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POQuestion about semaphore
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copied!<p>Given the following code, can you figure out what caused "You input 7 characters" showed up 3 times especially the last time?</p> <pre><code>#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"); 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); } </code></pre> <p>Test input/output:</p> <pre><code>$ cc -D_REENTRANT thread3a.c -o thread3a -lpthread $ ./thread3a Input some text. Enter ‘end’ to finish Excession You input 9 characters FAST You input 7 characters You input 7 characters You input 7 characters end Waiting for thread to finish... Thread joined </code></pre>

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