pthread_mutex_lock用法
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条件变量
条件变量是利用线程间共享的全局变量进行同步的一种机制,主要包括两个动作:一个线程等待"条件变量的条件成立"而挂起;另一个线程使"条件成立"(给出条件成立信号)。为了防止竞争,条件变量的使用总是和一个互斥锁结合在一起。
条件变量是利用线程间共享的全局变量进行同步的一种机制,主要包括两个动作:
1)一个线程等待"条件变量的条件成立"而挂起;
2)另一个线程使"条件成立"(给出条件成立信号)。
为了防止竞争,条件变量的使用总是和一个互斥锁结合在一起。
1.主要涉及到下面的函数:
int pthread_cond_init(pthread_cond_t *cond, pthread_condattr_t *cond_attr) ---动态创建条件变量
pthread_mutex_lock ---互斥锁上锁
pthread_mutex_unlock ----互斥锁解锁
pthread_cond_wait() / pthread_cond_timedwait -----等待条件变量,挂起线程,区别是后者,会有timeout时间,如 果到了timeout,线程自动解除阻塞,这个时间和 time()系统调用相同意义的。以1970年时间算起。
pthread_cond_signal ----激活等待列表中的线程,
pthread_cond_broadcast() -------激活所有等待线程列表中最先入队的线程
注意:1)上面这几个函数都是原子操作,可以为理解为一条指令,不会被其他程序打断
2)上面这个几个函数,必须配合使用。
3)pthread_cond_wait,先会解除当前线程的互斥锁,然后挂线线程,等待条件变量满足条件。一旦条件变 量满足条件,则会给线程上锁,继续执行pthread_cond_wait
2. 代码实例
编译:gcc thread_test.c -o thread_test -lpthread
------必须加上-lpthread,不然会报错,找不到线程的相关函数,gcc自身没有连接线程
#include<pthread.h>#include<unistd.h>#include<stdio.h>#include<stdlib.h>pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;/*初始化互斥锁*/pthread_cond_t cond = PTHREAD_COND_INITIALIZER;//init condvoid *thread1(void*);void *thread2(void*);int i = 1; //globalint main(void){ pthread_t t_a; pthread_t t_b;//two thread pthread_create(&t_a,NULL,thread2,(void*)NULL); pthread_create(&t_b,NULL,thread1,(void*)NULL);//Create thread printf("t_a:0x%x, t_b:0x%x:", t_a, t_b); pthread_join(t_b,NULL);//wait a_b thread end pthread_mutex_destroy(&mutex); pthread_cond_destroy(&cond); exit(0);}void *thread1(void *junk){ for(i = 1;i<= 9; i++){ pthread_mutex_lock(&mutex); //互斥锁 printf("call thread1 \n"); if(i%3 == 0) { pthread_cond_signal(&cond); //send sianal to t_b printf("thread1:******i=%d\n", i); } else printf("thread1: %d\n",i); pthread_mutex_unlock(&mutex);printf("thread1: sleep i=%d\n", i); sleep(1);printf("thread1: sleep i=%d******end\n", i); }}void *thread2(void*junk){ while(i < 9) { pthread_mutex_lock(&mutex); printf("call thread2 \n"); if(i%3 != 0) pthread_cond_wait(&cond,&mutex); //wait printf("thread2: %d\n",i); pthread_mutex_unlock(&mutex);printf("thread2: sleep i=%d\n", i); sleep(1);printf("thread2: sleep i=%d******end\n", i); }}
运行结果
[tandd@localhost test]$ ./thread_testcall thread2 t_a:0xb76f6b70, t_b:0xb6cf5b70:call thread1 thread1: 1thread1: sleep i=1thread1: sleep i=1******endcall thread1 thread1: 2thread1: sleep i=2thread1: sleep i=2******endcall thread1 thread1:******i=3thread1: sleep i=3thread2: 3thread2: sleep i=3thread1: sleep i=3******endcall thread1 thread1: 4thread1: sleep i=4thread2: sleep i=4******endcall thread2 thread1: sleep i=4******endcall thread1 thread1: 5thread1: sleep i=5thread1: sleep i=5******endcall thread1 thread1:******i=6thread1: sleep i=6thread2: 6thread2: sleep i=6thread1: sleep i=6******endcall thread1 thread1: 7thread1: sleep i=7thread2: sleep i=7******endcall thread2 thread1: sleep i=7******endcall thread1 thread1: 8thread1: sleep i=8thread1: sleep i=8******endcall thread1 thread1:******i=9thread1: sleep i=9thread2: 9thread2: sleep i=9thread1: sleep i=9******end[tandd@localhost test]$
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