Can I make gcc tell me when a calculation results in NaN or inf at runtime?

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Almost any floating-point operation or math library function that produces a NaN from non-NaN inputs should also signal the ‘invalid operation’ floating-point exception; similarly, a calculation that produces an infinity from finite inputs will typically signal either the ‘divide-by-zero’ or ‘overflow’ floating-point exception. So you want some way of turning these exceptions into a SIGFPE.

I suspect the answer will be highly system-dependent, since control of floating-point traps and flags is likely to be supplied by the platform C library rather than by gcc itself. But here’s an example that works for me, on Linux. It uses the feenableexcept function from fenv.h. The _GNU_SOURCE define is necessary for this function to be declared.

#define _GNU_SOURCE
#include <fenv.h>

int main(void) {
    double x, y, z;
    feenableexcept(FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW);

    x = 1e300;
    y = 1e300;
    z = x * y; /* should cause an FPE */

    return 0;
}

A caveat: I think it’s possible with some setups that the exception isn’t actually generated until the next floating-point operation after the one that (in theory) should have caused it, so you sometimes need a no-op floating-point operation (e.g. multiplying by 1.0) to trigger the exception.

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