---
name: "logb"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/logb"
---

# logb(3)

get exponent of a floating-point value

## NAME

logb, logbf, logbl - 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)](/feature_test_macros/7.md)):

**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)](/math_error/7.md) 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)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **logb**(), **logbf**(), **logbl**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

C99, POSIX.1-2001.

- **logb**()

  4\.3BSD (see IEEE.3 in the 4.3BSD manual).

## SEE ALSO

[**ilogb**(3)](/ilogb/3.md), [**log**(3)](/log/3.md)