/* cc thisfile.c -lthread -lpthread */
/* 
 * This version of the program uses semaphore to protect the
 * shared variable.
 */
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>
#include <semaphore.h>    /* for semphares */

#define NO_SHARE 0        /* 0 means not shared among processes, used
			   * in sem_init()
			   */

//#define NUM_THREADS 5000
#define NUM_THREADS 10

void *work(void *);   /* thread routine */

 /* global variables */
int v = 0;    // shared data
sem_t lock;   // semaphore lock


int  main(int argc, char *argv[]) {

  int i;
  int lock_value;                  /* value of the semaphore */
  pthread_t tid[NUM_THREADS];      /* array of thread IDs */

  sem_init(&lock, NO_SHARE, 1);    /* initialize the semaphore lock */
  sem_getvalue(&lock,&lock_value); /* retrieve the lock value */
  printf("lock value in main() is %d\n", lock_value);

  for ( i = 0; i < NUM_THREADS; i++)
    pthread_create(&tid[i], NULL, work, NULL);
  for ( i = 0; i < NUM_THREADS; i++)
    pthread_join(tid[i], NULL);
  printf("main() reporting that all %d threads have terminated\n", i);
  printf("v should be %d, it is %d\n", NUM_THREADS, v);

  return (0);
}  /* main */
void * work(void *arg)   {

  int lock_value;

  if (pthread_self() % 2 == 0) // even numbered thread, try to create race cond
    usleep(30);
  sem_wait(&lock);   // allows only one process to update v at a time
  sem_getvalue(&lock,&lock_value);
  v ++;              // CR
  sem_post(&lock);   // release the lock so others can come into the CR
  printf("lock value in work() is %d\n", lock_value);
  return (NULL);
}