# ilogbf (3)

# NAME

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

|| _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

|| /* 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:

- 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).

# 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

**log**(3),

**logb**(3),

**significand**(3)