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Section: Linux Programmer's Manual (3)
Updated: 20150419
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NAME
logb, logbf, logbl  get exponent of a floatingpoint 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():

_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
DESCRIPTION
These functions extract the exponent from the
internal floatingpoint representation of
x
and return it as a floatingpoint value.
The integer constant
FLT_RADIX,
defined in
<float.h>,
indicates the radix used for the system's floatingpoint 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 dividebyzero floatingpoint 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  MTSafe

CONFORMING TO
C99, POSIX.12001, POSIX.12008.
SEE ALSO
ilogb(3),
log(3)
COLOPHON
This page is part of release 4.04 of the Linux
manpages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
http://www.kernel.org/doc/manpages/.
Index
 NAME

 SYNOPSIS

 DESCRIPTION

 RETURN VALUE

 ERRORS

 ATTRIBUTES

 CONFORMING TO

 SEE ALSO

 COLOPHON

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