pthread_cond_wait用于等待某个特定的条件为真，pthread_cond_signal用于通知阻塞的线程某个特定的条件为真了。在调用者两个函数之前需要声明一个pthread_cond_t类型的变量，用于这两个函数的参数。
/*
是否熟悉POSIX多线程编程技术？如熟悉，编写程序完成如下功能：
1）有一int型全局变量g_Flag初始值为0；
2）在主线称中起动线程1，打印“this is thread1”，并将g_Flag设置为1
3）在主线称中启动线程2，打印“this is thread2”，并将g_Flag设置为2
4）线程序1需要在线程2退出后才能退出
5）主线程在检测到g_Flag从1变为2，或者从2变为1的时候退出
*/#include<stdio.h>
#include<stdlib.h>
#include<pthread.h>
#include<errno.h>
#include<unistd.h>typedef void* (*fun)(void*);int g_Flag=0;static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;void* thread1(void*);void* thread2(void*);/*
* when program is started, a single thread is created, called the initial thread or main thread.
* Additional threads are created by pthread_create.
* So we just need to create two thread in main().
*/int main(int argc, char** argv)
{ printf("enter main\n");
pthread_t tid1, tid2; int rc1=0, rc2=0;
rc2 = pthread_create(&tid2, NULL, thread2, NULL); if(rc2 != 0) printf("%s: %d\n",__func__, strerror(rc2));
rc1 = pthread_create(&tid1, NULL, thread1, &tid2); if(rc1 != 0) printf("%s: %d\n",__func__, strerror(rc1));
pthread_cond_wait(&cond, &mutex); printf("leave main\n"); exit(0);
}/*
* thread1() will be execute by thread1, after pthread_create()
* it will set g_Flag = 1;
*/void* thread1(void* arg)
{ printf("enter thread1\n"); printf("this is thread1, g_Flag: %d, thread id is %u\n",g_Flag, (unsigned int)pthread_self());
pthread_mutex_lock(&mutex); if(g_Flag == 2)
pthread_cond_signal(&cond);
g_Flag = 1; printf("this is thread1, g_Flag: %d, thread id is %u\n",g_Flag, (unsigned int)pthread_self());
pthread_mutex_unlock(&mutex);
pthread_join(*(pthread_t*)arg, NULL); printf("leave thread1\n");
pthread_exit(0);
}/*
* thread2() will be execute by thread2, after pthread_create()
* it will set g_Flag = 2;
*/void* thread2(void* arg)
{ printf("enter thread2\n"); printf("this is thread2, g_Flag: %d, thread id is %u\n",g_Flag, (unsigned int)pthread_self());
pthread_mutex_lock(&mutex); if(g_Flag == 1)
pthread_cond_signal(&cond);
g_Flag = 2; printf("this is thread2, g_Flag: %d, thread id is %u\n",g_Flag, (unsigned int)pthread_self());
pthread_mutex_unlock(&mutex); printf("leave thread2\n");
pthread_exit(0);
}