# 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

|| _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

|| /* 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.

*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)