# LOGB

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

SEE ALSO

COLOPHON

## NAME

logb, logbf,
logbl - get exponent of a floating-point value

## SYNOPSIS

**#include
<math.h>**

**double
logb(double** *x***); **

float logbf(float *x***); **

long double logbl(long double *x***);**

Link with
*-lm*.

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 versions <= 2.19: */ _BSD_SOURCE ||
_SVID_SOURCE

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

_ISOC99_SOURCE ||
_POSIX_C_SOURCE >= 200112L

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 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).

## CONFORMING TO

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

## SEE ALSO

**ilogb**(3),
**log**(3)

## COLOPHON

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