Testing how semaphores work - c

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).