#include <pthread.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#define THREADS 5
static int sum = 1;
pthread_mutex_t sum_lock;
void *updater(void *ptr){
  pthread_mutex_lock(&sum_lock);
  sum = sum + 1;
  pthread_mutex_unlock(&sum_lock);
  pthread_exit(NULL);
}
int main(){
  int i;
  pthread_t threads[THREADS];
  pthread_mutex_init(&sum_lock,NULL);
  for(i = 0; i < THREADS ; i++)
    pthread_create(&threads[i],NULL,updater,NULL);
  for(i = 0; i < THREADS ; i++)
    pthread_join(threads[i],NULL);
  fprintf(stderr, "sum = %d\n", sum);
  exit(EXIT_SUCCESS);
}