## table of contents

other versions

- buster 4.16-2
- buster-backports 5.02-1~bpo10+1
- testing 5.03-1
- unstable 5.03-1

ILOGB(3) | Linux Programmer's Manual | ILOGB(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

**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:

- 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)# COLOPHON¶

This page is part of release 5.02 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/.

2017-09-15 |