Re: Symlinks under /proc

[Igor Pechtchanski <pechtcha at cs dot nyu dot edu>]

On Mon, 31 Jan 2005, Corinna Vinschen wrote:

> On Jan 31 12:43, Igor Pechtchanski wrote:
> > On Mon, 31 Jan 2005, Corinna Vinschen wrote:
> > > On Jan 31 12:20, Igor Pechtchanski wrote:
> > > > That's cool.  Don't want to seem pushy, but how hard would it be
> > > > to go from the above to /dev/std{in,out,err}?  Come to think of
> > > > it, it may even be as easy as "ln -s /proc/self/fd/0 /dev/stdin",
> > > > etc...  That would really help some apps, especially those that
> > > > don't understand the "-" filename argument.
> > >
> > > Why don't you just try it?  Experiments are the bread and butter of
> > > progress, aren't they?
> >
> > D'oh!  After all this thought process, I looked on Linux, and that's
> > *exactly* how /dev/std* are implemented there.
>
> Talking about experiments, did you also try it on Cygwin if it works or
> did you just look on Linux how it's implemented there?

I tried it on the CVS HEAD.  Making /dev/stdin a symlink works wherever
/proc/self/fd/0 works (as expected).
Both work perfectly for programs invoked directly from the bash command
line.  Pipes are another story.  The following fails for me:

$ echo "blaaah" | perl -pe 's/aaa/bbb/g' /proc/self/fd/0
Can't open /proc/self/fd/0: No such file or directory.

This shows some more detail:

$ echo "blaaah" | perl -e 'print `ls -l /proc/self/fd/0 2>&1`,`ls -lL /proc/self/fd/0 2>&1`'
lrwxrwxrwx    1 igor     root            0 Jan 31 18:54 /proc/self/fd/0 -> pipe:[1700]
ls: /proc/self/fd/0: No such file or directory

Could it be that the pipe is closed before the child process completes?
But no, making the first process in the pipe longer-running doesn't help:

$ (echo "blaaah";sleep 10;echo "blaaarg") | perl -e 'print `ls -lL /proc/self/fd/0`'
$ (echo "blaaah";sleep 10;echo "blaaarg") | perl -e 'print `ls -l /proc/self/fd/0 2>&1`,`ls -lL /proc/self/fd/0 2>&1`'
lrwxrwxrwx    1 igor     root            0 Jan 31 18:54 /proc/self/fd/0 -> pipe:[1700]
ls: /proc/self/fd/0: No such file or directory

Doesn't look like the symlink target is valid.  I have the strace output,
but unfortunately, I don't have time at the moment to look at it in more
detail.  I can send it if needed.  Sorry I couldn't be more helpful.
	Igor
P.S. "uname -orv" shows "1.5.13(0.118/4/2) 2005-01-31 18:09 Cygwin"
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