Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!gorodish!guy
From: guy@gorodish.Sun.COM (Guy Harris)
Newsgroups: comp.lang.c
Subject: Re: exit(-1), 0 is sometimes magic
Message-ID: <39799@sun.uucp>
Date: 23 Jan 88 05:01:42 GMT
References: <502@cresswell.quintus.UUCP> <6935@brl-smoke.ARPA> <1179@wjvax.UUCP> <562@cresswell.quintus.UUCP>
Sender: news@sun.uucp
Lines: 24
Keywords: exit, zero, shells

> Beware, by the way.  Due to byte sex problems, even in UNIX, the
> result of system() may be shifted left by 8 (it is on a SUN).

Nope.  This has nothing to do with byte sex problems, and is true on all
non-broken UNIX implementations (even those with a byte size other than 8
bits).

On UNIX systems, "system()" calls "wait()" to wait for the shell that runs the
command to terminate; the value that "wait()" stuffs into the "int" pointed to
by its first argument is what "system()" returns.  The exit status of the shell
is always shifted left by 8 in the value returned by "wait()"; the low order 8
bits are used if the shell stops or is terminated by a signal.

(Ignore the fact that in BSD systems "wait" is nominally declared as returning
a "union wait".  The BSD *kernel* code still returns an "int", in the same way
that all other UNIX implementations do.  The "union wait" was, I presume, an
attempt to make it easier to pull the exit status apart.  Unfortunately, since
the order of bit fields in a structure is implementation-dependent, that
structure is not portable!  Note that the Sun version of that structure has
"#ifdef"s giving one definition for little-endian VAXes and another for
big-endian Sun-3s and Sun-4s.)
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com