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//===----------------------------------------------------------------------===//
//
// 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: libcpp-has-no-threads
// <condition_variable>
// class condition_variable;
// void notify_one();
// NOTE: `notify_one` is just a wrapper around pthread_cond_signal, but
// POSIX does not guarantee that one and only one thread will be woken:
//
// https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_signal.html
//
// Quote:
// Multiple Awakenings by Condition Signal
// On a multi-processor, it may be impossible for an implementation of
// pthread_cond_signal() to avoid the unblocking of more than one thread
// blocked on a condition variable. For example...
// NOTE: In previous versions of this test, `notify_one` was called WITHOUT
// holding the lock but POSIX says (in the aforementioned URL) that:
// ...if predictable scheduling behavior is required, then that mutex shall
// be locked by the thread calling pthread_cond_broadcast() or
// pthread_cond_signal().
#include <condition_variable>
#include <atomic>
#include <mutex>
#include <thread>
#include <cassert>
#include "test_macros.h"
std::condition_variable cv;
std::mutex mut;
std::atomic_int test1(0);
std::atomic_int test2(0);
std::atomic_int ready(2);
std::atomic_int which(0);
void f1()
{
std::unique_lock<std::mutex> lk(mut);
assert(test1 == 0);
--ready;
while (test1 == 0)
cv.wait(lk);
which = 1;
assert(test1 == 1);
test1 = 2;
}
void f2()
{
std::unique_lock<std::mutex> lk(mut);
assert(test2 == 0);
--ready;
while (test2 == 0)
cv.wait(lk);
which = 2;
assert(test2 == 1);
test2 = 2;
}
int main(int, char**)
{
std::thread t1(f1);
std::thread t2(f2);
{
while (ready > 0)
std::this_thread::yield();
// At this point:
// 1) Both f1 and f2 have entered their condition variable wait.
// 2) Either f1 or f2 has the mutex locked and is about to wait.
std::unique_lock<std::mutex> lk(mut);
test1 = 1;
test2 = 1;
ready = 1;
cv.notify_one();
}
{
while (which == 0)
std::this_thread::yield();
std::unique_lock<std::mutex> lk(mut);
if (test1 == 2) {
assert(test2 == 1);
t1.join();
test1 = 0;
} else {
assert(test1 == 1);
assert(test2 == 2);
t2.join();
test2 = 0;
}
which = 0;
cv.notify_one();
}
{
while (which == 0)
std::this_thread::yield();
std::unique_lock<std::mutex> lk(mut);
if (test1 == 2) {
assert(test2 == 0);
t1.join();
test1 = 0;
} else {
assert(test1 == 0);
assert(test2 == 2);
t2.join();
test2 = 0;
}
}
return 0;
}
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