Divide by zero does not crash

[Bill]

I noticed that my application does not crash when there is a divide by
zero. For example, the following test code:

int i;
i = 10/0;
printf("i = %d\n", i);

gives:

i = 0

It does not crash.

Is there a way to make the application crash when there is a divide by
zero? I am running Linux 2.4.26 on a powerpc (MPC8248). I compile
with powerpc-linux-gcc.

[Dirk Heinrichs]

Post by Bill
I noticed that my application does not crash when there is a divide by
int i;
i = 10/0;
printf("i = %d\n", i);
i = 0
It does not crash.

Because it's a "div" operation on integer.

Post by Bill
Is there a way to make the application crash when there is a divide by
zero? I am running Linux 2.4.26 on a powerpc (MPC8248). I compile
with powerpc-linux-gcc.

Use float instead of int.

HTH...

Dirk

[John W. Gillikin]

Post by Dirk Heinrichs

Post by Bill
I noticed that my application does not crash when there is a divide by
int i;
i = 10/0;
printf("i = %d\n", i);
i = 0
It does not crash.

Because it's a "div" operation on integer.

Post by Bill
Is there a way to make the application crash when there is a divide by
zero? I am running Linux 2.4.26 on a powerpc (MPC8248). I compile
with powerpc-linux-gcc.

Use float instead of int.
HTH...
Dirk

You will not get an abend with float either:

float f;
f = 10.0/0;
printf("f = %f\n", f);

./a.out
f = INF

An Abend is not good programming either, you should set a signal handle
for integer and floating point exceptions: see <sys/signal.h>

/* SIGFPE */
#define FPE_INTDIV 20
#define FPE_INTOVF 21
#define FPE_FLTDIV 22
#define FPE_FLTOVF 23
#define FPE_FLTUND 24
#define FPE_FLTRES 25
#define FPE_FLTINV 26
#define FPE_FLTSUB 27

Cheers, John.

[Dirk Heinrichs]

Post by John W. Gillikin

Post by Dirk Heinrichs

Post by Bill
I noticed that my application does not crash when there is a divide by
int i;
i = 10/0;
printf("i = %d\n", i);
i = 0
It does not crash.

Because it's a "div" operation on integer.

Post by Bill
Is there a way to make the application crash when there is a divide by
zero? I am running Linux 2.4.26 on a powerpc (MPC8248). I compile
with powerpc-linux-gcc.

Use float instead of int.

float f;
f = 10.0/0;
printf("f = %f\n", f);
./a.out
f = INF

Hmm, you're right. However, I get at least a warning at compile time.

Bye...

Dirk

[Michael Schnell]

Post by Bill
int i;
i = 10/0;

It's evaluated on compile time. So it can't crash (but it should result
in a compiler error or warning).

-Michael

[Bill]

On Feb 8, 7:48 am, Michael Schnell

Post by Michael Schnell

Post by Bill
int i;
i = 10/0;

It's evaluated on compile time. So it can't crash (but it should result
in a compiler error or warning).
-Michael

This does give a warning at compile time because the constant zero is
there. What concerns me is when the denominator is a variable that
takes the value of zero while the application is running. There is
neither a compilation warning nor any indication that a divide by zero
is occurring when the application is running.

[Gil Hamilton]

Post by Bill
On Feb 8, 7:48 am, Michael Schnell

Post by Michael Schnell

Post by Bill
int i;
i = 10/0;

It's evaluated on compile time. So it can't crash (but it should
result in a compiler error or warning).

This does give a warning at compile time because the constant zero is
there. What concerns me is when the denominator is a variable that
takes the value of zero while the application is running. There is
neither a compilation warning nor any indication that a divide by zero
is occurring when the application is running.

The behavior of integer division by 0 is undefined per the C Language
standard and, hence, implementation-dependent. On x86, division by 0
produces a hardware exception. On x86 linux, this exception is presented
to the program as a signal. However, a conforming implementation may
choose to do anything else that is convenient: nothing (leave quotient
unmodified), produce random value for the quotient, kill the process.

GH

[Unruh]

Post by Bill
On Feb 8, 7:48 am, Michael Schnell

Post by Michael Schnell

Post by Bill
int i;
i = 10/0;

It's evaluated on compile time. So it can't crash (but it should result
in a compiler error or warning).
-Michael

This does give a warning at compile time because the constant zero is
there. What concerns me is when the denominator is a variable that
takes the value of zero while the application is running. There is
neither a compilation warning nor any indication that a divide by zero
is occurring when the application is running.

So PUT IN A TEST!!!
if (x==0)
{
fprintf(stderr,"Divide by zero.\n");
exit;
}
else
i=10/x;

[Anton Ertl]

Post by Bill
On Feb 8, 7:48 am, Michael Schnell

Post by Michael Schnell

Post by Bill
int i;
i = 10/0;

It's evaluated on compile time. So it can't crash (but it should result
in a compiler error or warning).

The compiler could insert code that crashes (or produces a signal or
whatever behaviour is deemed appropriate) at run-time. But it does
not, because the behaviour when not doing an evaluation at
compile-time is not to crash or produce a signal. In contrast, on an
AMD64 box I also get a warning at compile-time, and a SIGFPE at
run-time.

Post by Bill
This does give a warning at compile time because the constant zero is
there. What concerns me is when the denominator is a variable that
takes the value of zero while the application is running. There is
neither a compilation warning nor any indication that a divide by zero
is occurring when the application is running.

Yes, that's a property of the PowerPC archictecture. AFAIK the theory
of the hardware designers was that the compiler should produce code
equivalent to:

q=n/d;
if (d == 0)
raise(SIGFPE);

The idea is that the check would be performed during the latency of
the division, so it would usually not cost extra time. So one could
make the hardware simpler by not putting in the check there. I may be
confusing PowerPC with another RISC architecture (most likely MIPS)
wrt this aspect, though.

But obviously gcc does not do this (there does not even seem to be an
option to turn this on). Do other C compilers for PowerPC (from IBM,
Apple or Motorola generate such code)?

Followups set to comp.os.linux.powerpc.

- anton

[Hagen Patzke]

Post by Bill
I noticed that my application does not crash when there is a divide
by zero.

I'd suggest to download the PowerPC ISA 2.05 from power.org.

On p.68 it states for integer divides, e.g. "divw":
"If an attempt is made to perform [...] <anything> ÷ 0 then the contents
of register RT are undefined [...]"

=> no exception

For _floating_point_ operations, on p.111 it states:
"A Zero Divide Exception occurs when a Divide instruction is executed
with a zero divisor value and a finite nonzero dividend value."