[alsa-devel] ALSA: pcm_direct.c SND_PCM_DIRECT_MAGIC

[Takashi Iwai]

On Wed, 23 Sep 2015 18:07:46 +0200,
Mike Bjorge wrote:
> 
> I see, interesting.
> 
> I am running into an issue where pcm_dmix.c is occasionally failing with an
> "unable to create IPC shm instance" (pcm_dmix.c at 1004) because in
> pcm_direct.c at 134, (dmix->shmptr->magic != SND_PCM_DIRECT_MAGIC) when
> buf.shm_nattch == 2.
> 
> Do you know if there are any likely causes that would result in
> dmix->shmptr->magic not being set correctly?

What value is read there?  If the value is completely different, it
means that some other programs use the same shmem for other purpose.
It'd be just a conflict of shmem key.


Takashi

> 
> Thanks for your help,
> Mike
> 
> 
> 
> On Wed, Sep 23, 2015 at 6:11 AM, Takashi Iwai <tiwai at suse.de> wrote:
> 
> > On Sat, 19 Sep 2015 01:13:42 +0200,
> > Mike Bjorge wrote:
> > >
> > > Hi Takashi,
> > >
> > > I know this is a bit out of the blue, but I was wondering if you could
> > > explain the point of SND_PCM_DIRECT_MAGIC and dmix->shmptr->magic?
> > >
> > > Specifically, what it's doing in pcm_direct.c from this commit?
> > >
> > http://git.alsa-project.org/?p=alsa-lib.git;a=blobdiff;f=src/pcm/pcm_direct.c;h=4e2f36cd275bfda1bc3453e569cf7fb44c317ef3;hp=46664a13d0a3368527b103ff13a08f3bcfe1ddbb;hb=eafb7ae5d8fa5ee52757ee0ee9137442d2c39326;hpb=194ff97ea68d66dac8e38671cbf73d52b42f3363
> >
> > It's to check the consistency of shared data via shmem.
> > If we change the data structure in an incompatible way, we'd change
> > the magic key so that each instance can detect it.
> >
> >
> > Takashi
> >
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