/* sem.h - CreateSem, P, V, DelSem */

typedef int *semaphore;  

semaphore CreateSem (int count);  /* Returns a new semaphore */

void P(semaphore s);              /* P, or wait, or down on semaphore */

void V(semaphore s);              /* V, or signal, or up on semaphore */

int DelSem(semaphore s);          /* Deletes an existing semaphore */

/* sem.c - CreateSem, P, V, DelSem */

/*
 This implementation of semaphores is strictly for kicks since
 it assumes the availability of pipes!
 A semaphore is represented by a pipe. The P operation is simulated
 by reading a character from the pipe. The V operation is simulated
 by writing a character to the pipe. The kind of the semaphore is
 specified when the semaphore is created. If the argument is 0, we get a
 blocking semaphore, if 1, a boolean semaphore, and if n>1, a counting
 semaphore.
 */

#include <stdio.h>
#include <sys/types.h>
#include "sem.h"

semaphore CreateSem (int count){
/* Create and return a new semaphore */
   int *fd;
   int i;

   fd = (int *)malloc(8); /* Allocating space for two filedescriptors */
   if (pipe(fd) < 0) {
      printf("Cannot create pipe.\n");
      exit(0);}
   for(i=0;i<count;i++)
      write(fd[1], "x", 1);
   return(fd);
}

void P(semaphore s){
   char c;
   read(s[0], &c, 1); 
}

void V(semaphore s){
   char c;
   write(s[1], &c, 1); 
}

int DelSem(semaphore s){
/* Delete an existing semaphore */
   close(s[0]);
   close(s[1]);
   free(s);
}