Question

I have a numerical method that could return nan or inf if there was an error, and for testing purposed I’d like to temporarily force it to return nan or inf to ensure the situation is being handled correctly. Is there a reliable, compiler-independent way to create values of nan and inf in C?

After googling for about 10 minutes I’ve only been able to find compiler dependent solutions.

Answer 1

You can test if your implementation has it:

#include <math.h>
#ifdef NAN
/* NAN is supported */
#endif
#ifdef INFINITY
/* INFINITY is supported */
#endif

The existence of INFINITY is guaranteed by C99 (or the latest draft at least), and “expands to a constant expression of type float representing positive or unsigned
infinity, if available; else to a positive constant of type float that overflows at translation time.”

NAN may or may not be defined, and “is defined if and only if the implementation supports quiet NaNs for the float type. It expands to a constant expression of type float representing a quiet NaN.”

Note that if you’re comparing floating point values, and do:

a = NAN;

even then,

a == NAN;

is false. One way to check for NaN would be:

#include <math.h>
if (isnan(a)) { ... }

You can also do: a != a to test if a is NaN.

There is also isfinite(), isinf(), isnormal(), and signbit() macros in math.h in C99.

C99 also has nan functions:

#include <math.h>
double nan(const char *tagp);
float nanf(const char *tagp);
long double nanl(const char *tagp);

(Reference: n1256).

Docs INFINITY
Docs NAN