Re: [alsa-devel] Direct write (using mmap_bigin and commit)

On Mon, 15 Nov 2010, Irfan Shaikh wrote:

Hello,

    I am reading an raw PCM  file and want to write on PCM device using direct write (interleaved). I have gone through PCM.c test application.

I am trying tio write PCM audio data as follows. I am unable to use snd_pcm_mmap_begin and snd_pcm_mmap_commit properly. Please help me to use using these APIs Question :

1. How can i write to Ring buffer area (any pcm audio data)? //Any example or explanation other than pcm.c
2. Do i need to write diffferently for each channel as shown in pcm.c

I am trying to do following but not succeded (reading pcm file and writing content on device) . ANy thing played on audio device would help me in understanding.

   snd_pcm_channel_area_t my_areas;
   snd_pcm_uframes_t offset, frames, size;
   snd_pcm_sframes_t avail, commitres;
   int result;
   char *buffer;

   while(1)
   {
       avail = snd_pcm_avail_update(handle);
       if(avail>=period_size)
       {
           size=periodsize
           while (size > 0) {
                   frames = size;
                   err = snd_pcm_mmap_begin(handle, &my_areas, &offset, &frames);
                   mybuffer=(char *)malloc(frames*2);
                   result = fread (mybuffer,1,(frames*2),pFile);
                   my_areas.addr=mybuffer;           // IS THIS CORRECT ? WILL THIS WORK ? How can i write my audio file data here in areas?

What's this? You just overwrite the read-only pointer value in my_areas. Use the ring buffer pointer and do memcpy or so. Use area returned from mmap_begin(), don't forget to add offset. Also, application must check, if my_areas description (buffer arrangement) is expected (check for sample step/offset).

The pcm.c is good example. Just try to understand what my_areas really does. And no, it's not necessary to handle just one sample, if you know the PCM ring buffer layout.

Jaroslav

----- Jaroslav Kysela perex@perex.cz Linux Kernel Sound Maintainer ALSA Project, Red Hat, Inc.