Message-ID: <Pine.LNX.4.44.0305301613300.5956-100000@gannet.stats>
Date: 2003-05-30T15:28:53Z
From: Brian Ripley
Subject: How to check if a pipe was successfully opened or not?
In-Reply-To: <000501c326b9$67413dc0$e502eb82@alpha.wehi.edu.au>
On Fri, 30 May 2003, Henrik Bengtsson wrote:
> Is there a way to detect if the opening of a connection to a pipe was
> successful or not? Here are two examples
>
> # Works
> > con <- pipe("ls")
> > res <- open(con, open="r")
> > print(res)
> NULL
>
> # Does not work
> > con <- pipe("unknown_command")
> > res <- open(con, open="r")
> > 'unknown_command' is not recognized as an internal or external
> command,
> operable program or batch file.
> > print(res)
> NULL
open() always returns NULL.
> Can I make my script recognize/detect that the latter failed? try() will
> not catch the error. The error message is not written to stdout so
> sink() won't "catch" it either. Does anyone know of a (cross-platform)
> way to test if "unknown_command" exists or not on the current system
> before calling pipe()/open()?
>
> I'm running R v1.7.0 on WinXP.
The C code called by open() has
fp = popen(con->description, mode);
if(!fp) {
warning("cannot open cmd `%s'", con->description);
return FALSE;
}
so presumably your system's popen is returning a FILE stream even though
the command cannot be opened. Not much we can do about that. Solaris
says
The popen() function returns a null pointer if files or
processes cannot be created.
but I think the problem is that the `process' is that launching the shell,
not that of the command. I am not using Windows, so cannot check there.
--
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595