Re: sio_write blocking?

On Tue, Feb 09, 2021 at 07:35:12PM -0800, Elijah Stone wrote:
> Hi all,
> 
> The manual page for sndio currently says:
> 
> > using the buffer usage to guess if sio_write() would block is false and
> > leads to unreliable programs
> 
> Why is this?  Is it perhaps talking about using bufsz instead of appbufsz?

Hi,

As soon as the hardware consumes buffered samples, space is freed and
the same amount could be transmitted with sio_write(). However the
code used to transmit the samples may block (eg. network layers may
block), and it's difficult to calculate in advance how many bytes
could be written without blocking even if we know the amount of free
buffer space.

> (Additionally, is there a way to tell how much can be written without
> blocking?  An equivalent to alsa's snd_pcm_avail or SNDCTL_DSP_GETOSPACE?)
> 

To answer your question, the space available in the hardware buffer
is:

	buf_avail = sio_par.bufsz - buf_used
	          = sio_par.bufsz - (samples_written - samples_played)

The numer of samples played is the sum of "deltas" passed to the
sio_onmove() call-back. If space is available, sio_write() still could
block.

The SNDCTL_DSP_GETOSPACE-style interfaces used to be used for two
different things:

(1) calculate the current play position: for instance, video players
    need it to synchronize video to audio. The current play position
    is easily obtained using sio_onmove()

(2) determine how many bytes to write. There's no sndio equivalent for
    this, the program could use poll() and non-blocking sio_write()
    which is the standard unix way to implement event loops.

Finally, note that network layers are fast, so if there's space
available in the hardware buffer, sio_write() may block, but not for
very long. This might be acceptable, depending on the program. For
low-latency real-time synths it is not acceptable. For players that
use large buffers it works, though that's not very elegant.

If you provide more details about the program, maybe I could point you
to code of a similar program

HTH

-- Alexandre