copysign, copysignf, copysignl

Defined in header <math.h>
float       copysignf( float x, float y );
(1) (since C99)
double      copysign( double x, double y );
(2) (since C99)
long double copysignl( long double x, long double y );
(3) (since C99)
Defined in header <tgmath.h>
#define copysign(x, y)
(4) (since C99)
1-3) Composes a floating point value with the magnitude of x and the sign of y.
4) Type-generic macro: If any argument has type long double, copysignl is called. Otherwise, if any argument has integer type or has type double, copysign is called. Otherwise, copysignf is called.


x, y - floating point values

Return value

If no errors occur, the floating point value with the magnitude of x and the sign of y is returned.

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

If y is -0, the result is only negative if the implementation supports the signed zero consistently in arithmetic operations.

Error handling

This function is not subject to any errors specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),


copysign is the only portable way to manipulate the sign of a NaN value (to examine the sign of a NaN, signbit may also be used).


#include <stdio.h>
#include <math.h>
int main(void)
    printf("copysign(1.0,+2.0)      = %+.1f\n", copysign(1.0,+2.0));
    printf("copysign(1.0,-2.0)      = %+.1f\n", copysign(1.0,-2.0));
    printf("copysign(INFINITY,-2.0) = %f\n",    copysign(INFINITY,-2.0));
    printf("copysign(NAN,-2.0)      = %f\n",    copysign(NAN,-2.0));

Possible output:

copysign(1.0,+2.0)      = +1.0
copysign(1.0,-2.0)      = -1.0
copysign(INFINITY,-2.0) = -inf
copysign(NAN,-2.0)      = -nan


  • C11 standard (ISO/IEC 9899:2011):
    • The copysign functions (p: 255)
    • 7.25 Type-generic math <tgmath.h> (p: 373-375)
    • F.10.8.1 The copysign functions (p: 529)
  • C99 standard (ISO/IEC 9899:1999):
    • The copysign functions (p: 236)
    • 7.22 Type-generic math <tgmath.h> (p: 335-337)
    • F.9.8.1 The copysign functions (p: 465)

See also

computes absolute value of a floating-point value (|x|)
checks if the given number is negative

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