File descriptors and Sound

File descriptors and Sound

[Rui Sousa]

Hi,

Can someone give me some pointers on how to get control
over /dev/audio by two different applications?

I realized that it's not possible to open /dev/audio
in two separate applications while trying to setup
speakfreely...

If the device is full-duplex how can I have one application
reading it and another writing?

Thanks in advance
-- 
Rui Sousa

Re: File descriptors and Sound

[Alan Cox]

> If the device is full-duplex how can I have one application
> reading it and another writing?

You either do file handle passing (funky) or normally one program runs the
other: ie

	open file
	fork()
	
	child:
		exec other app (which inherits file)
		exit

	parent:
		run code

Re: File descriptors and Sound

[Rui Sousa]

Alan Cox wrote:
> 
> > If the device is full-duplex how can I have one application
> > reading it and another writing?
> 
> You either do file handle passing (funky) or normally one program runs the
> other: ie
> 
>         open file
>         fork()
> 
>         child:
>                 exec other app (which inherits file)
>                 exit
> 
>         parent:
>                 run code

As I said before I'm trying to get speakfreely to work in full-duplex
mode.
This program has two components (two independent processes) that try to
access /dev/audio. The thing is that during execution they open/close
/dev/audio multiple times so I think that the second method wont work
(correct me if I am wrong).

Can you indicate me some (online) documentation concerning file handle
passing?

Thanks
-- 
Rui Sousa

Re: File descriptors and Sound

[Matthew Wilcox]

On Mon, Aug 09, 1999 at 03:57:08PM +0100, Rui Sousa wrote:
> Can you indicate me some (online) documentation concerning file handle
> passing?

man 7 unix in very recent manpage distributions (formerly part of netman).

-- 
Matthew Wilcox <willy@bofh.ai>
"Windows and MacOS are products, contrived by engineers in the service of
specific companies. Unix, by contrast, is not so much a product as it is a
painstakingly compiled oral history of the hacker subculture." - N Stephenson

Re: File descriptors and Sound

[Benno Senoner]

On Mon, 09 Aug 1999, Alan Cox wrote:
> > If the device is full-duplex how can I have one application
> > reading it and another writing?
> 
> You either do file handle passing (funky) or normally one program runs the
> other: ie
> 
> 	open file
> 	fork()
> 	
> 	child:
> 		exec other app (which inherits file)
> 		exit
> 
> 	parent:
> 		run code

ok,
but what if one wants to implement some sort of MIDI-thru util ?
(not related to /dev/audio but /dev/midi , but the problem is the same:
2 different apps want to access to the same device,
the one in O_RDONLY , the other in O_WRONLY mode)

For example:

/dev/midi00   external midi port
/dev/midi01   AWE64 internal synth

Assume the midi-thru util opens /dev/midi00 in O_RDONLY and /dev/midi01 in
O_WRONLY mode.
The app reads the incoming midi data from the midi00 device and applies certain
event filters and then writes the data to /dev/midi01. (the synth).

It would be nice if a sequencer was still able to to write to /dev/midi00 to
drive an external synth.

Since the sequencer tries to open /dev/midi00 
(assuming that the sequencer opens the device only in O_WRONLY mode),
 I can't use the fork() method to inherit filehandles.
Is there a way to do this ?

What is in your opinion the best way to implement a virtual midi device in
userspace ? Like the midi-thru example
Using a regular named pipe ?
But sequencers would not recognize this virtual device since they look in
/dev/sndstat for available devices.


regards,
Benno.