Using MMAP feature

Using MMAP feature

[alberto Villa]

Hello,

I'm going to use the MMAP feature of ALSA. I understood I have to use 
snd_pcm_mmap_begin and snd_pcm_mmap_commit functions, but I can't 
understand the meaning of some parameters in  the snd_pcm_mmap_begin 
function: when I call this function in order to have access to the 
capture stream on a call like this:

err = snd_pcm_mmap_begin(capture, &c_area, &c_offset, &c_frames);

it keeps to return c_frames = 0: is it right ? what does  this mean? is 
the buffer full so that I can read samples or the available contiguous 
space to be read is 0?

Thank you,

Alberto

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Re: Using MMAP feature

[Clemens Ladisch]

alberto Villa wrote:> Hello,
> I'm going to use the MMAP feature of ALSA.

Why do you need to do this?

> snd_pcm_mmap_begin(capture, &c_area, &c_offset, &c_frames);
>
> it keeps to return c_frames = 0: is it right ? what does this mean?

It means that there are no samples available at the moment.
(Or that you forgot to call snd_pcm_avail_update(), or that you
didn't set c_frames before the call.)


Regards,
Clemens

Re: Using MMAP feature

[Clemens Ladisch]

(please don't drop the mailing list)
(please configure your mailer to wrap lines)

alberto Villa wrote:
> Il 23/07/2012 12:38, Clemens Ladisch ha scritto:
>> alberto Villa wrote:
>>> Hello,
>>> I'm going to use the MMAP feature of ALSA.
>>
>> Why do you need to do this?
>
> I need to use MMAP because we are running on an embedded system and we
> want to optimize its operations.

What exactly do you want to optimize?  In most cases, mmap is no faster
than snd_pcm_write; using mmap make sense only if the samples are
generated dynamically without storing them in a userspace buffer.

> It was necessary to use snd_pcm_wait() before snd_pcm_begin(), may be
> this the reason?

If you never wait, the buffer will be filled eventually.


Regards,
Clemens