[alsa-devel] mm functions vs. normal IO

[Clemens Ladisch]

Reiner wrote:
> I was astonished that a simple wav playback example does not exist.

The alsa-lib package has test/pcm_min.c.

> What should be used:
>
> snd_pcm_mmap_commit
> (I think snd_pcm_mmap_writei is the same with an additional copy behind ?)

snd_pcm_mmap_writei is the same as calling snd_pcm_mmap_begin, copying
into the buffer, and calling snd_pcm_mmap_commit.

> or snd_pcm_writei

If you have to ask, use snd_pcm_writei.

> I think mmap will have better performance,

Only if the program generates samples on the fly and can write them
directly to the buffer in a format supported by the hardware.

If you already have the data in some buffer, copying it into the
device's buffer with snd_pcm_writei or snd_pcm_mmap_writei is just as
fast as when you copy it into the device's buffer by yourself.

> but will it work with all soundcards?

Not directly.  ALSA has mmap emulation for cards that don't support it.
And there are devices where only support mmap access is
SND_PCM_ACCESS_MMAP_COMPLEX.

> I'am also looking for a function to get the number of played frames.

ALSA doesn't count them.  This is nothing that a program couldn't do
trivially by itself.


HTH
Clemens