thread.h
+++ b/libcxx/include/__thread/thread.h
@@ -242,13 +242,13 @@ class _LIBCPP_EXPORTED_FROM_ABI thread {
_LIBCPP_HIDE_FROM_ABI void swap(thread& __t) _NOEXCEPT { std::swap(__t_, __t.__t_); }
- _LIBCPP_HIDE_FROM_ABI bool joinable() const _NOEXCEPT { return !__libcpp_thread_isnull(&__t_); }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI bool joinable() const _NOEXCEPT { return !__libcpp_thread_isnull(&__t_); }
void join();
void detach();
- _LIBCPP_HIDE_FROM_ABI id get_id() const _NOEXCEPT { return __libcpp_thread_get_id(&__t_); }
- _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() _NOEXCEPT { return __t_; }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI id get_id() const _NOEXCEPT { return __libcpp_thread_get_id(&__t_); }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() _NOEXCEPT { return __t_; }
- static unsigned hardware_concurrency() _NOEXCEPT;
+ [[__nodiscard__]] static unsigned hardware_concurrency() _NOEXCEPT;
};
inline _LIBCPP_HIDE_FROM_ABI void swap(thread& __x, thread& __y) _NOEXCEPT { __x.swap(__y); }
diff --git a/libcxx/include/barrier b/libcxx/include/barrier
index 41fbfb3e8fb7b..66f1012ccf4a9 100644
--- a/libcxx/include/barrier
+++ b/libcxx/include/barrier
@@ -158,7 +158,7 @@ class barrier {
public:
using arrival_token = typename __barrier_base<_CompletionF>::arrival_token;
- static _LIBCPP_HIDE_FROM_ABI constexpr ptrdiff_t max() noexcept { return __barrier_base<_CompletionF>::max(); }
+ [[nodiscard]] static _LIBCPP_HIDE_FROM_ABI constexpr ptrdiff_t max() noexcept { return __barrier_base<_CompletionF>::max(); }
_LIBCPP_HIDE_FROM_ABI explicit barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF())
: __b_(__count, std::move(__completion)) {
diff --git a/libcxx/include/latch b/libcxx/include/latch
index c3b8f62e9b50e..d5ff63a5fcfd2 100644
--- a/libcxx/include/latch
+++ b/libcxx/include/latch
@@ -70,7 +70,7 @@ class latch {
atomic<ptrdiff_t> __a_;
public:
- static _LIBCPP_HIDE_FROM_ABI constexpr ptrdiff_t max() noexcept { return numeric_limits<ptrdiff_t>::max(); }
+ [[nodiscard]] static _LIBCPP_HIDE_FROM_ABI constexpr ptrdiff_t max() noexcept { return numeric_limits<ptrdiff_t>::max(); }
inline _LIBCPP_HIDE_FROM_ABI constexpr explicit latch(ptrdiff_t __expected) : __a_(__expected) {
_LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
diff --git a/libcxx/include/mutex b/libcxx/include/mutex
index 0b81f1bb1c8a6..1c31bafc0da6b 100644
--- a/libcxx/include/mutex
+++ b/libcxx/include/mutex
@@ -234,7 +234,7 @@ public:
typedef __libcpp_recursive_mutex_t* native_handle_type;
- _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() { return &__m_; }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() { return &__m_; }
};
class _LIBCPP_EXPORTED_FROM_ABI timed_mutex {
diff --git a/libcxx/include/semaphore b/libcxx/include/semaphore
index 99c4ad24b35ec..d490d4f4abfb3 100644
--- a/libcxx/include/semaphore
+++ b/libcxx/include/semaphore
@@ -133,7 +133,7 @@ class counting_semaphore {
public:
static_assert(__least_max_value >= 0, "The least maximum value must be a positive number");
- static constexpr ptrdiff_t max() noexcept { return __least_max_value; }
+ [[nodiscard]] static constexpr ptrdiff_t max() noexcept { return __least_max_value; }
_LIBCPP_HIDE_FROM_ABI constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore_(__count) {
_LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
diff --git a/libcxx/test/libcxx/thread/nodiscard.verify.cpp b/libcxx/test/libcxx/thread/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..47ede0321ea77
--- /dev/null
+++ b/libcxx/test/libcxx/thread/nodiscard.verify.cpp
@@ -0,0 +1,94 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+// UNSUPPORTED: no-threads
+
+// Check that functions are marked [[nodiscard]]
+
+#include <barrier>
+#include <latch>
+#include <mutex>
+#include <semaphore>
+#include <thread>
+
+#include "test_macros.h"
+
+void test() {
+ // Threads
+ {
+ std::thread th;
+
+ th.joinable(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ th.get_id(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ th.native_handle(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ th.hardware_concurrency(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+#if TEST_STD_VER >= 20
+ {
+ std::jthread jt;
+
+ jt.joinable(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ jt.get_id(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ jt.native_handle(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ jt.get_stop_source(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ jt.get_stop_token(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ jt.hardware_concurrency(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+#endif
+
+ // Mutual exclusion
+
+ { // <mutex>
+ std::mutex m;
+
+ m.native_handle(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+ {
+ std::recursive_mutex m;
+
+ m.native_handle(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+
+ // Condition variables
+
+ { // <condition_variable>
+ std::condition_variable cv;
+
+ cv.native_handle(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+
+#if TEST_STD_VER >= 20
+
+ // Semaphores
+
+ { // <semaphor>
+ std::counting_semaphore<> cs{0};
+
+ cs.max(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ std::binary_semaphore bs{0};
+
+ bs.max(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+
+ // Latches and barriers
+
+ { // <barrier>
+ std::barrier<> b{94};
+
+ b.max(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+ { // <latch>
+ std::latch l{94};
+
+ l.max(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+
+#endif
+}
\ No newline at end of file
diff --git a/libcxx/test/std/thread/thread.jthread/nodiscard.verify.cpp b/libcxx/test/std/thread/thread.jthread/nodiscard.verify.cpp
deleted file mode 100644
index 2ef5cf874da90..0000000000000
--- a/libcxx/test/std/thread/thread.jthread/nodiscard.verify.cpp
+++ /dev/null
@@ -1,29 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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
-//
-//===----------------------------------------------------------------------===//
-//
-// UNSUPPORTED: no-threads
-// UNSUPPORTED: c++03, c++11, c++14, c++17
-
-// [[nodiscard]] bool joinable() const noexcept;
-// [[nodiscard]] id get_id() const noexcept;
-// [[nodiscard]] native_handle_type native_handle();
-// [[nodiscard]] stop_source get_stop_source() noexcept;
-// [[nodiscard]] stop_token get_stop_token() const noexcept;
-// [[nodiscard]] static unsigned int hardware_concurrency() noexcept;
-
-#include <thread>
-
-void test() {
- std::jthread jt;
- jt.joinable(); // expected-warning {{ignoring return value of function}}
- jt.get_id(); // expected-warning {{ignoring return value of function}}
- jt.native_handle(); // expected-warning {{ignoring return value of function}}
- jt.get_stop_source(); // expected-warning {{ignoring return value of function}}
- jt.get_stop_token(); // expected-warning {{ignoring return value of function}}
- jt.hardware_concurrency(); // expected-warning {{ignoring return value of function}}
-}
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