github.com/afumu/libc@v0.0.6/musl/src/math/copysign.c (about)

     1  #include "libm.h"
     2  
     3  double copysign(double x, double y) {
     4  	union {double f; uint64_t i;} ux={x}, uy={y};
     5  	ux.i &= -1ULL/2;
     6  	ux.i |= uy.i & 1ULL<<63;
     7  	return ux.f;
     8  }