Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!seismo!brl-adm!brl-smoke!gwyn
From: gwyn@brl-smoke.UUCP
Newsgroups: comp.unix.questions
Subject: Re: A couple questions
Message-ID: <5767@brl-smoke.ARPA>
Date: Thu, 16-Apr-87 09:55:12 EST
Article-I.D.: brl-smok.5767
Posted: Thu Apr 16 09:55:12 1987
Date-Received: Fri, 17-Apr-87 05:41:48 EST
References: <3164@jade.BERKELEY.EDU> <1296@decuac.DEC.COM> <80@eps2.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 30

In article <80@eps2.UUCP> jon@eps2.UUCP (Jonathan Hue) writes:
-In article <1296@decuac.DEC.COM>, avolio@gildor.dec.com (Frederick M. Avolio) writes:
->        if ((pid = fork()) == 0)		/* IF CHILD */
-> 		{	
->                 execl("/bin/sh", "sh", "-c", arg, 0);
-> 		printf("HELP! SOMEONE STOLE THE SHELL!\n");
-> 		exit(1);
-
-Use _exit() in this instance instead of exit() because there is the
-possibility of flushing the stdio buffers twice, once in the child and
-once in the parent, which wouldn't really be right.  You might see some
-output twice.  I would probably use an fputs to stderr here, to avoid
-buffering.  This was probably just an oversight on Frederick's followup.
-To really confuse people running ps, replace "sh" with a clever saying in
-double quotes.

The above advice, which is good as far as it goes, is insufficient.
Using _exit() will mean that the printf may not be seen (although
with default line-buffering to the terminal it probably would be),
since stdio buffer flushing would be skipped.  Before the fork(),
there should be an fflush(stdout) to clear out any buffered parent
data.  Error messages should probably be written to stderr rather
than stdout (stdout is often directed down a pipe).  Finally, it is
much better style to return an error indication to a higher level
of program control and let the higher level determine strategy (such
as whether to print an error message).

The fact that it is hard to get this stuff exactly right is why one
should use the library routines such as system() instead, whenever
possible.  (If the library routine is broken, get it fixed!)