c - 关于pthread_cond_wait的用法

Question

我试图把头放在pthread条件变量周围。
我看过一些使用pthread_cond_wait和pthread_cond_signal的代码示例，它们全部如下所示:

while (condition)
{
    // Assume that the mutex is locked before the following call
    pthread_cond_wait(&cond, &mutex);
}

是否有条件在条件上使用while循环？为什么不只使用一个if语句呢？

Answer 1

虚假唤醒。

参见Why does pthread_cond_wait have spurious wakeups?和https://en.wikipedia.org/wiki/Spurious_wakeup:

Spurious wakeup describes a complication in the use of condition variables as provided by certain multithreading APIs such as POSIX Threads and the Windows API.

Even after a condition variable appears to have been signaled from a waiting thread's point of view, the condition that was awaited may still be false. One of the reasons for this is a spurious wakeup; that is, a thread might be awoken from its waiting state even though no thread signaled the condition variable. For correctness it is necessary, then, to verify that the condition is indeed true after the thread has finished waiting. Because spurious wakeup can happen repeatedly, this is achieved by waiting inside a loop that terminates when the condition is true ...