signbit

Defined in header `<math.h>`
#define signbit( arg ) /* implementation defined */		(since C99)

Determines if the given floating-point number arg is negative. The macro returns an integral value.

Parameters

arg

floating-point value

Return value

Nonzero integral value if arg is negative, 0 otherwise.

Notes

This macro detects the sign bit of zeroes, infinities, and NaNs. Along with copysign, this macro is one of the only two portable ways to examine the sign of a NaN.

Example

#include <math.h>
#include <stdio.h>
 
int main(void)
{
    printf("signbit(+0.0) = %d\n", signbit(+0.0));
    printf("signbit(-0.0) = %d\n", signbit(-0.0));
}

Possible output:

signbit(+0.0) = 0
signbit(-0.0) = 128

References

C23 standard (ISO/IEC 9899:2024):

7.12.3.6 The signbit macro (p: TBD)

C17 standard (ISO/IEC 9899:2018):

7.12.3.6 The signbit macro (p: TBD)

C11 standard (ISO/IEC 9899:2011):

7.12.3.6 The signbit macro (p: 237)

C99 standard (ISO/IEC 9899:1999):

7.12.3.6 The signbit macro (p: 218)