
LOGB(3) Linux Programmer's Manual LOGB(3)
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.
CONFORMING TO
C99, POSIX.12001.
SEE ALSO
ilogb(3), log(3)
COLOPHON
This page is part of release 3.55 of the Linux manpages project. A description of the
project, and information about reporting bugs, can be found at
http://www.kernel.org/doc/manpages/.
20100920 LOGB(3) 
