LINUX信号量线程控制例子

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。

 

         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>

 

#define return_if_fail(p) /
         if(!p) { printf("[%s]:func error!", __func__); return; }

 

typedef struct _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);

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;
         }

         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);

         if(0 != ret) {
                  perror("pthread2 creation failed!"); 
         }

         pthread_join(pt_1, NULL);
         pthread_join(pt_2, NULL);

         sem_destroy(&thiz->sem);
 
         free(thiz); 
         thiz = NULL;
 
         return 0;
}

 

void info_init(PrivInfo *thiz) {
         return_if_fail(&thiz != NULL);

         thiz->lock_var = 0;
         thiz->end_time = time(NULL) + 10;

         sem_init(&thiz->sem, 0, 1);
 
         return;
}

 

void *pthread_function1(void *paramthiz) { 
         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);

         pthread_exit(NULL);
}

 

void *pthread_function2(void *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",

                                 thiz->lock_var);
  
                  sem_post(&thiz->sem);
                  printf("thread2 unlock./n");
 
                  sleep(3);
         }
  
         pthread_exit(NULL);
}