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

/* Define a mutex and initialize it. */
static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

static int counter = 0;	/* A global variable to protect. */

/* Function executed by each thread. */
static void *
increment(void *_)
{
    /* Remove the mutex_lock/unlock calls and observe what the program does.
	Run it several times. */

    int i;
    for (i = 0; i < 1000000; i++) {
	pthread_mutex_lock(&lock);
	counter++;
	pthread_mutex_unlock(&lock);
    }
}

int
main()
{
    int i;
#define NTHREADS	10
    pthread_t t[NTHREADS];
    for (i = 0; i < NTHREADS; i++)
	pthread_create(&t[i], NULL, increment, NULL);
    for (i = 0; i < NTHREADS; i++)
	pthread_join(t[i], NULL);
    if (counter != NTHREADS * 1000000)
	printf("Something went wrong, counter is %d\n", counter);
    else
	printf("Correct. counter value is %d\n", counter);
}