信号量一小段学习代码，看不懂输出，求教

Question

1、源代码
此程序为linux程序设计第四版书上的代码，学习信号量，书上说信号量增加了不只一次，没有别的解释，看不懂输出为什么是重复三次，求教大神。。。

#include
#include
#include
#include
#include
#include

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){
        printf("%s",work_area);
        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);
}

2、编译输出为
[pc@localhost test]$ ./test
Input some text.Enter 'end' to finish
asdf
you input 4 characters
FAST
FAST
you input 5 characters
you input 5 characters
you input 5 characters
end

Waiting for thread to finish...
Thread joined
[pc@localhost test]$

求教了，感谢感谢