pthread_cond_wait和while的问题

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);
       <span style="color: #ff6666;"> while (workq == NULL)	
        	pthread_cond_wait(&qready, &qlock);</span>
        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);
}