Example of mutual exclusion problem in POSIX threads

mutual_exclusion.c

// Mutual Exclusion Example
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define NUM_THREADS	500
#define N_SUM 		100000

void* sum(void *t)
{
   long* n = (long*)t;
   int i;

   for (i=0; i<N_SUM ; i++)
   {
      (*n)++;
   }

   pthread_exit((void*) t);
}

int main ()
{
	printf("Number of threads: %d\nEvery thread will sum %d times.\n", NUM_THREADS, N_SUM);

	pthread_t thread[NUM_THREADS];
	pthread_attr_t attr;
	int rc;

	// variable that will be access for every thread
	long n = 0;
	void *status;

	/* Initialize and set thread detached attribute */
	pthread_attr_init(&attr);
	pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

	long t;
	for(t=0; t<NUM_THREADS; t++) {
		rc = pthread_create(&thread[t], &attr, sum, (void *)&n); 
	}

	/* Free attribute and wait for the other threads */
	pthread_attr_destroy(&attr);
	for(t=0; t<NUM_THREADS; t++) {
		rc = pthread_join(thread[t], &status);
	}

	printf("Right value:\t%d;\nResult:\t\t%ld.\n", NUM_THREADS*N_SUM, n);
	pthread_exit(NULL);
}