"No space left on device" error using SystemV shared memory

clalfa · 07-07-2007, 09:17 AM

Hi everyone,

I am using SystemV shared memory and semaphores to let some processes exchange information.
I've used shmget function to create a shared memory area, shmat to attach it to each process, then shmdt and shmctl to detach and deallocate the memory I've used; as for semaphores, I've used semget and semctl.
The problem is, after I've launched the program 8-9 times apparently without problems, when I try to execute the program semget returns an error: no space left on device. What leaves me astonished is the fact that I've actually detached AND released all of the memory I've used, by calling shmdt in each process and shmctl in main process, and by using semctl only in main process.
Does anyone have any idea ???

I'm getting pretty messed up with that program, so please help me!!

thanks

wjevans_7d1@yahoo.co · 07-07-2007, 11:34 AM

The problem you describe is not with shared memory, because you say the error occurs with semget(), which is concerned with semaphores, not shared memory. You probably have a semaphore leak.

In your description, you say that you delete all the shared memory, but do you delete all semaphores that you use?

That would be something to double-check.

Also, to help you figure this out, do this at the command line:

Code:

ipcs -a
man ipcs

Hope this helps.

clalfa · 07-07-2007, 06:14 PM

Hi wjevans!
First of all thank you for answering my post, I really appreciated your help!
I run my program a few times, then checked the ipc status with ipcs (that's REALLY useful

).
The output shows that each semaphore I've used in my program still exists!! Not only, I've also discovered that even shared memory areas aren't deallocated when the program is terminated: they're still in the list provided by ipcs, with 0 processes attached to them.
Basically, detachment is successful, but shared memory isn't really deallocated. Any idea?
This is how I call shmget, shmat, shmctl (to deallocate when detachment is done) and shmdt:

coord_buf_s_id = shmget (IPC_PRIVATE, COORD_DIM * sizeof(pos), IPC_CREAT | S_IRUSR | S_IWUSR );
coord_buf = shmat ( coord_buf_s_id, NULL, SHM_RND );
shmctl (coord_buf_s_id, IPC_RMID, NULL);
shmdt (coord_buf);

Since ipcs reports no precess is attached to shared segments, I suppose there's some kind of problem with shmctl...
As for semaphores, there's no need to attach/deattach, so I just call a
coord_mutex = semget (IPC_PRIVATE, 1, IPC_CREAT | S_IRUSR | S_IWUSR );
when program begins, then call
semctl (coord_mutex, 1, IPC_RMID, arg);
before the last process of the program exits.

Is my code OK?

Thank you in advance

wjevans_7d1@yahoo.co · 07-08-2007, 07:48 AM

Here's my suggestion.

Run the following program. It allocates, attaches, removes, and detaches a shared memory segment.

When it has announced a new shmid, go to another terminal window and run ipcs. That should show you the new shmid.

Then go back to this program and press the <Enter> key. When you run ipcs again, you should notice that the new shmid has disappeared.

When you have confidence that the shared memory facility isn't broken (at least it wasn't for me), figure out what went wrong in your program.

Rinse and repeat for semaphores.

Code:

#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
  int   coord_buf_s_id;

  char *coord_buf;

  char  answer[8];

  coord_buf_s_id=shmget(IPC_PRIVATE,
                        123,
                        IPC_CREAT|S_IRUSR|S_IWUSR
                       );

  if(coord_buf_s_id==-1)
  {
    perror("shmget()");

    exit(1);
  }

  coord_buf=shmat(coord_buf_s_id,
                  NULL,
                  SHM_RND
                 );

  if((int)coord_buf==-1)
  {
    perror("shmat()");

    exit(1);
  }

  printf("shmid %d has been created; press <Enter> to continue. >  ",
         coord_buf_s_id
        );

  fgets(answer,sizeof(answer),stdin);

  if(shmctl(coord_buf_s_id,
            IPC_RMID,
            NULL
           )
    )
  {
    perror("shmctl(,IPC_RMID,NULL)");

    exit(1);
  }

  if(shmdt(coord_buf))
  {
    perror("shmdt()");

    exit(1);
  }

  return 0;

} /* main() */

clalfa · 07-08-2007, 10:59 AM

I've done what you suggested, and solved the problems with shared memory segments: the semctl was to be called BEFORE any detachment...
Unfortunately, the problem with semaphores remains: I've tried running a program like the one you've posted, and everything is ok. ipcs shows the semaphore is allocated, then deallocated when the process is terminated.
I thought the reason why semaphores were not deallocated was the fact that I did not use the flag SEM_UNDO when doing a wait or a signal: I thought there could be a process waiting on a waiting queue of that semaphore which blocked the deallocation, but then when I tried using SEM_UNDO nothing changed.
I really haven't got the faintes idea of what is wrong with my program...

Anyhow thank you so much for your help !!

wjevans_7d1@yahoo.co · 07-09-2007, 07:34 AM

I hesitate to suggest this, partly because I know you've already thought of it and are already working on it, but:

If all else fails, have you thought of taking a destroyable copy of your program and ripping out chunks of it one at a time, testing it each time, as it becomes more and more like your (well-behaved) semaphore test program, until it works, and then focusing on what you ripped out that fixed the problem?

A lot of work, but sometimes that's the only thing that will work.