Inconsistent results from divide-by-zero

[Bob Altman]

Hi all,

We have some native C++ code that (inadvertently) performs a divide-by-zero
operation. This code is baked into two different executables, both built by
VS 2005. In one case, the divide-by-zero produces a result of positive
infinity (the debugger displays the value of the variable on the left side
of the equal sign as "#IND00"). In the other case, the divide-by-zero
results in a runtime exception.

We've looked through the project properties for some setting that might turn
divide-by-zero exceptions into "infinity" results, but the closest thing we
can find is /fp:except (which is disabled in both projects, and which would
only have the effect - if I understand it correctly - of deferring the
divide-by-zero exception until the next floating point operation).

Can someone give me a clue as to why the two projects behave differently?

TIA - Bob

 

[David Lowndes]

Can someone give me a clue as to why the two projects behave differently?

Do either of the projects make use of _control87, _controlfp, or
__control87_2 ?

Dave

 

[Bob Altman]

Do either of the projects make use of _control87, _controlfp, or
__control87_2 ?

That sounds promising, but, no, none of the projects use that stuff.

 

[David Lowndes]

Do either of the projects make use of _control87, _controlfp, or

That sounds promising, but, no, none of the projects use that stuff.

Is it possible that one of the projects pulls in some static library
or DLL component that may use those facilities?

Dave

 

[Rong-Chun Zhang [MSFT]]

David, thanks for your valuable input.

Hello Bob,

Thanks for using Microsoft Newsgroup Support Service, my name is Rongchun
Zhang[MSFT] and I will work on this issue with you.

Based on my understanding, division by zero can be divide into two types:
floating point and integer. Floating point division by zero has a
well-defined result. In IEEE 754 arithmetic, a ¡Â 0 is positive infinity
when a is positive, negative infinity when a is negative, and NaN (not a
number) when a = 0. However, integer division by zero is usually handled
differently from floating point since there is no integer representation
for the result. Usually, it generates an exception when attempting to
divide an integer by zero. More information, please check:

http://en.wikipedia.org/wiki/Division_by_zero

Since I don't know how your code works, I would like to suggest that you
check if it is a floating point division or an integer division.

Also, Windows NT has all floating point exceptions (see
http://msdn.microsoft.com/en-us/library/aa226618(VS.60).aspx) turned off by
default. However, the floating-point control state can be changed (as David
pointed out, it might be enabled by other dll). The following KB show you
how to enable all floating point exception.

http://support.microsoft.com/kb/94998

I suggest you try to disable the division by zero exception first in your
code and see if the run time error still happens.

int cw = _control87( 0,0 );
cw |= EM_ZERODIVIDE;
_control87( cw, MCW_EM );

Let me know if this helps. If not, it would be helpful if you can provide
more detailed information about your code and projects, so that we can
investigate the issue locally. It is not necessary that you send out the
whole of your project. We just need a simplest sample to reproduce the
problem. You can remove any confidential information or business details
from it. I appreciate your work on providing these information.

If you have any additional questions and concerns, feel free to contact me.

Best regards,
Rongchun Zhang ([email protected], remove 'online.')
Microsoft Online Community Support

Delighting our customers is our #1 priority. We welcome your comments and
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