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How to fix GCC undefined reference to `_finite' or implicit declaration of function ‘_finite’

Problem:

If you’re trying to compile Windows code on Linux, you will often see messages like

linker_warning.txt
lmmin.c:261:10: warning: implicit declaration of function ‘_finite’; did you mean ‘finite’? [-Wimplicit-function-declaration]
  116 |     if(!_finite(myvalue)){

or

linker_output.txt
/usr/bin/ld: mymath.c:(.text+0x1057): undefined reference to `_finite'

and your code won’t compile.

Solution

_finite is a function that is only available on Windows. In order to use it on Linux using GCC or G++, one option is to use isfinite() from math.h:

finite_macro.c
#include <math.h>

#define _finite(v) (isfinite((v)))

In case that function is not available (like on some Microcontroller platforms), you can use __builtin_isfinite(). Note that glibc defines isfinite() as an alias for __builtin_isfinite(). Use it like this:

finite_builtin.c
#define _finite(v) (__builtin_isfinite(v))

In case you want to have code that is compatible with both platforms (Windows and Linux), use

finite_compat.c
#ifdef __linux__
#define _finite(v) (__builtin_isfinite(v))
#endif

or

finite_compat_alt.c
#include <math.h>

#ifdef __linux__
#define _finite(v) (isfinite((v)))
#endif
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