Date: Mon, 18 May 2015 13:55:07 +0200 From: Polytropon <freebsd@edvax.de> To: gyliamos@gmail.com Cc: freebsd-questions@freebsd.org Subject: Re: Strange return codes from old but good C program Message-ID: <20150518135507.ada90d25.freebsd@edvax.de> In-Reply-To: <slrnmlhru5.190.varro@anukis.local> References: <20150517204503.V69409@sola.nimnet.asn.au> <20150517124223.GA82704@ozzmosis.com> <5558A2D0.8080207@hiwaay.net> <20150517171713.09b01ec4.freebsd@edvax.de> <slrnmlhru5.190.varro@anukis.local>
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On Sun, 17 May 2015 19:45:09 +0000 (UTC), Will Parsons wrote:
> I don't have the actual C standard, but Harbison & Steele's "C - A
> Reference Manual" (which I think can be relied on) states "If the end
> of the body of *main* is reached without returning, it is treated as
> if *return 0;* were executed".
In that case, no random return codes should appear. It also
doesn't meet the little test I've wrote (which again matches
with the initially described problem).
I have written (haha) the following "test case":
% cat returntest.c
main() {
int i, j;
for(i = 0, j = 0; i < 100; i++)
j += i;
}
There are two "error" in it: main() doesn't have a return
type assigned, so per standard (int) will be assumed. And
there is no return statement.
Compiler is system's gcc (older system, obviously):
% cc -Wall -o returntest returntest.c
returntest.c:1: warning: return type defaults to 'int'
returntest.c: In function 'main':
returntest.c:5: warning: control reaches end of non-void function
This is what we expect.
But the program can be run, and we see:
% ./returntest ; echo $?
99
The return code is somehow assigned to the 'i' variable.
Why? Probably because it's stored in a register, and this
register is being used by the exit() and _exit() chain to
represent the return code.
Funnily, when the program is modified:
main() {
int i, j;
for(i = 0, j = 0; i > -100; i--)
j += i;
}
The compiler warnings are the same. This is the result now:
% ./returntest ; echo $?
157
None of them looks like an implicit "return 0;". I am not
judging Harbison & Steele, I'm just observing things. :-)
--
Polytropon
Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...
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