Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!auspex!guy
From: guy@auspex.auspex.com (Guy Harris)
Newsgroups: comp.unix.programmer
Subject: Re: How to determine the destination of standard output?
Message-ID: <5845@auspex.auspex.com>
Date: 9 Feb 91 19:31:21 GMT
References: <1991Feb6.204432.2850@cubmol.bio.columbia.edu> <32310001@hpfcdc.HP.COM>
Organization: Auspex Systems, Santa Clara
Lines: 25

>To check if it is going to a pipe, you may have to use the
>stat(2),stat(5) (or actually fstat()) system call on file descriptor 1,
>and look at the st_mode field, in particular, the S_IFIFO bit field
>to tell if stdout is a pipe.

Well, that works on many systems, but not all systems; a better test for
"is this a pipe" may be to do "lseek(fd, 0L, 1)" and see if it fails
setting "errno" to ESPIPE.

>The st_mode field will also tell you if it is going to a character or
>block device, or if it is a regular file...

Yup, and note that the correct way to test for a given file type is

	if ((statbuf.st_mode & S_IFMT) == <the type>)

*NOT*

	if (statbuf.st_mode & <the type>)

Most of you probably already know that, but every so often I see code
written by somebody who didn't, so....

If you have a POSIX-conforming system, you should use the "S_ISxxx"
macros to test for a given file type instead.