# ilogb man page

ilogb, ilogbf, ilogbl — get integer exponent of a floating-point value

## Synopsis

**#include <math.h>**

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

`Link with `

*-lm*.

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

**ilogb**():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

|| _XOPEN_SOURCE >= 500

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

**ilogbf**(), **ilogbl**():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

## Description

These functions return the exponent part of their argument as a signed integer. When no error occurs, these functions are equivalent to the corresponding logb(3) functions, cast to *int*.

## Return Value

On success, these functions return the exponent of *x*, as a signed integer.

If *x* is zero, then a domain error occurs, and the functions return **FP_ILOGB0**.

If *x* is a NaN, then a domain error occurs, and the functions return **FP_ILOGBNAN**.

If *x* is negative infinity or positive infinity, then a domain error occurs, and the functions return **INT_MAX**.

## Errors

See math_error(7) for information on how to determine whether an error has occurred when calling these functions.

The following errors can occur:

## Attributes

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

Interface | Attribute | Value |
---|---|---|

ilogb(), ilogbf(), ilogbl() | Thread safety | MT-Safe |

## Conforming to

C99, POSIX.1-2001, POSIX.1-2008.

## Bugs

Before version 2.16, the following bugs existed in the glibc implementation of these functions:

- The domain error case where
*x*is 0 or a NaN did not cause*errno*to be set or (on some architectures) raise a floating-point exception. - The domain error case where
*x*is an infinity did not cause*errno*to be set or raise a floating-point exception.

## See Also

## Colophon

This page is part of release 4.13 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 https://www.kernel.org/doc/man-pages/.

## Referenced By

The man pages ilogbf(3) and ilogbl(3) are aliases of ilogb(3).