linux用信号量进行同步

信号量一般常用来保护一段代码，使其每次只能被一个执行线程运行。

以下这段程序在主线程中，我们等待直到有文本输入，然后调用sem_post增加信号量的值，这将立刻令另一个线程从sem_wait的等待中返回并开始执行。在统计完字符个数后，它再次调用sem_wait并再次被阻塞，直到主线程再次调用sem_post增加信号量的值为止。

#include<stdio.h>#include<unistd.h>#include<stdlib.h>#include<string.h>#include<pthread.h>#include<semaphore.h>void *thread_function(void *arg);sem_t bin_sem;#define WORK_SIZE  1024char work_area[WORK_SIZE];int main(){  int res;  pthread_t a_thread;  void *thread_result;    res=sem_init(&bin_sem,0,0);  if(res!=0)  {    perror("Semaphore initialization failed");    exit(EXIT_FAILURE);  }  res=pthread_create(&a_thread,NULL,thread_function,NULL);  if(res!=0)  {    perror("Thread creation failed");    exit(EXIT_FAILURE);  }  printf("Input some text,Enter 'end' to finish\n");  while(strncmp("end",work_area,3)!=0)  {     fgets(work_area,WORK_SIZE,stdin);     sem_post(&bin_sem);  }  printf("\nwaiting for thread to finish...\n");  res=pthread_join(a_thread,&thread_result);  if(res!=0)  {    perror("Thread join failed");    exit(EXIT_FAILURE);  }  printf("Thread joined\n");  sem_destroy(&bin_sem);  exit(EXIT_SUCCESS);}void *thread_function(void *arg){  sem_wait(&bin_sem);  while(strncmp("end",work_area,3)!=0)  {     printf("You input %d characters\n",strlen(work_area)-1);     sem_wait(&bin_sem);  }  pthread_exit(NULL);}

以上程序还存在细微错误，假如将以上程序修改为:

#include<stdio.h>#include<unistd.h>#include<stdlib.h>#include<string.h>#include<pthread.h>#include<semaphore.h>void *thread_function(void *arg);sem_t bin_sem;#define WORK_SIZE  1024char work_area[WORK_SIZE];int main(){  int res;  pthread_t a_thread;  void *thread_result;    res=sem_init(&bin_sem,0,0);  if(res!=0)  {    perror("Semaphore initialization failed");    exit(EXIT_FAILURE);  }     res=pthread_create(&a_thread,NULL,thread_function,NULL);  if(res!=0)  {    perror("Thread creation failed");    exit(EXIT_FAILURE);  }  printf("Input some text,Enter 'end' to finish\n");    while(strncmp("end",work_area,3)!=0)  {     if(strncmp(work_area,"FAST",4)==0)     {                sem_post(&bin_sem);        strcpy(work_area,"Wheeee...");     }     else     {         fgets(work_area,WORK_SIZE,stdin);              }     sem_post(&bin_sem);  }  printf("\nwaiting for thread to finish...\n");  res=pthread_join(a_thread,&thread_result);  if(res!=0)  {    perror("Thread join failed");    exit(EXIT_FAILURE);  }  printf("Thread joined\n");  sem_destroy(&bin_sem);  exit(EXIT_SUCCESS);}void *thread_function(void *arg){  sem_wait(&bin_sem);  while(strncmp("end",work_area,3)!=0)  {     printf("You input %d characters\n",strlen(work_area)-1);     sem_wait(&bin_sem);  }  pthread_exit(NULL);}

如果输入FAST，就会发现新线程被连续调用三次。

问题在于，我们的程序依赖其接收文本输入的时间足够长，这样另一个线程才有时间在主线程还未准备好给它更多的单词去统计之前统计处工作区中字符的个数。当我们试图连续快速的给它两组不同的单词时，第二个线程没有时间去执行，但信号量已经被增加不止一次。

为了解决上面的问题，我们可以再增加一个信号量，让主线程等待统计线程完成字符个数的统计后再继续执行。

程序如下：

#include<stdio.h>#include<unistd.h>#include<stdlib.h>#include<string.h>#include<pthread.h>#include<semaphore.h>void *thread_function(void *arg);sem_t bin_sem;sem_t bin_sem2;#define WORK_SIZE  1024char work_area[WORK_SIZE];int main(){  int res;  pthread_t a_thread;  void *thread_result;    res=sem_init(&bin_sem,0,0);  if(res!=0)  {    perror("Semaphore initialization failed");    exit(EXIT_FAILURE);  }  res=sem_init(&bin_sem2,0,1);  if(res!=0)  {    perror("Semaphore initialization failed");    exit(EXIT_FAILURE);  }    res=pthread_create(&a_thread,NULL,thread_function,NULL);  if(res!=0)  {    perror("Thread creation failed");    exit(EXIT_FAILURE);  }  printf("Input some text,Enter 'end' to finish\n");  sem_wait(&bin_sem2);  while(strncmp("end",work_area,3)!=0)  {     if(strncmp(work_area,"FAST",4)==0)     {        printf("***\n");        sem_post(&bin_sem);        strcpy(work_area,"Wheeee...");     }     else     {         fgets(work_area,WORK_SIZE,stdin);         sem_post(&bin_sem);     }     printf("&&&\n");     //sem_post(&bin_sem);     sem_wait(&bin_sem2);  }  printf("\nwaiting for thread to finish...\n");  res=pthread_join(a_thread,&thread_result);  if(res!=0)  {    perror("Thread join failed");    exit(EXIT_FAILURE);  }  printf("Thread joined\n");  sem_destroy(&bin_sem);  sem_destroy(&bin_sem2);  exit(EXIT_SUCCESS);}void *thread_function(void *arg){  sem_wait(&bin_sem);  while(strncmp("end",work_area,3)!=0)  {     printf("You input %d characters\n",strlen(work_area)-1);     sem_post(&bin_sem2);     sem_wait(&bin_sem);  }  pthread_exit(NULL);}