copysignl(3) -- Linux man page
NAMEcopysign, copysignf, copysignl - copy sign of a number
#include <math.h> double copysign(double x, double y);
float copysignf(float x, float y);
long double copysignl(long double x, long double y);
DESCRIPTIONThe copysign() functions return a value whose absolute value matches that of x, but whose sign matches that of y. If x is a NaN, then a NaN with the sign of y is returned.
NOTESThe copysign() functions may treat a negative zero as positive.
CONFORMING TOC99, BSD 4.3. This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).