linux 線程同步

最近看《UNIX環境高級編程》多線程同步，看到他舉例說條件變量pthread_cond_t怎麼用，愣是沒有看懂，只好在網上找了份代碼，跑了跑，才弄明白

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
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_a, NULL);/*等待進程t_a結束*/
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<=6;i++)
{
pthread_mutex_lock(&mutex);/*鎖住互斥量*/
printf("thread1: lock %d/n", __LINE__);
if(i%3==0){
printf("thread1:signal 1 %d/n", __LINE__);
pthread_cond_signal(&cond);/*條件改變，發送信號，通知t_b進程*/
printf("thread1:signal 2 %d/n", __LINE__);
sleep(1);
}
pthread_mutex_unlock(&mutex);/*解鎖互斥量*/
printf("thread1: unlock %d/n/n", __LINE__);
sleep(1);
}
}
void *thread2(void *junk)
{
while(i<6)
{
pthread_mutex_lock(&mutex);
printf("thread2: lock %d/n", __LINE__);
if(i%3!=0){
printf("thread2: wait 1 %d/n", __LINE__);
pthread_cond_wait(&cond,&mutex);/*解鎖mutex，並等待cond改變*/
printf("thread2: wait 2 %d/n", __LINE__);
}
pthread_mutex_unlock(&mutex);
printf("thread2: unlock %d/n/n", __LINE__);
sleep(1);
}
}

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
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_a, NULL);/*等待進程t_a結束*/
    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<=6;i++)
    {
        pthread_mutex_lock(&mutex);/*鎖住互斥量*/
        printf("thread1: lock %d/n", __LINE__);
        if(i%3==0){
            printf("thread1:signal 1  %d/n", __LINE__);
            pthread_cond_signal(&cond);/*條件改變，發送信號，通知t_b進程*/
            printf("thread1:signal 2  %d/n", __LINE__);
            sleep(1);
        }
        pthread_mutex_unlock(&mutex);/*解鎖互斥量*/
        printf("thread1: unlock %d/n/n", __LINE__);
        sleep(1);
    }
}
void *thread2(void *junk)
{
    while(i<6)
    {
        pthread_mutex_lock(&mutex);
        printf("thread2: lock %d/n", __LINE__);
        if(i%3!=0){
            printf("thread2: wait 1  %d/n", __LINE__);
            pthread_cond_wait(&cond,&mutex);/*解鎖mutex，並等待cond改變*/
            printf("thread2: wait 2  %d/n", __LINE__);
        }
        pthread_mutex_unlock(&mutex);
        printf("thread2: unlock %d/n/n", __LINE__);
        sleep(1);
    }
}

編譯：

[X61@horizon threads]$ gcc thread_cond.c -lpthread -o tcd

以下是程序運行結果：

[X61@horizon threads]$ ./tcd 
thread1: lock 30
thread1: unlock 40

thread2: lock 52
thread2: wait 1 55
thread1: lock 30
thread1: unlock 40

thread1: lock 30
thread1:signal 1 33
thread1:signal 2 35
thread1: unlock 40

thread2: wait 2 57
thread2: unlock 61

thread1: lock 30
thread1: unlock 40

thread2: lock 52
thread2: wait 1 55
thread1: lock 30
thread1: unlock 40

thread1: lock 30
thread1:signal 1 33
thread1:signal 2 35
thread1: unlock 40

thread2: wait 2 57
thread2: unlock 61

這裏的兩個關鍵函數就在pthread_cond_wait和pthread_cond_signal函數。

本例中：

線程一先執行，獲得mutex鎖，打印，然後釋放mutex鎖，然後阻塞自己1秒。

線程二此時和線程一應該是併發的執行 ，這裏是一個要點，爲什麼說是線程此時是併發的執行，因爲此時不做任何干涉的話，是沒有辦法確定是線程一先獲得執行還是線程二先獲得執行，到底那個線程先獲得執行，取決於操作系統的調度，想刻意的讓線程2先執行，可以讓線程2一出來，先sleep一秒。
這裏併發執行的情況是，線程一先進入循環，然後獲得鎖，此時估計線程二執行,阻塞在
pthread_mutex_lock(&mutex);
這行語句中，直到線程1釋放mutex鎖
pthread_mutex_unlock(&mutex);/*解鎖互斥量*/
然後線程二得已執行，獲取metux鎖，滿足if條件，到pthread_cond_wait (&cond,&mutex);/*等待*/
這裏的線程二阻塞，不僅僅是等待cond變量發生改變，同時釋放mutex鎖 ，因爲當時看書沒有注意，所以這裏卡了很久。
mutex鎖釋放後，線程1終於獲得了mutex鎖，得已繼續運行，當線程1的if（i%3==0）的條件滿足後，通過pthread_cond_signal發送信號，告訴等待cond的變量的線程（這個情景中是線程二），cond條件變量已經發生了改變。

不過此時線程二並沒有立即得到運行 ，因爲線程二還在等待mutex鎖的釋放，所以線程一繼續往下走，直到線程一釋放mutex鎖，線程二才能停止等待，打印語句，然後往下走通過pthread_mutex_unlock(&mutex)釋放mutex鎖，進入下一個循環。