Question on SysV Non-blocking semaphore

by Damo » Sat, 15 Jul 2000 04:00:00

Is there anyone know how to write a non-blocking semaphores function?
I read the book written by Steven R (Unix Network Programming 2) that
there exist a non-blocking semaphores. I found the code of that
try_wait function.
But i can't use it, it only give error.
Does anybody know why?

#include "semaphore.h"

int sem_trywait(sem_t *sem)
{
struct sembuf op;

if (sem->sem_magic != SEM_MAGIC) {
errno = EINVAL;
return(-1);
}

op.sem_num = 0;
op.sem_op = -1;
op.sem_flg = IPC_NOWAIT;
if (semop(sem->sem_semid, &op, 1) < 0)
return(-1);
return(0);

Quote:}

Top

1. mandatory to use SEM_UNDO for non-blocking Semaphores ?

Hi,

Tried the man pages but found the man pages for
semaphores pretty "loaded" for me to grasp, hence
this posting...

I needed a non-blocking semaphores so used "semop"
with "IPC_NOWAIT".

--snip--
struct sembuf v = { 0, -1, IPC_NOWAIT};
if((ret = semop(sem_id, &v, 1)) < 0) {
// error handling...
}
--snip--

This impl caused some of my processes to stall since the
process which called semop first didn't really release the
semaphore even though I released it explicitly using the
following code fragment in a release funtion.

--snip--
struct sembuf p = { 0, 1, SEM_UNDO };
if((ret = semop(sem_id, &p, 1)) < 0) {
// error handling...
}
--snip--

if while opening the semaphore I use "IPC_NOWAIT | SEM_UNDO"
things seem to work fine.
Is it mandatory to use SEM_UNDO with IPC_NOWAIT for non-blocking
semaphores?
Another confusing this is that for simple examples (2 simple
processes just taking/releasing semaphores) this seesm to work
nicely without SEM_UNDO :(

Would appreciate any help/pointers :)

thanks,
Deven

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