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

**#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

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || /* Since glibc 2.19: */ _DEFAULT_SOURCE || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

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`

.

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**.

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

The following errors can occur:

- Domain error:
`x`

is 0 or a NaN An invalid floating-point exception (

**FE_INVALID**) is raised, and`errno`

is set to**EDOM**(but see BUGS).

- Domain error:
`x`

is an infinity An invalid floating-point exception (

**FE_INVALID**) is raised, and`errno`

is set to**EDOM**(but see BUGS).

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

Interface | Attribute | Value |

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

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

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.

log(3), logb(3), significand(3)

This page is part of release 4.15 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/.