### 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**(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE; or
*cc -std=c99* **logbf**(), **logbl**(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; 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 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*.

### CONFORMING TO

C99, POSIX.1-2001.

### SEE ALSO

### COLOPHON

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