除了上次讲的互斥锁,使用信号量也就是操作系统中所提到的PV原语,也能达到互斥和同步的效果,这就是下面要说的信号量线程控制。
PV原语是对整数计数器信号量sem的操作,一次P操作可使sem减一,而一次V操作可是sem加一。进程(或线程)根据信号量的值来判断是否对公共资源具有访问权限。当信号量的值大于零或等于零的时候,该进程(或线程)具有对公共资源访问的权限,否则,当信号量的值小于时,该进程(或线程)就会被阻塞,直到信号量的值大于或等于一。
在LINUX中,实现了POSIX的无名信号量,主要用于线程间的互斥同步,下面将简单介绍一些函数接口:
sem_init
功能: 用于创建一个信号量,并初始化信号量的值。
函数原型: int sem_init (sem_t* sem, int pshared, unsigned int value);
int sem_wait (sem_t* sem);
int sem_trywait (sem_t* sem);
int sem_post (sem_t* sem);
int sem_getvalue (sem_t* sem);
int sem_destroy (sem_t* sem);
功能:
sem_wait和sem_trywait相当于P操作,它们都能将信号量的值减一,两者的区别在
于若信号量的值小于零时,sem_wait将会阻塞进程,而sem_trywait则会立即返回。
sem_post相当于V操作,它将信号量的值加一,同时发出唤醒的信号给等待的进程
(或线程)。
sem_getvalue 得到信号量的值。
sem_destroy 摧毁信号量。
程序实例如下:
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
#include <errno.h>
#include <semaphore.h>
#include <stdio.h>
#include <pthread.h>
#include <errno.h>
#include <semaphore.h>
#define return_if_fail(p) \
if(!p) { printf("[%s]:func error!", __func__); return; }
if(!p) { printf("[%s]:func error!", __func__); return; }
typedef struct _PrivInfo {
sem_t sem;
int lock_var;
time_t end_time;
}PrivInfo;
sem_t sem;
int lock_var;
time_t end_time;
}PrivInfo;
void info_init(PrivInfo *thiz);
void *pthread_function1(void *paramthiz);
void *pthread_function2(void *paramthiz);
void *pthread_function1(void *paramthiz);
void *pthread_function2(void *paramthiz);
int main (int argc, char** argv) {
pthread_t pt_1 = 0;
pthread_t pt_2 = 0;
int ret = 0;
PrivInfo *thiz = NULL;
thiz = (PrivInfo*)malloc(sizeof(PrivInfo));
if(NULL == thiz) {
return -1;
}
return -1;
}
info_init(thiz);
ret = pthread_create(&pt_1, NULL, pthread_function1, (void*)thiz);
if(0 != ret) {
perror("pthread1 creation failed!");
}
ret = pthread_create(&pt_2, NULL, pthread_function2,(void*)thiz);
perror("pthread1 creation failed!");
}
ret = pthread_create(&pt_2, NULL, pthread_function2,(void*)thiz);
if(0 != ret) {
perror("pthread2 creation failed!");
}
perror("pthread2 creation failed!");
}
pthread_join(pt_1, NULL);
pthread_join(pt_2, NULL);
pthread_join(pt_2, NULL);
sem_destroy(&thiz->sem);
free(thiz);
thiz = NULL;
return 0;
}
free(thiz);
thiz = NULL;
return 0;
}
void info_init(PrivInfo *thiz) {
return_if_fail(&thiz != NULL);
return_if_fail(&thiz != NULL);
thiz->lock_var = 0;
thiz->end_time = time(NULL) + 10;
thiz->end_time = time(NULL) + 10;
sem_init(&thiz->sem, 0, 1);
return;
}
return;
}
void *pthread_function1(void *paramthiz) {
int i = 0;
int i = 0;
PrivInfo *thiz = (PrivInfo *)paramthiz;
while(time(NULL) < thiz->end_time) {
sem_wait(&thiz->sem);
printf("thread1 get the lock.\n");
for(; i<2; i++) {
thiz->lock_var ++;
sleep(3);
}
sem_post(&thiz->sem);
printf("thread1 unlock.\n");
}
sleep(1);
sem_wait(&thiz->sem);
printf("thread1 get the lock.\n");
for(; i<2; i++) {
thiz->lock_var ++;
sleep(3);
}
sem_post(&thiz->sem);
printf("thread1 unlock.\n");
}
sleep(1);
pthread_exit(NULL);
}
}
void *pthread_function2(void *paramthiz) {
PrivInfo *thiz = (PrivInfo *)paramthiz;
PrivInfo *thiz = (PrivInfo *)paramthiz;
while(time(NULL) < thiz->end_time) {
sem_wait(&thiz->sem);
printf("thread2 get the lock. The lock_var = %d.\n",
sem_wait(&thiz->sem);
printf("thread2 get the lock. The lock_var = %d.\n",
thiz->lock_var);
sem_post(&thiz->sem);
printf("thread2 unlock.\n");
sleep(3);
}
pthread_exit(NULL);
}
sem_post(&thiz->sem);
printf("thread2 unlock.\n");
sleep(3);
}
pthread_exit(NULL);
}
本文转自jazka 51CTO博客,原文链接:http://blog.51cto.com/jazka/234585,如需转载请自行联系原作者