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Date: Fri, 15 Feb 2019 10:31:22 +0000
From: "jounijl@...oo.co.uk" <jounijl@...oo.co.uk>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: "Arithmetic exception" with modulus operator '%'
Exactly. To be complite:
The host machine prints: "Floating point exception" and outputs a core
file. Uses: /lib/libc.so.7
The Alpine prints: "Arithmetic exception". Uses: /lib/ld-musl-x86_64.so.1
Solaris 10 prints: "Arithmetic exception". Uses: /lib/libc.so.1 ;
/lib/libm.so.2
Ubuntu prints: "Floating point exception" and outputs a core file. Uses:
/lib/x86_64-linux-gnu/libc.so.6
To the question "what do you except":
Of course the behaviour is similar to others and this is correct. As in
programs the behaviour would be best like this: number%zero would be the
number it self when number/zero is undefined or infinity (maby set the
number to the largest known number). To change this, some mathematical
evaluation would be needed. Answer: mod 0: Convenient would be the
number it self ?
Maby the core file has to be enabled somehow. Attached is a complite
test program. The word "trap" is unfamiliar to me. I think it means just
checking with an 'if'-like comparison.
Thank you for the replies. It helped. The simple facts are not always
obvious.
Jouni
the file:
$ cat m.c
#include <stdio.h>
int main( int argc, char *argv[] );
int main( int argc, char *argv[] ){
unsigned int z = 0;
unsigned int w = 0;
//floating point exception: fprintf( stderr, "\n 0 %% 0 = %i",
(int) z%w );
z = 1;
//floating point exception: fprintf( stderr, "\n 1 %% 0 = %i",
(int) z%w );
z = 0; w = 1;
fprintf( stderr, "\n 0 %% 1 = %i", (int) z%w );
z = 1; w = 1;
fprintf( stderr, "\n 1 %% 1 = %i", (int) z%w );
}
On 14.2.2019 23.58, Rich Felker wrote:
> On Fri, Feb 15, 2019 at 03:35:23AM +0000, jounijl@...oo.co.uk wrote:
>> As in the headline. Program stops and prints "Arithmetic exception"
>> at the line where the modulus operator '%' is.
>>
>> I'm compiling in Alpine linux with clang installed from apk:s:
>>
>> clang -c test.c
>> clang -o test test.o
>>
>> The code is:
>>
>> ----- snip -----
>> unsigned int unum = 0;
>> unsigned int umod = 0;
>> unsigned int ures = 0;
>> ures = unum % umod; // <-- this one
>> ----- /snip -----
>>
>> The variables have some values other than 0.
> I don't follow. You say they have some value other than 0, but the
> above example snippet has them as zero, and if they're 0 it's
> undefined behavior and a fault of some sort is a typical result. What
> did you expect to happen?
>
> Rich
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