cond, condition (条件量)，某个线程拿到 mutex，但发现某些条件没达到（比如队列中无任务），则需要 block，直到条件达到位置，此时 cond 就有作用了。
pthread_signal, 只唤醒所有等待 cond 线程中的一个。
pthread_broadcast, 唤醒所有等待 cond 的线程。
其实还是一个个唤醒，一个被唤醒，释放 mutex，然后另一个被唤醒。
cond 典型的用法如下：
pthread_mutex_lock(&m);
while ( cond is not ok )
{
pthread_cond_wait(&cond, &m);
}
... do sth.
pthread_mutex_unlock(&m);
为何这里需要一个 while 呢？因为 pthread_broadcast 会唤醒所有等待 cond 的线程，比如队列中只有一个任务，而唤醒了三个线程，则只有一个线程能拿到任务，而另两个需要继续 wait on cond。
cond 也有 process-shared 的属性，意义不大，忽略了。而 clock 属性是实时系统(realtime)才用的，略过不表。
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#include <stdio.h>
#include <unistd.h>
#include <time.h>
#include <pthread.h>
int global_val = 0;
pthread_mutex_t global_mutex;
pthread_cond_t global_cond;
void *mythr(void *arg)
{
printf("thr #%d, before lock, time = %d\n", (int)arg, time(NULL));
pthread_mutex_lock(&global_mutex);
while ( global_val > 0 )
{
printf("thr #%d, before cond wait, time = %d\n", (int)arg, time(NULL));
pthread_cond_wait(&global_cond, &global_mutex);
}
global_val = 1;
printf("thr #%d, cond, time = %d\n", (int)arg, time(NULL));
pthread_mutex_unlock(&global_mutex);
return (void *)0;
}
int main()
{
pthread_t pid1, pid2, pid3, pid4;
pthread_cond_init(&global_cond, NULL);
pthread_mutex_init(&global_mutex, NULL);
pthread_create(&pid1, NULL, mythr, (void *)1);
pthread_create(&pid2, NULL, mythr, (void *)2);
pthread_create(&pid3, NULL, mythr, (void *)3);
pthread_create(&pid4, NULL, mythr, (void *)4);
sleep(5);
global_val = 0;
pthread_cond_broadcast(&global_cond);
sleep(5);
global_val = 0;
pthread_cond_signal(&global_cond);
sleep(5);
global_val = 0;
pthread_cond_signal(&global_cond);
pthread_join(pid1, NULL);
pthread_join(pid2, NULL);
pthread_join(pid3, NULL);
pthread_join(pid4, NULL);
return 0;
}
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$ ./a.out
thr #1, before lock, time = 1262783837
thr #3, before lock, time = 1262783837
thr #4, before lock, time = 1262783837
thr #2, before lock, time = 1262783837
thr #3, cond, time = 1262783837
thr #4, before cond wait, time = 1262783837
thr #2, before cond wait, time = 1262783837
thr #1, before cond wait, time = 1262783837
// broadcast, all wakeup
thr #4, cond, time = 1262783842
thr #2, before cond wait, time = 1262783842
thr #1, before cond wait, time = 1262783842
// signal, single wakeup
thr #2, cond, time = 1262783847
thr #1, cond, time = 1262783852
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