/* criticalsection2.c with semaphore operations implemented using a pipe */
#include <signal.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <signal.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
#define THREADS 5

int sem[2]; /* pipe file descriptor for use as a semaphore */

void P( int sem[] ){
  char buf[1];

  read( sem[0], &buf, 1 );
}

void V( int sem[] ){
  char buf[1] = { 'a' };

  write( sem[1], &buf, 1 );
}

void *threadfunc( void * arg ) { /* function for each thread */
  int *me;

  me = (int *) arg;

  P( sem );
  printf( "Thread %d running!! Line 1\n", *me );
  printf( "Thread %d running!! Line 2\n", *me );
  printf( "Thread %d running!! Line 3\n", *me );
  V( sem );

  pthread_exit( NULL );
}

int main( void ) {
  int i;
  pthread_t tid[THREADS];
  int id[THREADS];

  pipe( sem );               /* create "semaphore" and */
  V( sem );    /* initialise to 1 */

  for( i=0; i<THREADS; i++ ) /* set up thread ids */
    id[i] = i;
  for( i=0; i<THREADS; i++ ) /* run the threads */
    pthread_create( &tid[i], NULL, threadfunc, (void *) &id[i] );
  for( i=0; i<THREADS; i++ ) /* wait for them to finish */
    pthread_join( tid[i], NULL );
  return 0;
}