shmat returning NULL with 0 sized segment

shmat returning NULL with 0 sized segment

[Dave Gilbert]

Hi,
  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.

(Linux/Alpha 2.4.0-test8)

Back to trying to find out why it decided to allocate a  0 byte chunk....

Dave


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Re: shmat returning NULL with 0 sized segment

[Christoph Rohland]

Dave Gilbert <gilbertd@treblig.org> 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.

Greetings
                Christoph

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