Re: Tesing a fifo without opening it?

["Dominik Vogt" <dominik.vogt@xxxxxx>]

> > 05   if read -t REQUEST < "$1"; then
> [...]
> > Now, the problem is that input redirection in line 5 blocks
> > until the main script also opens the fifo.  So the -t flag of
> > read does not "work" here (because read is not even executed
> > until the fifo can be opened).  I want to check if data is
> > available in the fifo without blocking, but I can't find a way
> > around blocking in the input redirection.
> [...]
> 
> On Linux (at least, though the behavior is not clearly defined
> by POSIX)
> 
> read -t REQUEST <> "$1"
> 
> should work.

That prevents the shell from blocking, the reading shell does
not receive any data and the writing shell is killed by SIGPIPE:

1. Run "echo foo > fifo" in one shell W
2. W blocks when opening the fifo (no reader open)
3. Run "read -t 0 REQUEST <> fifo" in shell R
4. R opens the fifo without blocking.
5. W opens the fifo.
6. R runs "read -t 0" and notices that no data is available.
7. R closes the fifo.
8. W tries to write "foo" to the pipe.
9. As the pipe has already been closed, W gets SIGPIPE and dies.

So what is actually required is opening the shell non-blocking,
but blocking for input if the pipe could be opened.

I think I'm asking for something that is not possible without
precise control over the underlying syscalls.

> Otherwise, other approaches: open with O_NONBLOCK (though you'd
> need to do it in something else as I don't think zsh has an
> interface to open with O_NONBLOCK).

Right.  I'm not aware that there is such an interface.

> Or you could do
> 
> : > "$1" & read...
> 
> though it would have other side effects.

Allright, this approach works:

  $ : > fifo & read -t 1 REQUEST <> fifo; kill %

Thanks for helping!

Ciao

Dominik ^_^  ^_^

P.S.:  Please don't cc me; I'm on the list.
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