pthread_mutex simple example

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

const long kSize = 1e7L;

long ctr = 0;
pthread_mutex_t mtx;
void *func1(void *a) {
    for (long i = 0; i < kSize; ++i) {
        pthread_mutex_lock(&mtx);
        ++ctr;
        pthread_mutex_unlock(&mtx);
    }
    return NULL;
}

void *func2(void *a) {
    for (long i = 0; i < kSize; ++i) {
       pthread_mutex_lock(&mtx);
       --ctr;
       pthread_mutex_unlock(&mtx);
    }
    return NULL;
}

int main(void) {
    clock_t start = clock();
    if (pthread_mutex_init(&mtx, PTHREAD_MUTEX_NORMAL) != 0) {
        fprintf(stderr, "pthread_mutex_init(): Error.\n");
        return -1;
    }
    pthread_t th1, th2;
    if (pthread_create(&th1, NULL, func1, NULL) != 0 ||
        pthread_create(&th2, NULL, func2, NULL) != 0) {
        fprintf(stderr, "pthread_create(): Error.\n");
        return -1;
    }

    pthread_join(th1, NULL);
    pthread_join(th2, NULL);
    pthread_mutex_destroy(&mtx);

    printf("Counter: %ld \t CPU time: %ld ms\n",
            ctr, (clock() - start) * 1000L / CLOCKS_PER_SEC);
}