ilogb(), ilogbf(), ilogbl() — Integer unbiased exponent

Standards

Standards / Extensions	C or C++	Dependencies
XPG4.2 C99 Single UNIX Specification, Version 3 C++ TR1 C99	both	z/OS® V1R7

Format

#define _XOPEN_SOURCE_EXTENDED 1
#include <math.h>

int ilogb(double x);

C99:

#define _ISOC99_SOURCE
#include <math.h>

int ilogbf(float x);
int ilogbl(long double x);

C++ TR1 C99:

#define _TR1_C99
#include <math.h>

int ilogb(float x); 
int ilogb(long double x);

General description

The ilogb() functions returns the unbiased exponent of its argument x as an integer.

Note: The following table shows the viable formats for these functions. See IEEE binary floating-point for more information about IEEE Binary Floating-Point.

Function	Hex	IEEE
ilogb	X	X
ilogbf	X	X
ilogbl	X	X

Returned value

If successful, the ilogb() functions return the unbiased exponent of x as an integer.

If x is 0, the value FP_ILOGB0 is returned.

if x is a NaN, ilogb() will return FP_ILOGBNAN

if x is infinity, ilogb() will return INT_MAX

If the correct value is greater than {INT_MAX}, {INT_MAX} is returned and a domain error occurs.

If the correct value is less than {INT_MIN}, {INT_MIN} is returned and a domain error occurs.

Special behavior for hex: This function will return the unbiased exponent minus 1 (Because hex representation has no hidden bit, this treatment is needed to satisfy the logb() inequality).

Error Code: Description
EDOM: The x argument is zero, NaN, or ±inf, or the correct value is not representable as an integer.