# logb - Man Page

get exponent of a floating-point value

## Library

Math library (*libm*, *-lm*)

## Synopsis

#include <math.h> double logb(doublex); float logbf(floatx); long double logbl(long doublex);

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

**logb**():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 500 || /* Since glibc 2.19: */ _DEFAULT_SOURCE || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

**logbf**(), **logbl**():

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

## Description

These functions extract the exponent from the internal floating-point representation of *x* and return it as a floating-point value. The integer constant **FLT_RADIX**, defined in *<float.h>*, indicates the radix used for the system's floating-point representation. If **FLT_RADIX** is 2, **logb(***x***)** is equal to **floor(log2(***x***))**, except that it is probably faster.

If *x* is subnormal, **logb**() returns the exponent *x* would have if it were normalized.

## Return Value

On success, these functions return the exponent of *x*.

If *x* is a NaN, a NaN is returned.

If *x* is zero, then a pole error occurs, and the functions return -**HUGE_VAL**, -**HUGE_VALF**, or -**HUGE_VALL**, respectively.

If *x* is negative infinity or positive infinity, then positive infinity is returned.

## 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:

- Pole error:
*x*is 0 A divide-by-zero floating-point exception (

**FE_DIVBYZERO**) is raised.

These functions do not set *errno*.

## Attributes

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

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

logb(), logbf(), logbl() | Thread safety | MT-Safe |

## Standards

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

## See Also

## Referenced By

The man pages logbf(3) and logbl(3) are aliases of logb(3).