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Added more explanation for i/o access

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Jaroslav Kysela 2002-03-27 18:39:42 +00:00
parent 708847237b
commit e131f156b5
1 changed files with 12 additions and 4 deletions
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16
src/pcm/pcm.c
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@ -224,18 +224,26 @@ embedded poll waiting implementation.
\subsection alsa_pcm_rw Read / Write transfer \subsection alsa_pcm_rw Read / Write transfer
There are two versions of read / write routines. The first expects the There are two versions of read / write routines. The first expects the
interleaved samples at input, and the second one expects non-interleaved interleaved samples at input (#SND_PCM_ACCESS_RW_INTERLEAVED access method),
(samples in separated buffers) at input. There are these functions for and the second one expects non-interleaved (samples in separated buffers -
interleaved transfers: \link ::snd_pcm_writei \endlink, #SND_PCM_ACCESS_RW_NONINTERLEAVED access method) at input. There are these
functions for interleaved transfers: \link ::snd_pcm_writei \endlink,
\link ::snd_pcm_readi \endlink. For non-interleaved transfers, there are \link ::snd_pcm_readi \endlink. For non-interleaved transfers, there are
these functions: \link ::snd_pcm_writen \endlink and \link ::snd_pcm_readn these functions: \link ::snd_pcm_writen \endlink and \link ::snd_pcm_readn
\endlink. \endlink.
\subsection alsa_mmap_rw Direct Read / Write transfer (via mmap'ed areas) \subsection alsa_mmap_rw 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. Access
#SND_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.
\par
There are two functions for this kind of transfer. Application can get an There are two functions for this kind of transfer. Application can get an
access to memory areas via \link ::snd_pcm_mmap_begin \endlink function. access to memory areas via \link ::snd_pcm_mmap_begin \endlink function.
This functions returns the areas (single area is equal to a channel) This function returns the areas (single area is equal to a channel)
containing the direct pointers to memory and sample position description containing the direct pointers to memory and sample position description
in \link ::snd_pcm_channel_area_t \endlink structure. After application in \link ::snd_pcm_channel_area_t \endlink structure. After application
transfers the data in the memory areas, then it must be acknowledged transfers the data in the memory areas, then it must be acknowledged