NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

SEE ALSO

COLOPHON

## NAME

## SYNOPSIS

## DESCRIPTION

## RETURN VALUE

## ERRORS

## ATTRIBUTES

## CONFORMING TO

## SEE ALSO

## COLOPHON

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

SEE ALSO

COLOPHON

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

**#include
<math.h>**

**double
logb(double** *x***);
float logbf(float**

long double logbl(long double

Link with
*−lm*.

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

**logb**():

_BSD_SOURCE || _SVID_SOURCE ||
_XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED ||
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;

or *cc -std=c99*

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

_BSD_SOURCE || _SVID_SOURCE ||
_XOPEN_SOURCE >= 600 || _ISOC99_SOURCE ||
_POSIX_C_SOURCE >= 200112L;

or *cc -std=c99*

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.

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.

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*.

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

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

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

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