The short answer is you need to do this...
#ifdef __cplusplus
extern "C" {
#endif
float64_t f_add(float64_t a, float64_t b); // Returns a+b . Special case: -INF + INF = NaN
float64_t f_sub(float64_t a, float64_t b); // Returns a-b . Special case: INF - INF = NaN
float64_t f_mult(float64_t fa, float64_t fb); // Returns a*b . Special case: +/-INF * 0 = NaN
float64_t f_div(float64_t x, float64_t y); // Returns a/b . Special cases: x/0=NaN , INF/INF = NaN , x/INF = 0 if x!=+/-INF
#ifdef __cplusplus
}
#endif
The long answer...