extern int pthread_cond_init __P ((pthread_cond_t *__cond,__const pthread_condattr_t *__cond_attr));
其中cond是一個指向結構pthread_cond_t的指針，cond_attr是一個指向結構pthread_condattr_t的指針。結構 pthread_condattr_t是條件變量的屬性結構，和互斥鎖一樣我們可以用它來設置條件變量是進程內可用還是進程間可用，默認值是 PTHREAD_ PROCESS_PRIVATE，即此條件變量被同一進程內的各個線程使用。注意初始化條件變量只有未被使用時才能重新初始化或被釋放。釋放一個條件變量的函數爲pthread_cond_destroy（pthread_cond_t cond）。
也可以靜態的初始化條件變量
pthread_cond_t my_condition = PTHREAD_COND_INITIALIZER;
函數pthread_cond_wait（）使線程阻塞在一個條件變量上。它的函數原型爲：
extern int pthread_cond_wait __P ((pthread_cond_t *__cond,pthread_mutex_t *__mutex));
調用這個函數時,線程解開mutex指向的鎖並被條件變量cond阻塞。線程可以被函數pthread_cond_signal和函數 pthread_cond_broadcast喚醒線程被喚醒後，它將重新檢查判斷條件是否滿足，如果還不滿足，一般說來線程應該仍阻塞在這裏，被等待被下一次喚醒。這個過程一般用while語句實現。
通過下面的程序來理解:
__________________華麗的CODE分割線_________________________
#include
#include
#include
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; /*初始化互斥鎖*/
pthread_cond_t cond = PTHREAD_COND_INITIALIZER; //初始化條件變量
void *thread1(void *);
void *thread2(void *);
int i=1;
int main(void)
{
pthread_t t_a;
pthread_t t_b;
pthread_create(&t_a,NULL,thread1,(void *)NULL);/*創建進程t_a*/
pthread_create(&t_b,NULL,thread2,(void *)NULL); /*創建進程t_b*/
pthread_join(t_b, NULL);/*等待進程t_b結束*/
pthread_mutex_destroy(&mutex);
pthread_cond_destroy(&cond);
exit(0);
}
void *thread1(void *junk)
{
for(i=1;i<=9;i++)
{
pthread_mutex_lock(&mutex);//
if(i%3==0)
pthread_cond_signal(&cond);/*條件改變，發送信號，通知t_b進程*/
else
printf("thead1:%d\n",i);
pthread_mutex_unlock(&mutex);//*解鎖互斥量*/
printf("Up Unlock Mutex\n");
sleep(1);
}
}
void *thread2(void *junk)
{
while(i<9)
{
pthread_mutex_lock(&mutex);
if(i%3!=0)
pthread_cond_wait(&cond,&mutex);/*等待*/
printf("thread2:%d\n",i);
pthread_mutex_unlock(&mutex);
printf("Down Ulock Mutex\n");
sleep(1);
}
}
___________________至關重要的絢爛的結果分割線_________________
thead1:1
Up Unlock Mutex
thead1:2
Up Unlock Mutex
Up Unlock Mutex
thread2:3
Down Ulock Mutex
thead1:4
Up Unlock Mutex
thead1:5
Up Unlock Mutex
Up Unlock Mutex
thread2:6
Down Ulock Mutex
thead1:7
Up Unlock Mutex
thead1:8
Up Unlock Mutex
Up Unlock Mutex
thread2:9
Down Ulock Mutex
_________________HOW IT WORKS________
i不是三的倍數的時候.
thread2條件變量阻塞,釋放Mutex
thread1加鎖,打印thread1:i,釋放鎖,打印"Up Unlock Mutex"
i爲3的倍數的時候,
thread1,加鎖,條件變量通知,喚醒條件變量阻塞線程,打印"Up Unlock Mutex"
thread2,被喚醒,加鎖,打印"thread2:i",釋放鎖,打印"Down Ulock Mutex"
所以說函數pthread_cond_wait()調用時,不僅對條件變量起作用,還對互斥鎖有作用!
++++++++++++++++唯美的好書推薦線++++++++++++
"Beiginning Linux Programming" by Neil Matthrew & Richard Stones
You can choose the English version or Chinese version, but I recommend the English one. Maybe it' easier to understand what the authers say than the Chinese one , even if you're Chinese.