Question on SysV Non-blocking semaphore

Question on SysV Non-blocking semaphore

Post 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;

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



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


  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".

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

  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.

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

  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
   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 :)


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