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);

}