Using semaphore in C

Question

This is my first time using semaphores and I was wondering if I implemented them to the best of their abilities in my code....

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include<semaphore.h>
#include <math.h>

#define NR_THREAD 2

int sum=0;
unsigned int seed;
sem_t sem0;


void* func0(void *param) {
    int tmp;
    int i;
    for(i=0; i<10; i++) {
        sem_wait (&sem0);
        tmp=sum;
        tmp++;
        if(rand_r(&seed)%2) {
            printf("sleeping for 1 sec\n");
            sleep(1);
        } else {
            printf("sleeping for 2 sec\n");
            sleep(2);
        }
        sem_post (&sem0);
        sem_wait (&sem0);
        sum=tmp;
        if(rand_r(&seed)%2) {
            printf("sleeping for 1 sec\n");
            sleep(1);
        } else {
            printf("sleeping for 2 sec\n");
            sleep(2);
        }
        sem_post (&sem0);
    }

    return (void*)0;
}

int main() {
    int i;
    int error;
    pthread_t *tid;
    pthread_attr_t attr;

    seed=(unsigned int)time(NULL);

    if((tid=(pthread_t*)calloc(NR_THREAD, sizeof(pthread_t)))==NULL) {
        fprintf(stderr, "calloc() failed\n");
        exit(1);
    }
    sem_init(&sem0, 0, 1);
    if(sem_init(&sem0, 0, 1)==-1) {
        perror("Failed to sem_init() sem0");
        exit(1);
    }

    pthread_attr_init(&attr);
    for(i=0; i<NR_THREAD; i++) {
        if((error=pthread_create(&tid[i], &attr, func0, 
            (void*)0))) {
            fprintf(stderr, "pthread_create() failed: %d %d\n", 
                i, error);
            tid[i]=pthread_self();
        }
    }

    for(i=0; i<NR_THREAD; i++) {
        if(pthread_equal(pthread_self(), tid[i]))
            continue;

        if((error = pthread_join(tid[i], NULL))) {
            fprintf(stderr, "pthread_join() failed: %d %d\n", 
                i, error);
        }
    }

    printf("Final sum= %d\n", sum);

    free(tid);

    if(sem_destroy(&sem0)==-1) {
        perror("Failed to sem_destroy() sem0");
        exit(2);
    }

    return 0;
}

It runs and gets the correct answer, but I was wondering if there were better places to put them or not. Also, I'm getting a warning, I'm receiving an implicit declaration warning on if(rand_r(&seed)%2) { and I'm not sure how to go about fixing this problem.

This is a demo in which two threads each sleep 10 times.

Answer 1

I presume that you expected the result to be 20. Unfortunately this code may produce anything from 2 to 20.

Here is a scenario how 2 can be produced:

thread 1 grabs the semaphore (its tmp becomes 1)
thread 1 posts the semaphore in the mid-loop

thread 2 grabs the semaphore (sum is still 0)
thread 2 runs the loop for 9 times (sum becomes 9)
thread 2 posts the semaphore at the loop end

thread 1 takes over and completes its first loop
sum becomes 1
thread 1 posts the semaphore at the loop end

thread 2 takes over; its tmp becomes 2
thread 2 posts the semaphore in the mid-loop

thread 1 takes over and runs to completion

thread 2 (its tmp still 2) completes the second half of the loop
sum becomes 2

In your code the threads sleep while holding the semaphore (so sleeping is pretty much irrelevant). Contexts may only switch in the tiny window between sem_post and sem_wait. Most likely it never happens (scheduler doesn't have to switch threads), which explains why you are getting 20. Try to increase the window, say

    sem_post();
    sleep();
    sem_wait();

and see what happens.