copysign.c\math\src - musl - musl - an implementation of the standard library for Linux-based systems

#include "libm.h"
double copysign(double x, double y) {
	union {double f; uint64_t i;} ux={x}, uy={y};
	ux.i &= -1ULL/2;
	ux.i |= uy.i & 1ULL<<63;
	return ux.f;
}