pthread_cond_wait和while的问题-优快云博客

本文介绍了Linux下条件变量的正确使用方式，并通过示例对比了常见错误与正确做法的区别。文章强调了在条件发生变化前，等待线程可能并未真正被唤醒的问题，并给出了避免此问题的代码实现。

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如果使用linux下的条件变量的话通常意义上，会编写这样的代码：

#include <pthread.h> struct msg { struct msg *m_next; /* ... more stuff here ... */ }; struct msg *workq; pthread_cond_t qready = PTHREAD_COND_INITIALIZER; pthread_mutex_t qlock = PTHREAD_MUTEX_INITIALIZER; void process_msg(void) { struct msg *mp; for (;;) { pthread_mutex_lock(&qlock); pthread_cond_wait(&qready, &qlock); mp = workq; workq = mp->m_next; pthread_mutex_unlock(&qlock); /* now process the message mp */ } } void enqueue_msg(struct msg *mp) { pthread_mutex_lock(&qlock); mp->m_next = workq; workq = mp; pthread_mutex_unlock(&qlock); pthread_cond_signal(&qready); }
但其实这里的代码是存在问题的，可以参考：

http://download.oracle.com/docs/cd/E19253-01/819-7051/sync-44265/index.html

http://www.cppblog.com/converse/archive/2009/01/15/72064.aspx

也就唤醒的线程重新获取互斥锁并从pthread_cond_wait()返回之前，条件可能会发生变化。等待线程可能并未真正唤醒。正确的写法：

#include <pthread.h> struct msg { struct msg *m_next; /* ... more stuff here ... */ }; struct msg *workq; pthread_cond_t qready = PTHREAD_COND_INITIALIZER; pthread_mutex_t qlock = PTHREAD_MUTEX_INITIALIZER; void process_msg(void) { struct msg *mp; for (;;) { pthread_mutex_lock(&qlock); while (workq == NULL) pthread_cond_wait(&qready, &qlock); mp = workq; workq = mp->m_next; pthread_mutex_unlock(&qlock); /* now process the message mp */ } } void enqueue_msg(struct msg *mp) { pthread_mutex_lock(&qlock); mp->m_next = workq; workq = mp; pthread_mutex_unlock(&qlock); pthread_cond_signal(&qready); }