Gabriel Paubert wrote:
> On Mon, Jan 19, 2009 at 10:33:40AM +0100, Wolfgang Grandegger wrote:
>> Hello,
>>
>> I want to provoke a "floating point exception" by doing a division by 0.
>> The expection does come as expected on my x86 PC:
>>
>> $ ./divby0
>> Floating point exception
>>
>> but not on my MPC5200 board. Any idea why? I think the processor can
>> handle all floating-point exceptions. I'm using Linux 2.6.28 and the
>> ELDK v4.2.
>
> No, the PPC continue happily after performing an _integer_ division by zero.
> It is in the architecture specification AFAIR and allowed by C and other
> language definitions.
>
> This is not exactly a floating point exception per se: it has a different
> vector on x86, that the Linux kernel remaps to a floating point exception,
> although numeric exception would be more correct. PPC implements all
> standard IEEE754 floating point exceptions (you have to enable them
> with fesetenv() or something equivalent, but it's the same on all
> architectures).
>
> Even the C++ headers acknowledge this, see for example:
> /usr/include/c++/4.3/powerpc-linux-gnu/bits/cpu_defines.h
> (or a similar file on your system, note that older versions
> of GCC got it wrong, but I can't remember when it was fixed).
I get the "Floating point exception" signal when I explicitly enable
them with fesetenv() as you suggested:
$ cat divby0.c
#include <stdio.h>
#define __USE_GNU
#include <fenv.h>
int main(int argc, char* argv[])
{
double fa = 10., fb = 0., fc;
int ia = 10, ib = 0, ic;
printf("excepts=%#x\n", fegetexcept());
fesetenv(FE_NOMASK_ENV);
printf("excepts=%#x\n", fegetexcept());
ic = ia / ib;
printf("ic=%d\n", ic);
fc = fa / fb;
printf("c=%f\n", fc);
return 0;
}
$ ./divby0
excepts=0
excepts=0x3e000000
ic=0
Floating point exception
See "man fesetenv" for further information. On the x86 PC, I even get a
SIGFPE signal for the integer division by 0.
Thanks for the quick help.
Wolfgang.
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