Strange div-by-zero in alsa-lib 1.0.20's snd_pcm_mmap_begin()

Strange div-by-zero in alsa-lib 1.0.20's snd_pcm_mmap_begin()

[Daniel Chen]

Hi,

In https://bugs.launchpad.net/ubuntu/+source/alsa-lib/+bug/412677
we're debugging an issue where snd_pcm_mmap_begin(), at line 6409 with

*offset = *pcm->appl.ptr % pcm->buffer_size;

appears to have pcm->buffer_size == 0. What's the correct approach in
handling this corner case?

Thanks,
Dan

Re: Strange div-by-zero in alsa-lib 1.0.20's snd_pcm_mmap_begin()

[Takashi Iwai]

At Sun, 16 Aug 2009 18:24:35 -0400,
Daniel Chen wrote:
> 
> Hi,
> 
> In https://bugs.launchpad.net/ubuntu/+source/alsa-lib/+bug/412677
> we're debugging an issue where snd_pcm_mmap_begin(), at line 6409 with
> 
> *offset = *pcm->appl.ptr % pcm->buffer_size;
> 
> appears to have pcm->buffer_size == 0. What's the correct approach in
> handling this corner case?

We could add a sanity check in the function, of course.
But relying on it doesn't sound nice.

At least, the caller should be surely at the certain state that the
buffer has been set up, i.e. checking whether snd_pcm_state() returns
SETUP or better condition.


Takashi

Re: Strange div-by-zero in alsa-lib 1.0.20's snd_pcm_mmap_begin()

[Lennart Poettering]

On Mon, 17.08.09 08:49, Takashi Iwai (tiwai@suse.de) wrote:

> 
> At Sun, 16 Aug 2009 18:24:35 -0400,
> Daniel Chen wrote:
> > 
> > Hi,
> > 
> > In https://bugs.launchpad.net/ubuntu/+source/alsa-lib/+bug/412677
> > we're debugging an issue where snd_pcm_mmap_begin(), at line 6409 with
> > 
> > *offset = *pcm->appl.ptr % pcm->buffer_size;
> > 
> > appears to have pcm->buffer_size == 0. What's the correct approach in
> > handling this corner case?
> 
> We could add a sanity check in the function, of course.
> But relying on it doesn't sound nice.
> 
> At least, the caller should be surely at the certain state that the
> buffer has been set up, i.e. checking whether snd_pcm_state() returns
> SETUP or better condition.

Hmm, this bug is triggered in PA apparently. PA doesn't call
snd_pcm_mmap_begin() before the setup finished completely. Not sure
what's going on here, but this smells as if pcm->buffer_size is not
properly initialized.

Daniel, does this happen right-away on PA startup? Or does it
happen sometime while playing?

Could you get us the output of the PA startup phase when running
"pulsaudio -vvvv"? This should show us to which value the buffer_size
is initialized in the snd_pcm_t.

Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net
http://0pointer.net/lennart/           GnuPG 0x1A015CC4

Re: Strange div-by-zero in alsa-lib 1.0.20's snd_pcm_mmap_begin()

[Daniel Chen]

Hi Lennart,

On Mon, Aug 17, 2009 at 10:36 AM, Lennart Poettering<mznyfn@0pointer.de> wrote:
> Could you get us the output of the PA startup phase when running
> "pulsaudio -vvvv"? This should show us to which value the buffer_size
> is initialized in the snd_pcm_t.

The requested debug output is available at
http://launchpadlibrarian.net/30422583/pulseaudio .

Thanks,
Dan

Re: Strange div-by-zero in alsa-lib 1.0.20's snd_pcm_mmap_begin()

[Lennart Poettering]

On Mon, 17.08.09 12:16, Daniel Chen (seven.steps@gmail.com) wrote:

> 
> Hi Lennart,
> 
> On Mon, Aug 17, 2009 at 10:36 AM, Lennart Poettering<mznyfn@0pointer.de> wrote:
> > Could you get us the output of the PA startup phase when running
> > "pulsaudio -vvvv"? This should show us to which value the buffer_size
> > is initialized in the snd_pcm_t.
> 
> The requested debug output is available at
> http://launchpadlibrarian.net/30422583/pulseaudio .

Hmm, that output shows that buffer_size is initialized to 16384, and
the SIGFPE is not thrown.

Could you please reproduce this issue and tell us what buffer_size is
then?

Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net
http://0pointer.net/lennart/           GnuPG 0x1A015CC4