Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site hou3c.UUCP
Path: utzoo!linus!philabs!seismo!harpo!floyd!clyde!burl!hou3c!ka
From: ka@hou3c.UUCP (Kenneth Almquist)
Newsgroups: net.bugs.usg,net.unix-wizards
Subject: popen bug
Message-ID: <118@hou3c.UUCP>
Date: Tue, 22-Nov-83 12:01:30 EST
Article-I.D.: hou3c.118
Posted: Tue Nov 22 12:01:30 1983
Date-Received: Fri, 25-Nov-83 04:18:44 EST
Organization: Bell Labs, Holmdel, NJ
Lines: 23

When a program is invoked from cron, the standard input is not openned,
which will confuse some programs.  In particular, it will confuse any
program that uses popen to write to a program, at least under System V,
unless you install this fix.  Popen forks off a child and then redirects
the standard input or output of the child with code that looks like:

		(void) close(stdio);
		(void) fcntl(yourside, 0, stdio);
		(void) close(yourside);

That code fails if stdio and yourside are identical; the fix is to add
a test for this case:

		if (yourside != stdio) {
			(void) close(stdio);
			(void) fcntl(yourside, 0, stdio);
			(void) close(yourside);
		}

By the way, the zero in the call to fcntl is the F_DUPFD function which
is the USG means of duplicating a file descriptor.
				Kenneth Almquist
				{mhtsa,ihnp4}!hou3c!ka