/*
    pthreads_without_race.c
    Jeff Ondich, 29 May 2019

    A simple threaded program with a shared integer, using a mutex
    to prevent the obvious race condition.

    To compile:

        gcc -Wall -o pthreads_without_race pthreads_without_race.c

    On some systems, you might need to do this:

        gcc -Wall -o pthreads_without_race pthreads_without_race.c -lpthread
*/

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

int gSharedInteger = 0;
pthread_mutex_t gLock = PTHREAD_MUTEX_INITIALIZER;

void *threadMainA(void *arg);
void *threadMainB(void *arg);

int main() {
    pthread_t threadA, threadB;

    /* Spawn the threads. */
    if (pthread_create(&threadA, NULL, threadMainA, 0) != 0) {
        perror("Can't create thread A");
        exit(1);
    }

    if (pthread_create(&threadB, NULL, threadMainB, 0) != 0) {
        perror( "Can't create thread B" );
        exit( 1 );
    }

    pthread_join(threadA, NULL);
    pthread_join(threadB, NULL);

    return 0;
}

void *threadMainA(void *arg) {
    while(1) {
        pthread_mutex_lock(&gLock);
        gSharedInteger = 25;
        pthread_mutex_unlock(&gLock);
    }

    return NULL;
}

void *threadMainB(void *arg) {
    while(1) {
        pthread_mutex_lock(&gLock);
        gSharedInteger = 37;
        if (gSharedInteger != 37) {
            fprintf(stderr, "Uh-oh\n");
            fflush(stderr);
        }
        pthread_mutex_unlock(&gLock);
    }

    return NULL;
}