```LOGB(3)                    Linux Programmer's Manual                   LOGB(3)

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);

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  floating-point
representation of x and return it as a floating-point value.  The inte-
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.

CONFORMING TO
C99, POSIX.1-2001.