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