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

[Irfan Shaikh]

Based on your input and pcm.c i wrote folloeing code. But its showing seg fault as pointed by comment in code below. Please tell me what wrong i am doing and how do i correct.

/###################################################################################/
        unsigned int count = (alsaParams->frames/(((alsaParams->bit_per_sample)*(alsaParams->num_channel)) / 8));
        unsigned char *data = alsaParams->data;
        snd_pcm_sframes_t frames_available, frames_transmit;
        const snd_pcm_channel_area_t *area;
        snd_pcm_uframes_t offset;
        char *outbuffer;
        int result;

        /*available space in audio-buffer to write to*/
        frames_available = snd_pcm_avail_update(alsaParams->pcm_handle);

        /*
         * prepare the areas
         * and get back area, offset and frames_transmit
         * frames_transmit gives back the REAL value of available frames which could be written to in the mmaped-area
         * frames_transmit are less than frames_available
         */
        frames_transmit = frames_available;

        snd_pcm_mmap_begin(alsaParams->pcm_handle, &area, &offset, &frames_transmit);

        /*calculates the mmaped-buffer which could be written to*/
        outbuffer = ((char *) area->addr + (area->first + area->step * offset)/ 8);    // ********** SEGFAULT ************

        /*gives back the virtually written (writable) frames which should be always == frames_transmit*/
        result = snd_pcm_mmap_commit(alsaParams->pcm_handle, offset, frames_transmit);

        memcpy(outbuffer, data, count);  //data is actual data

I think if above code works fine should play sound.
/###################################################################################/

Thanks in Advance,
Irfan

________________________________________
From: Jaroslav Kysela [perex at perex.cz]
Sent: Monday, November 15, 2010 6:52 PM
To: Irfan Shaikh
Cc: alsa-devel at alsa-project.org
Subject: 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 at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.


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