/* example of comparing overhead of thread vs. process creation
 */

#define _XOPEN_SOURCE 500
#define _REENTRANT

/* Why is the definition of _REENTRANT necessary here?
 */

#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/time.h>

void * thread_body (void *arg) {
  return (void *) NULL;
}

#define US_PER_SEC 1000000

long ticks_per_sec;
long usec_per_tick;
long n_iterations;

clock_t before;
clock_t after;

void check_elapsed_time (clock_t *d) {
   *d = after - before;
   fprintf (stdout, "number of iterations = %ld\n", n_iterations);
   fprintf (stdout, "elapsed time = %ld ticks\n", *d);
   fprintf (stdout, "elapsed time = %ld us\n", *d * usec_per_tick);
   fprintf (stdout, "avg time per iteration = %ld us\n",
     (*d * usec_per_tick) / n_iterations);
}

int
main(int argc, char **argv) {
   int i;
   pthread_attr_t attrs;
   pthread_t thread;
   void * value_ptr;
   pid_t child;
   int status;
   clock_t d1, d2;

   fprintf (stdout, "------------------------------\n");
   fprintf (stdout, "system clock ticks per second = %ld\n",
     (long) CLOCKS_PER_SEC);
   usec_per_tick = US_PER_SEC / CLOCKS_PER_SEC;
   fprintf (stdout, "microseconds per clock tick = %ld\n", usec_per_tick);

   fprintf (stdout, "----------processes-----------\n");
   n_iterations = 1000;
   before = clock();
   for (i = 1; i < n_iterations; i++) {  
      if ((child = fork())) { /* we are the parent */
        waitpid (child, &status, 0);
      } else { /* we are the child */
        exit (0);
      }
   }
   after = clock();
   check_elapsed_time (&d1);

   fprintf (stdout, "-----------threads------------\n");
   n_iterations = 1000;
   before = clock();
   for (i = 1; i < n_iterations; i++) {  
     pthread_attr_init (&attrs);
     /* use the default attributes */
     pthread_create (
       &thread,     /* place to store the id of new thread */
       &attrs,      /* thread creation attributes */
       thread_body, /* function for thread to execute */
       NULL);       /* pass no parameter */
     pthread_join(thread, &value_ptr);
     pthread_attr_destroy (&attrs);
   }
   after = clock();
   check_elapsed_time (&d2);

   /* Why do we have the loops above? */

   fprintf (stdout, "------------------------------\n");
   if (d2 > 0.0)
   fprintf (stdout, "performance ratio = %d\n", (int) (d1/d2));

   exit (0);
}