/*******************************************************************************
 * semaphore.c
 * This file is part of "Lab 4: sum of prime numbers of integer numbers"
 * Copyright (C) 2009 - c506001 (email: c506001@cse.hcmut.edu.vn )
 *
 * Content:
 * --------
 * Implementation of semaphore
 *
 * System and Networking Department
 * Faculty of Computer Science and Engineering
 * Ho Chi Minh City University of Technology
 ******************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

/**
 * Function name: seminit
 * Description:
 *    Create new semaphore
 *    Then initialize its value is 1
 * Return value:
 *    semid: If successful
 *     exit: Otherwise
 **/
int seminit() {
  int semid;
  
  // create new semaphore
  if ((semid=semget(IPC_PRIVATE,1,0666|IPC_EXCL))==-1){
    perror("Error in creating a new semaphore");
    exit(1);
  }

  // initialize semaphore's value is 1
  union {
    int val;
    struct semid_ds *buf;
    ushort *array;
  } carg;

  //TODO: initialize carg
  
  if (semctl(semid,0,SETVAL,carg)==-1){
    perror("Error in initializing semaphore");
    exit(1);
  }
    
  return semid;
}

/**
 * Function name: P
 * Description:
 *    Wait for semaphore's value greater than 0
 *    Decrease semaphore's value by 1
 * Return value:
 *    exit if error in decreasing semaphore's value
 **/
void P(int semid){
  struct sembuf pbuf;
  //TODO: initialize pbuf
  
  // decrease semaphore's value by 1
  if (semop(semid,&pbuf,1)==-1) {
    perror("Error in decreasing semaphore's value"); 
    exit(1);
  }
}

/**
 * Function name: V
 * Description:
 *    Signal processes waiting for semaphore
 *    Increase semaphore's value by 1
 * Return value:
 *    exit if error in increasing semaphore's value
 **/
void V(int semid){
  struct sembuf vbuf;
  //TODO: initialize vbuf
  
  // increase semaphore's value by 1
  if (semop(semid,&vbuf,1)==-1) {
    perror("Error in increasing semaphore's value"); 
    exit(1);
  }
}

/**
 * Function name: semrem
 * Description:
 *    Remove an existing semaphore
 * Return value:
 *     0: If successful
 *    -1: Otherwise
 **/
int semrem(int semid){
	return semctl(semid,0,IPC_RMID,0);
}