Hello,

from what I have learned so far ALSA io plugins are implemented as shared libraries with their entry point defined with the SND_PCM_PLUGIN_DEFINE_FUNC() macro.

However, each plugin instance can only work for either the SND_PCM_STREAM_PLAYBACK or SND_PCM_STREAM_CAPTURE stream. Hence this is why io plugins supporting playback and capture usually set up their callback functions depending on what type of stream is used.

For example in the pulseaudio io plugin, this is done with:

pcm->io.callback = stream == SND_PCM_STREAM_PLAYBACK ? &pulse_playback_callback : &pulse_capture_callback;

* If such a plugin is used for capture AND playback at the same time, it seems that two instances of the plugin need to be running, right ?

* Are these instances running as separate processes (i.e. fork()) or are they implemented as threads ? How is this implemented internally in the ALSA code ?

In my plugin I'm planning that both the playback and the capture plugin instances talk to a common data processing thread over shared buffers. The reason for this is that the data processing threads needs to have access to the capture and playback samples *at the same time*. I'm guessing that the plugin instances run with different PIDs so that I would need to use common IPC mechanisms in order to let them communicate with each other, right ?

Hence my first approach would be to set up a common shared memory together with a semaphore so that only one process can access the shared memory at a time. Any better ideas ?

cheers, Stefan