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  1024
char 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);
}

#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  1024
char 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);
  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_wait(&bin_sem);
  }
  pthread_exit(NULL);
}