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// UNSUPPORTED: no-threads
10
11// This test occasionally fails on Android.
12// UNSUPPORTED: LIBCXX-ANDROID-FIXME
13
14// <condition_variable>
15
16// class condition_variable;
17
18// void notify_one();
19
20
21// NOTE: `notify_one` is just a wrapper around pthread_cond_signal, but
22// POSIX does not guarantee that one and only one thread will be woken:
23//
24// https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_signal.html
25//
26// Quote:
27// Multiple Awakenings by Condition Signal
28// On a multi-processor, it may be impossible for an implementation of
29// pthread_cond_signal() to avoid the unblocking of more than one thread
30// blocked on a condition variable. For example...
31
32
33
34// NOTE: In previous versions of this test, `notify_one` was called WITHOUT
35// holding the lock but POSIX says (in the aforementioned URL) that:
36// ...if predictable scheduling behavior is required, then that mutex shall
37// be locked by the thread calling pthread_cond_broadcast() or
38// pthread_cond_signal().
39
40
41#include <condition_variable>
42#include <atomic>
43#include <mutex>
44#include <thread>
45#include <cassert>
46
47#include "make_test_thread.h"
48#include "test_macros.h"
49
50
51std::condition_variable cv;
52std::mutex mut;
53
54std::atomic_int test1(0);
55std::atomic_int test2(0);
56std::atomic_int ready(2);
57std::atomic_int which(0);
58
59void f1()
60{
61 std::unique_lock<std::mutex> lk(mut);
62 assert(test1 == 0);
63 --ready;
64 while (test1 == 0)
65 cv.wait(lock&: lk);
66 which = 1;
67 assert(test1 == 1);
68 test1 = 2;
69}
70
71void f2()
72{
73 std::unique_lock<std::mutex> lk(mut);
74 assert(test2 == 0);
75 --ready;
76 while (test2 == 0)
77 cv.wait(lock&: lk);
78 which = 2;
79 assert(test2 == 1);
80 test2 = 2;
81}
82
83int main(int, char**)
84{
85 std::thread t1 = support::make_test_thread(f1);
86 std::thread t2 = support::make_test_thread(f2);
87 {
88 while (ready > 0)
89 std::this_thread::yield();
90 // At this point:
91 // 1) Both f1 and f2 have entered their condition variable wait.
92 // 2) Either f1 or f2 has the mutex locked and is about to wait.
93 std::unique_lock<std::mutex> lk(mut);
94 test1 = 1;
95 test2 = 1;
96 ready = 1;
97 cv.notify_one();
98 }
99 {
100 while (which == 0)
101 std::this_thread::yield();
102 std::unique_lock<std::mutex> lk(mut);
103 if (test1 == 2) {
104 assert(test2 == 1);
105 t1.join();
106 test1 = 0;
107 } else {
108 assert(test1 == 1);
109 assert(test2 == 2);
110 t2.join();
111 test2 = 0;
112 }
113 which = 0;
114 cv.notify_one();
115 }
116 {
117 while (which == 0)
118 std::this_thread::yield();
119 std::unique_lock<std::mutex> lk(mut);
120 if (test1 == 2) {
121 assert(test2 == 0);
122 t1.join();
123 test1 = 0;
124 } else {
125 assert(test1 == 0);
126 assert(test2 == 2);
127 t2.join();
128 test2 = 0;
129 }
130 }
131
132 return 0;
133}
134

source code of libcxx/test/std/thread/thread.condition/thread.condition.condvar/notify_one.pass.cpp