cmultithreadingoperating-systemsemaphore

Question about semaphore


Given the following code, can you figure out what caused "You input 7 characters" showed up 3 times especially the last time?

#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);
}

Test input/output:

$ 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

Solution

  • After typing "FAST" :

    I think the main thread have the priority till is blocked when it calls fgets(..stdin..), then the thread function can run and consume the semaphore token.

    Here a trace of what happens.

    > Input some text.
      Main thread :
        work_area = ?;
        bin_sem = 0;
    
      thread function :
        wait on semaphore;
    
    < Excession
      Main thread :
        work_area = Excession;
        bin_sem = 1;
    
      thread function :
        work_area == Excession; (!= end)
    > You input 9 characters; (1)
        bin_sem = 0;
        wait on semaphore;
    
      Main thread :
        work_area == Excession; (!= end)
        work_area == Excession; (!= FAST)
    < FAST
        bin_sem = 1;
        work_area == FAST; (!= end)
        work_area == FAST;
        bin_sem = 2;
        work_area == Wheeee...;
        bin_sem = 3;
        wait on stdin;
    
      thread function :
        work_area == Wheeee...; (!= end)
    > You input 7 characters; (Why seven?)
        bin_sem = 3-1 = 2;
    > You input 7 characters; (Why seven?)
        bin_sem = 2-1 = 1;
    > You input 7 characters; (Why seven?)
        bin_sem = 1-1 = 0;
        wait on semaphore;