Thread: Semaphore Problem

Hi All,
I am finding problem with the semaphores. Please have a look at the following code.

Code:

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

int main()
{
           key_t key;
           key=ftok("test.txt",1);
           printf("Key:%d\n",key);

            int semid;
            if(semid=semget(key, 1, IPC_CREAT)) 
                     printf("Semaphore ID:%d\n",semid);
            else
                     perror("Error in creating sem:\n");

            semaphore_wait(semid, 1);
            printf("\nInside Semaphore\nHit a Key to release");
            getchar();
            semaphore_signal(semid, 1);
}


int semaphore_wait(int sem_id, int sem_num)
{
  struct sembuf sem_b;

  sem_b.sem_num = sem_num;
  sem_b.sem_op = -1;
  sem_b.sem_flg = SEM_UNDO;

       if (semop(sem_id, &sem_b, 1) == -1)
            perror("\nError in sem_wait:");
      else
      {
             printf("\nSemaphore in wait);
             return(1);
       }
}

int semaphore_signal(int sem_id, int sem_num)
{
  struct sembuf sem_b;
 
  sem_b.sem_num = sem_num;
  sem_b.sem_op = 1;
  sem_b.sem_flg = SEM_UNDO;
  
  if (semop(sem_id, &sem_b, 1) == -1)
        perror("\nError in sem_signal");
  else
        {
         printf("Semaphore release");
         return(1);
         }
}

When I executed the following code it gives the Error as
------------------------------------------
Key:16999020
Semaphore ID:1114113

Error in sem_wait:: File too large

Inside Semaphore
Hit a Key to release
------------------------------------------

Whats the wrong in my code? Can anybody help me out please?

Thanks in advance.

Regards,
Cprogrammer.

If you create n semaphores then You have sem_numbers 0 to n-1.
And in semaphore wait you called sem_op=-1
Initially when creates semaphores its value is initialized to 0
and you are decreamenting this value that's why it is not working..

Code:

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

int main()
{
           key_t key;
           key=ftok("test.txt",1);
           printf("Key:%d\n",key);

            int semid;
            if(semid=semget(key, 1, IPC_CREAT)) 
                     printf("Semaphore ID:%d\n",semid);
            else
                     perror("Error in creating sem:\n");

            semaphore_wait(semid, 0);
            printf("\nInside Semaphore\nHit a Key to release");
            getchar();
            semaphore_signal(semid, 0);
}


int semaphore_wait(int sem_id, int sem_num)
{
  struct sembuf sem_b[2];

  sem_b[0].sem_num = sem_num;
  sem_b[0].sem_op = 0;
  //this is for it will wait untill the value become Zero
  sem_b[0].sem_flg = 0;
  sem_b[1].sem_num= sem_num;
  sem_b[1].sem_op = 1;
  //It will increment the semaphore value 
  sem_b[1].sem_flg = SEM_UNDO;

       if (semop(sem_id, &sem_b, 1) == -1)
            perror("\nError in sem_wait:");
      else
      {
             printf("\nSemaphore in wait);
             return(1);
             exit(0); 
       }
}

int semaphore_signal(int sem_id, int sem_num)
{
  struct sembuf sem_b;
 
  sem_b.sem_num = sem_num;
  sem_b.sem_op = -1;
 //It will decrement the semaphore value
  sem_b.sem_flg = SEM_UNDO;
  
  if (semop(sem_id, &sem_b, 1) == -1)
        perror("\nError in sem_signal");
  else
        {
         printf("Semaphore release");
         return(1);
         exit(0);
         }
}