threadtest.c

/**
 * File: threadtest.c
 * Author: Jesus Diaz Vico @ INCIBE (Spanish National Cybersecurity Institute).
 * Date: March 3rd, 2015.
 * Description: Example of source code with race conditions.
 *
 * Compile with:
 *    gcc -g -lpthread -o threadtest threadtest.c
 * and then use:
 *    valgrind --tool=helgrind ./threadtest
 *
 * For studying the solution, uncomment lines  23, 30, 34, 46, 50, 63 and 77
 * and repeat the process.
 */

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

int balance;
/* pthread_mutex_t lock; */

static void* thread_credit(void *arg) {

    int i, final;

    for (i=0; i<100; i++) {
        /* pthread_mutex_lock(&lock); */
        final = balance + 100;
        usleep(rand() % 100);
        balance = final;
        /* pthread_mutex_unlock(&lock); */
    }

    return;

}

static void* thread_debit(void *arg) {

    int i, final;

    for (i=0; i<100; i++) {
        /* pthread_mutex_lock(&lock); */
        final = balance - 100;
        usleep(rand() % 100);
        balance = final;
        /* pthread_mutex_unlock(&lock); */
    }

    return;

}

int main () {

    pthread_t t1, t2;

    srand(time(NULL));

    /* assert(!pthread_mutex_init(&lock, NULL)); */

    /* Create thread #1 */
    assert (!pthread_create(&t1, NULL, thread_credit, NULL));

    /* Create thread #2 */
    assert (!pthread_create(&t2, NULL, thread_debit, NULL));

    /* Wait for them to finish */
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    fprintf(stdout, "Final balance: %d\n", balance);

    /* pthread_mutex_destroy(&lock); */

    return 0;

}