C++

Reference

<cmath> (math.h)

macro/function
<cmath> <ctgmath>

signbit

macro
signbit(x)
function
bool signbit (float x);bool signbit (double x);bool signbit (long double x);
Sign bit
Returns whether the sign of x is negative.

This can be also applied to infinites, NaNs and zeroes (if zero is unsigned, it is considered positive).

In C, this is implemented as a macro that returns an int value. The type of x shall be float, double or long double.
In C++, it is implemented with function overloads for each floating-point type, each returning a bool value.

Parameters

x
A floating-point value.

Return value

A non-zero value (true) if the sign of x is negative; and zero (false) otherwise.

Example

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/* signbit example */
#include <stdio.h>      /* printf */
#include <math.h>       /* signbit, sqrt */

int main()
{
  printf ("signbit(0.0)       : %d\n",signbit(0.0));
  printf ("signbit(1.0/0.0)   : %d\n",signbit(1.0/0.0));
  printf ("signbit(-1.0/0.0)  : %d\n",signbit(-1.0/0.0));
  printf ("signbit(sqrt(-1.0)): %d\n",signbit(sqrt(-1.0)));
  return 0;
}

Output:

signbit(0.0)      : 0
signbit(1.0/0.0)  : 0
signbit(-1.0/0.0) : 1
signbit(sqrt(-1.0): 1


See also

isinf
Is infinity (macro/function)
isnormal
Is normal (macro/function)
isnan
Is Not-A-Number (macro/function)
fpclassify
Classify floating-point value (macro/function)