1//===----------------------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include <__mutex/once_flag.h>
10#include <__utility/exception_guard.h>
11
12#ifndef _LIBCPP_HAS_NO_THREADS
13# include <__thread/support.h>
14#endif
15
16#include "include/atomic_support.h"
17
18_LIBCPP_BEGIN_NAMESPACE_STD
19
20// If dispatch_once_f ever handles C++ exceptions, and if one can get to it
21// without illegal macros (unexpected macros not beginning with _UpperCase or
22// __lowercase), and if it stops spinning waiting threads, then call_once should
23// call into dispatch_once_f instead of here. Relevant radar this code needs to
24// keep in sync with: 7741191.
25
26#ifndef _LIBCPP_HAS_NO_THREADS
27static constinit __libcpp_mutex_t mut = _LIBCPP_MUTEX_INITIALIZER;
28static constinit __libcpp_condvar_t cv = _LIBCPP_CONDVAR_INITIALIZER;
29#endif
30
31void __call_once(volatile once_flag::_State_type& flag, void* arg, void (*func)(void*)) {
32#if defined(_LIBCPP_HAS_NO_THREADS)
33
34 if (flag == once_flag::_Unset) {
35 auto guard = std::__make_exception_guard([&flag] { flag = once_flag::_Unset; });
36 flag = once_flag::_Pending;
37 func(arg);
38 flag = once_flag::_Complete;
39 guard.__complete();
40 }
41
42#else // !_LIBCPP_HAS_NO_THREADS
43
44 __libcpp_mutex_lock(&mut);
45 while (flag == once_flag::_Pending)
46 __libcpp_condvar_wait(&cv, &mut);
47 if (flag == once_flag::_Unset) {
48 auto guard = std::__make_exception_guard(ex: [&flag] {
49 __libcpp_mutex_lock(&mut);
50 __libcpp_relaxed_store(&flag, once_flag::_Unset);
51 __libcpp_mutex_unlock(&mut);
52 __libcpp_condvar_broadcast(&cv);
53 });
54
55 __libcpp_relaxed_store(&flag, once_flag::_Pending);
56 __libcpp_mutex_unlock(&mut);
57 func(arg);
58 __libcpp_mutex_lock(&mut);
59 __libcpp_atomic_store(&flag, once_flag::_Complete, _AO_Release);
60 __libcpp_mutex_unlock(&mut);
61 __libcpp_condvar_broadcast(&cv);
62 guard.__complete();
63 } else {
64 __libcpp_mutex_unlock(&mut);
65 }
66
67#endif // !_LIBCPP_HAS_NO_THREADS
68}
69
70_LIBCPP_END_NAMESPACE_STD
71

source code of libcxx/src/call_once.cpp