/*
 *  Illustrate two threads working the same function
 */
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <sys/time.h>     // for gettimeofday
int max;                  // number of times to increment the variable per thread
volatile int balance = 0; // shared global variable
struct timeval start, end;
//
// syntax is:  int gettimeofday ( struct timeval *tp ,  struct timezone *tz )
//                   only timeval important timezone not needed, can be NULL
//    timeval struct has:  
//    tv_sec : It is the number of seconds since the epoch.
//    tv_usec :It is additional microseconds after number of seconds calculation since the epoch. .
//
//
void * mythread(void *arg) {
   char *letter = arg;
   int i; // stack (private per thread)
   printf("%s: begin [addr of i: %p]\n", letter, &i);
   for (i = 0; i < max; i++) {
      balance = balance + 1; // shared among threads
   }
   printf("%s: done\n", letter);
   return NULL;
}

int main(int argc, char *argv[]) {
   if (argc != 2) {
      fprintf(stderr, "usage: t1 <loopcount>\n");
      exit(1);
   }
   gettimeofday(&start, NULL);
   printf("seconds : %ld microseconds : %ld\n", start.tv_sec, start.tv_usec);
   max = atoi(argv[1]);
   pthread_t p1, p2;
   printf("main: begin [balance = %d] [%p]\n", balance, &balance);
   pthread_create(&p1, NULL, mythread, "A");  // P is capitalized for our textbook only!
   pthread_create(&p2, NULL, mythread, "B");  // p is lower case out in the wild.
   pthread_join(p1, NULL); // join waits for the threads to finish
   pthread_join(p2, NULL);
   printf("main: done\n [balance: %d]\n [should: %d]\n", balance, max*2);
   gettimeofday(&end, NULL);
   printf("seconds : %ld microseconds : %ld\n", end.tv_sec, end.tv_usec);
   printf("elapsed time is: %ld seconds and %ld microseconds\n", 
           end.tv_sec - start.tv_sec, end.tv_usec - start.tv_usec);
   return 0;
}