Question

Today I was tracking down a floating point exception in some code I had just written. It took a little while to find because it was actually caused by taking an integer mod zero. Obviously doing anything mod zero is not going to be defined but I thought it was strange that the error was so misleading. What is it within the C++ modulo operator that would use floating point for two integers? (I’m using gcc 4.3.2)

Here’s a simple program to demonstrate the error.

int main()
{
    int a=3,b=0;
    int c=a%b;
    return 0;
}

Answer 1

The operation triggers SIGFPE:

SIG is a common prefix for signal
names; FPE is an acronym for
floating-point exception. Although
SIGFPE does not necessarily involve
floating-point arithmetic, there is no
way to change its name without
breaking backward compatibility.

GDB is a bit clearer about this and calls it “Arithmetic exception”:

(gdb) run
Starting program: /home/emil/float

Program received signal SIGFPE, Arithmetic exception.
0x0804837d in main () at float.c:4
4           int c=a%b;