[SOLVED] A Question About Thread Behaviour

Hi There;
I am facing a thread question.The question is what will be the output of a0, a1 and a2? It is clear that a0=500 and a1=100.The result is as expected. However, value of a2 is changing whenever I run the code. Why? Here is the code:

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

#define NTH 5

pthread_mutex_t mutex;
int a0,a1,a2;

void * foo(void * arg)
{

	int tindex,i;
	int x1 = 0;
	int x2 = 7;

	tindex = (int) arg;

	for(i = 0 ; i < 100 ; i++)
	{
		pthread_mutex_lock(&mutex);

		a0++;
		x1++;


		pthread_mutex_unlock(&mutex);
		x2 += tindex;
	}

	pthread_mutex_lock(&mutex);

	a1 = x1;
	a2 = x2;

	pthread_mutex_unlock(&mutex);
	pthread_exit(0);

	return NULL;
}


int main(void)
{

	int i;
	pthread_t tid[NTH];

	a0 = a1 = a2 = 0;

	pthread_mutex_init(&mutex,NULL);

	for(i=0 ; i<NTH ; ++i)
	{
		pthread_create(&tid[i] , NULL , foo , (void *) i);
	}

	for(i=0 ; i<NTH ; ++i)
	{
		pthread_join(tid[i] , NULL );
	}

	printf("a0=%d a1=%d a2=%d \n",a0,a1,a2);

	return 0;


}

Thanks in advance.

I suspect it is because x2 is incremented outside the mutex leading to race condition.

The value of a2 at the end is (7 + 100 * T), where T is the index (0 to NTH-1) of the thread which happened to exit last. Since you can't predict which thread will exit last, the value could be 7, 107, 207, 307, or 407 (with NTH=5).