/* Compile, for example: gcc pthread_example.c -lpthread -o pthread_example */
#include <pthread.h>
#include <stdio.h>

/* variable sum is shared by the threads */
int sum;
/* thread */
void *runner(void *param);

/* First thread */
main(int argc, char *argv[])
{
  /* Thread identifier */
  pthread_t tid;
  /* Attributes for a thread (stack size, scheduling ...) */
  pthread_attr_t attr;
  if (argc !=2){
      fprintf(stderr,"Usage: %s <integer_value>\n",argv[0] );
      exit();
  }
  if (atoi(argv[1]) < 0) {
     fprintf(stderr,"(%d) <integer_value> must be >= 0\n",atoi(argv[1]));
     exit(); 
  }

  /* Set attributes (default attributes are used) */
  pthread_attr_init(&attr);
  /* Create a separate thread */
  /* Pass a thread identifier (tid), attributes (attr), name of the   */
  /* function where the new thread will begin execution (runner),     */
  /* and the integer parameter provided on the command line (argv[1]) */
  /* Now program has two threads: main and runner */
  pthread_create(&tid,&attr,runner,argv[1]);
  /* main waits runner thread using pthread_join */  
  pthread_join(tid,NULL);
  /* print results of summing */
  printf("Sum = %d\n",sum);
}

/* Second thread */
void *runner(void *param)
{
  int upper = atoi(param);
  int i;
  sum = 0;
  if (upper > 0) {
     for (i=1; i<=upper; i++)
          sum +=i;
  }
  /* runner thread completes here */   
  pthread_exit(0);
}