Direct Access (MMAP) Functions

Overview

See the PCM (digital audio) interface page for more details. More…

// global functions

int
snd_pcm_mmap_begin(
    snd_pcm_t* pcm,
    const snd_pcm_channel_area_t** areas,
    snd_pcm_uframes_t* offset,
    snd_pcm_uframes_t* frames
);

snd_pcm_sframes_t
snd_pcm_mmap_commit(
    snd_pcm_t* pcm,
    snd_pcm_uframes_t offset,
    snd_pcm_uframes_t frames
);

snd_pcm_sframes_t
snd_pcm_mmap_writei(
    snd_pcm_t* pcm,
    const void* buffer,
    snd_pcm_uframes_t size
);

snd_pcm_sframes_t
snd_pcm_mmap_readi(
    snd_pcm_t* pcm,
    void* buffer,
    snd_pcm_uframes_t size
);

snd_pcm_sframes_t
snd_pcm_mmap_writen(
    snd_pcm_t* pcm,
    void** bufs,
    snd_pcm_uframes_t size
);

snd_pcm_sframes_t
snd_pcm_mmap_readn(
    snd_pcm_t* pcm,
    void** bufs,
    snd_pcm_uframes_t size
);

Detailed Documentation

See the PCM (digital audio) interface page for more details.

Global Functions

int
snd_pcm_mmap_begin(
    snd_pcm_t* pcm,
    const snd_pcm_channel_area_t** areas,
    snd_pcm_uframes_t* offset,
    snd_pcm_uframes_t* frames
)

Application request to access a portion of direct (mmap) area.

It is necessary to call the snd_pcm_avail_update() function directly before this call. Otherwise, this function can return a wrong count of available frames.

The function should be called before a sample-direct area can be accessed. The resulting size parameter is always less or equal to the input count of frames and can be zero, if no frames can be processed (the ring buffer is full).

See the snd_pcm_mmap_commit() function to finish the frame processing in the direct areas.

The function is thread-safe when built with the proper option.

Parameters:

pcm	PCM handle
areas	Returned mmap channel areas
offset	Returned mmap area offset in area steps (== frames)
frames	mmap area portion size in frames (wanted on entry, contiguous available on exit)

Returns:

0 on success otherwise a negative error code

snd_pcm_sframes_t
snd_pcm_mmap_commit(
    snd_pcm_t* pcm,
    snd_pcm_uframes_t offset,
    snd_pcm_uframes_t frames
)

Application has completed the access to area requested with snd_pcm_mmap_begin.

You should pass this function the offset value that snd_pcm_mmap_begin() returned. The frames parameter should hold the number of frames you have written or read to/from the audio buffer. The frames parameter must never exceed the contiguous frames count that snd_pcm_mmap_begin() returned. Each call to snd_pcm_mmap_begin() must be followed by a call to snd_pcm_mmap_commit().

Example:

 double phase = 0;
 const snd_pcm_area_t *areas;
 snd_pcm_sframes_t avail, size, commitres;
 snd_pcm_uframes_t offset, frames;
 int err;

 avail = snd_pcm_avail_update(pcm);
 if (avail < 0)
   error(avail);
 // at this point, we can transfer at least 'avail' frames

 // we want to process frames in chunks (period_size)
 if (avail < period_size)
   goto _skip;
 size = period_size;
 // it is possible that contiguous areas are smaller, thus we use a loop
 while (size > 0) {
   frames = size;

   err = snd_pcm_mmap_begin(pcm_handle, &areas, &offset, &frames);
   if (err < 0)
     error(err);
   // this function fills the areas from offset with count of frames
   generate_sine(areas, offset, frames, &phase);
   commitres = snd_pcm_mmap_commit(pcm_handle, offset, frames);
   if (commitres < 0 || commitres != frames)
     error(commitres >= 0 ? -EPIPE : commitres);

   size -= frames;
 }
_skip:

Look to the Sine-wave generator example for more details about the generate_sine function.

The function is thread-safe when built with the proper option.

Parameters:

pcm	PCM handle
offset	area offset in area steps (== frames)
frames	area portion size in frames

Returns:

count of transferred frames otherwise a negative error code

snd_pcm_sframes_t
snd_pcm_mmap_writei(
    snd_pcm_t* pcm,
    const void* buffer,
    snd_pcm_uframes_t size
)

Write interleaved frames to a PCM using direct buffer (mmap)

If the blocking behaviour is selected, then routine waits until all requested bytes are played or put to the playback ring buffer. The count of bytes can be less only if a signal or underrun occurred.

If the non-blocking behaviour is selected, then routine doesn’t wait at all.

Parameters:

pcm	PCM handle
buffer	frames containing buffer
size	frames to be written
-EBADFD	PCM is not in the right state (SND_PCM_STATE_PREPARED or SND_PCM_STATE_RUNNING)
-EPIPE	an underrun occurred
-ESTRPIPE	a suspend event occurred (stream is suspended and waiting for an application recovery)

Returns:

a positive number of frames actually written otherwise a negative error code

snd_pcm_sframes_t
snd_pcm_mmap_readi(
    snd_pcm_t* pcm,
    void* buffer,
    snd_pcm_uframes_t size
)

Read interleaved frames from a PCM using direct buffer (mmap)

If the blocking behaviour was selected, then routine waits until all requested bytes are filled. The count of bytes can be less only if a signal or underrun occurred.

If the non-blocking behaviour is selected, then routine doesn’t wait at all.

Parameters:

pcm	PCM handle
buffer	frames containing buffer
size	frames to be written
-EBADFD	PCM is not in the right state (SND_PCM_STATE_PREPARED or SND_PCM_STATE_RUNNING)
-EPIPE	an overrun occurred
-ESTRPIPE	a suspend event occurred (stream is suspended and waiting for an application recovery)

Returns:

a positive number of frames actually read otherwise a negative error code

snd_pcm_sframes_t
snd_pcm_mmap_writen(
    snd_pcm_t* pcm,
    void** bufs,
    snd_pcm_uframes_t size
)

Write non interleaved frames to a PCM using direct buffer (mmap)

If the blocking behaviour is selected, then routine waits until all requested bytes are played or put to the playback ring buffer. The count of bytes can be less only if a signal or underrun occurred.

If the non-blocking behaviour is selected, then routine doesn’t wait at all.

Parameters:

pcm	PCM handle
bufs	frames containing buffers (one for each channel)
size	frames to be written
-EBADFD	PCM is not in the right state (SND_PCM_STATE_PREPARED or SND_PCM_STATE_RUNNING)
-EPIPE	an underrun occurred
-ESTRPIPE	a suspend event occurred (stream is suspended and waiting for an application recovery)

Returns:

a positive number of frames actually written otherwise a negative error code

snd_pcm_sframes_t
snd_pcm_mmap_readn(
    snd_pcm_t* pcm,
    void** bufs,
    snd_pcm_uframes_t size
)

Read non interleaved frames to a PCM using direct buffer (mmap)

If the blocking behaviour was selected, then routine waits until all requested bytes are filled. The count of bytes can be less only if a signal or underrun occurred.

If the non-blocking behaviour is selected, then routine doesn’t wait at all.

Parameters:

pcm	PCM handle
bufs	frames containing buffers (one for each channel)
size	frames to be written
-EBADFD	PCM is not in the right state (SND_PCM_STATE_PREPARED or SND_PCM_STATE_RUNNING)
-EPIPE	an overrun occurred
-ESTRPIPE	a suspend event occurred (stream is suspended and waiting for an application recovery)

Returns:

a positive number of frames actually read otherwise a negative error code