# Linux 2.6 - man page for logbf (linux section 3)

```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);

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).

+-------------------------+---------------+---------+
|Interface		 | Attribute	 | Value   |
+-------------------------+---------------+---------+
|logb(), logbf(), logbl() | Thread safety | MT-Safe |
+-------------------------+---------------+---------+
CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.

SEE ALSO
ilogb(3), log(3)

2017-09-15								   LOGB(3)```