Re: Why do I get broken pipe on write to a pcm in statePREPARED?

[Takashi Iwai <tiwai@suse.de>]

At Tue, 17 Sep 2002 11:05:44 +0200 (METDST),
Clemens Ladisch wrote:
> 
> Anders Torger wrote:
> > On Monday 16 September 2002 21.31, you [Abramo Bagnara] wrote:
> > > I think that the best behaviour is the current and it's also the
> > > simplest to describe and to understand: poll/select never blocks when
> > > there is nothing to wait.
> > >
> > > ... and in PREPARED state definitely there's nothing to wait from
> > > sound card.
> >
> > I don't agree. (...)
> >
> > Also, as we have noted, there is no real use for the current behaviour
> > (at least no-one has said what it is), while there is use for the
> > proper work-as-all-other-file-descriptors behaviour.
> 
> IMHO the current behaviour is the proper behaviour as implemented by other
> file descriptors, and as mandated by POSIX.

good point.  referring to POSIX helps our decision.


> <http://www.opengroup.org/onlinepubs/007904975/functions/write.html>
> says regarding pipes, FIFOs and sockets:
> | The write() function shall fail if:
> | (...)
> | [EPIPE]
> | An attempt is made to write to a pipe or FIFO that is not open for
> | reading by any process, or that only has one end open.
> | (...)
> | A write was attempted on a socket that is shut down for writing, or is
> | no longer connected.
> 
> 
> > It makes sense to block, there may be another thread, or even another
> > process starting the sound card.
> 
> In the cases cited above, there may be another thread/process which will
> open the other end of the FIFO, or connect to the socket. But write() only
> looks at the state of the file descriptor at the time the call is made,
> and does not take into regard what _might_ happen in the future.
> 
> The 'prepared' state is equivalent to the EPIPE cases above because it's a
> state in which the pcm device is _not_ reading data.

but are you sure that this feature is really implemented?
on my system, write() to an FIFO which is not opened for read doesn't
fail, for example,
	% mkfifo /tmp/foo
	% cat /dev/random > /tmp/foo
and cat is blocked, not failed.

i'll check how poll() behaves...


Takashi


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