Date: Mon, 11 Feb 2013 11:45:57 +0200 From: Peter Pentchev <roam@ringlet.net> To: Erich Dollansky <erichsfreebsdlist@alogt.com> Cc: freebsd-doc@freebsd.org Subject: Re: missing information in man 3 system Message-ID: <20130211094557.GB5801@straylight.m.ringlet.net> In-Reply-To: <20130211120438.730ce5d3@X220.ovitrap.com> References: <20130211120438.730ce5d3@X220.ovitrap.com>
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On Mon, Feb 11, 2013 at 12:04:38PM +0700, Erich Dollansky wrote:
> Hi,
>=20
> man 3 system says:
>=20
> RETURN VALUES
>=20
> The system() function returns the exit status of the shell as returned
> by waitpid(2), or -1 if an error occurred when invoking fork(2) or
> waitpid(2). A return value of 127 means the execution of the shell
> failed.
[snip]
>=20
> system () returns the result from 'error.c' in the result but shifted
> by 8 bits to the left.
>=20
> I could not find this behaviour in the FreeBSD documentation.
>=20
> So, is this the intended behaviour of system ()?
>=20
> Should the documentation be uptdated?
>=20
> Or did I simply miss something?
Yes, this is indeed the intended behavior of system(3) - it does say "as
returned by waitpid(2)" :) Look at the manual page for the waitpid
system call (in section 2, as noted by the number in parentheses) - what
it returns is not the exact exit code of the finished program, it
encodes a bit more information there. This return code ought to be
examined using the <sys/wait.h> macros described in the wait/waitpid
manual page; you need to do something like:
if (WIFEXITED(code)) {
printf("The program exited with code %d\n",
WEXITSTATUS(code));
} else if (WIFSIGNALED(code)) {
printf("The program was terminated by signal %d\n",
WTERMSIG(code));
} else if (WIFSTOPPED(code)) {
/**
* You will only get here if you explicitly ask to be
* notified about child processes being stopped using
* the flags of the waitpid() syscall; you will never
* get here when examining the exit code of system(3).
*/
printf("The program was stopped by signal %d\n",
WSTOPSIG(code));
} else {
printf("The program neither exited, nor was "
"terminated or stopped by a signal; what in "
"the world does wait() return value %d mean?!\n",
code);
}
Yes, it may seem a bit convoluted at first glance, but it is indeed
necessary for the purpose of providing all the information in a single
return value. Don't worry, this is something that everyone stumbles
into when making their first steps in system programming :) (pun
unintended ;)
Hope that helps!
G'luck,
Peter
--=20
Peter Pentchev roam@ringlet.net roam@FreeBSD.org p.penchev@storpool.com
PGP key: http://people.FreeBSD.org/~roam/roam.key.asc
Key fingerprint 2EE7 A7A5 17FC 124C F115 C354 651E EFB0 2527 DF13
This sentence claims to be an Epimenides paradox, but it is lying.
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