http://www.alsa-project.org/alsa-doc/alsa-lib/pcm.html
PCM formats
The full list of formats present the snd_pcm_format_t type. The 24-bit linear samples use 32-bit physical space, but the sample is stored in the lower three bytes. Some hardware does not support processing of full range, thus you may get the significant bits for linear samples via snd_pcm_hw_params_get_sbits() function. The example: ICE1712 chips support 32-bit sample processing, but low byte is ignored (playback) or zero (capture). The function snd_pcm_hw_params_get_sbits() returns 24 in this case.
ALSA transfers
There are two methods to transfer samples in application. The first method is the standard read / write one. The second method, uses the direct audio buffer to communicate with the device while ALSA library manages this space itself. You can find examples of all communication schemes for playback in Sine-wave generator example. To complete the list, we should note that snd_pcm_wait() function contains embedded poll waiting implementation.
Read / Write transfer
There are two versions of read / write routines. The first expects the interleaved samples at input (SND_PCM_ACCESS_RW_INTERLEAVED access method), and the second one expects non-interleaved (samples in separated buffers - SND_PCM_ACCESS_RW_NONINTERLEAVED access method) at input. There are these functions for interleaved transfers:snd_pcm_writei() snd_pcm_readi(). For non-interleaved transfers, there are these functions: snd_pcm_writen() and snd_pcm_readn().
Direct Read / Write transfer (via mmap'ed areas)
Three kinds of organization of ring buffer memory areas exist in ALSA API. Access SND_PCM_ACCESS_MMAP_INTERLEAVED has interleaved samples. AccessSND_PCM_ACCESS_MMAP_NONINTERLEAVED expects continous sample areas for one channel. Access SND_PCM_ACCESS_MMAP_COMPLEX does not fit to interleaved and non-interleaved ring buffer organization.
There are two functions for this kind of transfer. Application can get an access to memory areas via snd_pcm_mmap_begin() function. This function returns the areas (single area is equal to a channel) containing the direct pointers to memory and sample position description in snd_pcm_channel_area_t structure. After application transfers the data in the memory areas, then it must be acknowledged the end of transfer via snd_pcm_mmap_commit() function to allow the ALSA library update the pointers to ring buffer. This kind of communication is also called "zero-copy", because the device does not require to copy the samples from application to another place in system memory.
If you like to use the compatibility functions in mmap mode, there are read / write routines equaling to standard read / write transfers. Using these functions discards the benefits of direct access to memory region. See the snd_pcm_mmap_readi(), snd_pcm_writei(), snd_pcm_readn() and snd_pcm_writen() functions.
