copysign man page

copysign, 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);

Link with -lm.

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

copysign(), copysignf(), copysignl():

   || /* Since glibc 2.19: */ _DEFAULT_SOURCE
   || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE


These functions return a value whose absolute value matches that of x, but whose sign bit matches that of y.

For example, copysign(42.0, -1.0) and copysign(-42.0, -1.0) both return -42.0.

Return Value

On success, these functions return a value whose magnitude is taken from x and whose sign is taken from y.

If x is a NaN, a NaN with the sign bit of y is returned.


No errors occur.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
copysign(), copysignf(), copysignl() Thread safety MT-Safe

Conforming to

C99, POSIX.1-2001, POSIX.1-2008. This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).


On architectures where the floating-point formats are not IEEE 754 compliant, these functions may treat a negative zero as positive.

See Also



This page is part of release 4.09 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at

Referenced By


copysignf(3) and copysignl(3) are aliases of copysign(3).

2016-03-15 GNU Linux Programmer's Manual