# logb - Man Page

get exponent of a floating-point value

## Library

Math library (libm, -lm)

## 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():

```    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| _XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE```

logbf(), logbl():

```    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 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 similar to floor(log2(fabs(x))), except that the latter may give an incorrect integer due to intermediate rounding.

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).

InterfaceAttributeValue

## Standards

C11, POSIX.1-2008.

## History

C99, POSIX.1-2001.

logb()

4.3BSD (see IEEE.3 in the 4.3BSD manual).