Testing how semaphores work.

Question

I am trying to test the action of semaphores by calling them in my program with manual switches. I have the functions ready to go, but am failing to codinate them during the calling. I using UNIX system. Please NOTE: the function defitions are fine, put them here for quick refence.The problem is with the caline below these function. I will be greatful for any assistance.

//----------------semmaphore.h ---------------------------------
#define SEM_NAME "semaphore.h",'a'
#define SEM_MAX   3
#define FREE      0
#define DATA      1
#define ROOM      2
#define S_WAIT   -1
#define S_SIGNAL  1
#define NO_EVENT -1

 int sem_config(int, int);
 int sem_wait(int, int);
 int sem_signal(int, int);

  //----------------semmaphore.c ---------------------------------

#include <assert.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

#include "semaphore.h"

static int sem_id;

static union semun 
{
    int val; /* value for SETVAL */
    struct semid_ds *buf; /* buffer for IPC_STAT & IPC_SET */
    ushort array[1]; 
} sem_attr;

  static struct sembuf asem[1];
  static key_t s_key;

 int sem_config(int event, int init_val)
    {
int x;
s_key = ftok(SEM_NAME);
if ( -1 == (sem_id = semget(s_key, SEM_MAX, 0666|IPC_CREAT ) ) {
    perror("semget");
    return -1;
}
  if ( event == NO_EVENT )
    return 0;   
sem_attr.val = init_val;
if ( -1 == semctl(sem_id, event, SETVAL, sem_attr ) )
{
    perror("semctl SETVAL");
    return -1;
}
if ( -1 == ( x = semctl(sem_id, event, GETVAL, sem_attr ) ) )
  {
    perror("semctl GETVAL");
    return -1;
  }
    assert( x == init_val );
    return 0;
  }
//------------------------------------------------------------
int sem_wait(int event, int nwaits)
 {
asem[0].sem_num = event;
asem[0].sem_op = nwaits * S_WAIT;
asem[0].sem_flg = 0;
if ( event == NO_EVENT )    /*remove semaphore set*/
    if ( -1 == semctl(sem_id, 0, IPC_RMID, sem_attr ) )
    {
        perror("semctl IPC_RMID");
        return -1;
    }
    else
        return 0;
if ( -1 == semop(sem_id, asem, 1 ) )
{
    perror("semop");
    return -1;
}
return 0;
   }
 //------------------------------------------------------------
 int sem_signal(int event, int nsignals)
    {
  asem[0].sem_num = event;
  asem[0].sem_op = nsignals * S_SIGNAL;
  asem[0].sem_flg = 0;
  if ( event == NO_EVENT )
    if ( -1 == semctl(sem_id, 0, IPC_RMID, sem_attr ) )
    {
        perror("semctl IPC_RMID");
        return -1;
    }
    else
        return 0;
if ( -1 == semop(sem_id, asem, 1 ) )
{
    perror("semop");
    return -1;
}
return 0;
   }
   //========================PROBLEM STARTS HERE=================
#include <stdio.h>
#include "semaphore.h"

main()
 {
 char op, discard;
 int event, nval;
 do {
   printf("Enter semaphore operation ");
   printf("s(ignal)/w(ait)/i(nit)/f(ind)/c(leanup)");
   scanf("%c%c", &op, &discard);
   printf("Enter semaphore no and initial value :");
   scanf("%d%d%c",&event,&nval,&discard);
   switch ( op )
    {
       case 'i':
             // Get semaphore for the forks
             sem_config(event, nval); 
             printf("Initialized semaphore\n");
             break;

    case 'f':
             break;

    case 's':
             sem_signal(event, 1) 
             break;

    case 'w':
             sem_wait(event, nval)
             break;

    case 'c':
             break;

    default: 
             if ( -1 == sem_wait(NO_EVENT, 0) )
             {
               printf("semctl IPC_RMID failed\n");
               exit(1);
             }
             printf("done\n");
             exit(0);
    }
 } while (1);
  return 0; 
}

Answer 1

Semaphores are for interprocess communication. So you need at least two processes. Testing them with only a single process is pointless :-)

Start your program twice and then type the appropriate commands (wait in #1, signal in #2, etc).