Race Condition by example

Consider the following code

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

#define NUM_THREADS 4

void *threadFunc(void *pArg) { /* thread main */
	int *p = (int*)pArg;
	int myNum = *p;
	printf("Thread number %d\n", myNum);
	return 0;
}

int main(void) {
	int i;
	pthread_t tid[NUM_THREADS];
	
	for(i = 0; i < NUM_THREADS; i++) { /* create/fork threads */
		pthread_create(&tid[i], NULL, threadFunc, &i);
	}
	
	for(i = 0; i < NUM_THREADS; i++) { /* wait/join threads */
		pthread_join(tid[i], NULL);
	}
	return 0;
}

In the above example, one would normally expect that the output of the program would be

Thread Number 0
Thread Number 1
Thread Number 2
Thread Number 3

Or the above in a jumbled order because we cannot guarantee the order of execution of print. Now there is also this condition possible where we can expect a result something like this

Thread Number 0
Thread Number 2
Thread Number 2
Thread Number 3

Now how is that possible ?

The main reason is that the variable i is available for change even while its value is being read. In detail, suppose we reach a condition where i = 1 ; and threadFunc starts getting exectuted. As per the code of threadFunc, the pointer p is assigned the value of address of i, and variable myNum is being assigned the value of variable whose address is stored by p pointer, which we expect to be value of i, and hence you expect Thread Number 1 to be printed. But here before printing that, the value of i has been changed by the main thread to 2 by incrementing it. So when you are expecting that you are assigning 1 to myNum because p contains address of i, whose value is 1, is actually no longer 1 but has been incremented to 2; by the main thread.

This is called Data Race or Race condition where a variable is being read by one thread, and the variable’s value is being written(altered) by other thread. Now how do we avoid it Store it in a variable(an array preferably here) which isn’t altered and pass it as argument in pthread_create. Because though i will be altered the value which we want has been copied into a array cell which is not going to be changed. The code would be in this manner:

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

#define NUM_THREADS 4

void *threadFunc(void *pArg) { /* thread main */
	int myNum = *((int*)pArg);
	printf("Thread number %d\n", myNum);
	return 0;
}

int main(void) {

	int i;
	int tNum[NUM_THREADS];
	pthread_t tid[NUM_THREADS];
	
	for(i = 0; i < NUM_THREADS; i++) { /* create/fork threads */
		tNum[i] = i;
		pthread_create(&tid[i], NULL, threadFunc, &tNum[i]);
	}
	
	for(i = 0; i < NUM_THREADS; i++) { /* wait/join threads */
		pthread_join(tid[i], NULL);
	}

	return 0;
}