/* Stolen and edited, with utmost gratitude, from: 
     http://grace.evergreen.edu/~sherri/sos/concurrency/secondEdition/chap14/maincriticalsem.c
*/

#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

#define TEN_MILLION 10000000L
#define BUFSIZE 1024

void *threadout(void *args) {
   char buffer[BUFSIZE];
   char *c;
   sem_t *semlockp;
   struct timespec sleeptime;

   semlockp = (sem_t *)args;
   sleeptime.tv_sec = 0;
   sleeptime.tv_nsec = TEN_MILLION;
   snprintf(buffer, BUFSIZE, "This is a thread from process %ld\n",
           (long)getpid());
   c = buffer;

   /****************** entry section *******************************/
   /* Now we create a mutual exclusion semaphore to make it right: */
   while (sem_wait(semlockp) == -1)        /* Entry section */
      if(errno != EINTR) {
         fprintf(stderr, "Thread failed to lock semaphore\n");
         return NULL;
      }
   /****************** start of critical section *******************/
   while (*c != '\0') {
      fputc(*c, stderr);
      c++;
      nanosleep(&sleeptime, NULL);
   }
   /****************** exit section ********************************/
   if (sem_post(semlockp) == -1)         /* Exit section */
      fprintf(stderr, "Thread failed to unlock semaphore\n");
   /****************** remainder section ***************************/
   return NULL;
}








int main(int argc, char *argv[]) {
   int error;
   int i;
   int n;
   sem_t semlock;
   pthread_t *tids;

   if (argc != 2){   /* check for valid number of command-line arguments */
      fprintf (stderr, "Usage: %s numthreads\n", argv[0]);
      return 1;
   }

   n = atoi(argv[1]);
   tids = (pthread_t *)calloc(n, sizeof(pthread_t));
   if (tids == NULL) {
      perror("Failed to allocate memory for thread IDs");
      return 1;
   }
   if (sem_init(&semlock, 0, 1) == -1) {
      perror("Failed to initialize semaphore");
      return 1;
   }
   for (i = 0; i < n; i++)
      if (error = pthread_create(tids + i, NULL, threadout, &semlock)) {
         fprintf(stderr, "Failed to create thread:%s\n", strerror(error));
         return 1;
      }
   for (i = 0; i < n; i++)
      if (error = pthread_join(tids[i], NULL)) {
         fprintf(stderr, "Failed to join thread:%s\n", strerror(error));
         return 1;
      }
   return 0;
}