Re: shmat returning NULL with 0 sized segment

From: Christoph Rohland (
Date: Tue Dec 26 2000 - 04:31:37 EST

Dave Gilbert <> writes:

> I'm trying to debug a weird problem with Xine - its screwing up its use
> of shared memory for regions I haven't sussed yet. One odd consequence is
> that it has apparently successfully managed to allocate a 0 byte chunk of
> shared memory; shmat is then called with shmaddr=0 and shmflg=0; the
> result of shmat is 0
> Is this what shmat is supposed to do in this (admittedly odd)
> circumstance? The error behaviour is defined in the man page as returning
> -1 on error.

Yes, this should be competely legal and wanted. Some programs use
shmget (..,0,..) to test if the segment is there. Apparently Xine does
this while setting the IPC_CREATE flag. This is legal on 2.4 (wasn't
in 2.2) and gives you a 0 byte segment.

shmat will give you then the legal address 0 like mmap would.


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