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

static int counter;
static pthread_mutex_t l1 = PTHREAD_MUTEX_INITIALIZER; // protects counter
static pthread_mutex_t l2 = PTHREAD_MUTEX_INITIALIZER; // protects counter again

static void *thread(void * _tn) {
    for (int i = 0; i < 1000000; i++) {
        pthread_mutex_lock(&l1);
        pthread_mutex_lock(&l2);        
        counter++;
        pthread_mutex_unlock(&l2);        
        pthread_mutex_unlock(&l1);
    }
    return NULL;
}

int main() {
    int i, N = 10;
    pthread_t t[N];
    for (i = 0; i < N; i++)
        pthread_create(t + i, NULL, thread, NULL);        
    for (i = 0; i < N; i++)
        pthread_join(t[i], NULL);
    printf("counter = %d\n", counter);
    return 0;
}