---
name: "ilogbl"
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/ilogb"
---

# ilogbl(3)

get integer exponent of a floating-point value

Part of [ilogb(3)](/ilogb/3.md).

## NAME

ilogb, ilogbf, ilogbl - get integer exponent of a floating-point value

## LIBRARY

Math library (*libm*, *-lm*)

## SYNOPSIS

```
#include <math.h>
```

**int ilogb(double** *x***);** **int ilogbf(float** *x***);** **int ilogbl(long double** *x***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**ilogb**():

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

**ilogbf**(), **ilogbl**():

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

## DESCRIPTION

These functions return the exponent part of their argument as a signed integer. When no error occurs, these functions are equivalent to the corresponding [**logb**(3)](/logb/3.md) functions, cast to *int*.

## RETURN VALUE

On success, these functions return the exponent of *x*, as a signed integer.

If *x* is zero, then a domain error occurs, and the functions return **FP\_ILOGB0**.

If *x* is a NaN, then a domain error occurs, and the functions return **FP\_ILOGBNAN**.

If *x* is negative infinity or positive infinity, then a domain error occurs, and the functions return **INT\_MAX**.

## 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:

- Domain error: *x* is 0 or a NaN

  An invalid floating-point exception (**FE\_INVALID**) is raised, and *errno* is set to **EDOM** (but see BUGS).

- Domain error: *x* is an infinity

  An invalid floating-point exception (**FE\_INVALID**) is raised, and *errno* is set to **EDOM** (but see BUGS).

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **ilogb**(), **ilogbf**(), **ilogbl**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

C99, POSIX.1-2001.

## BUGS

Before glibc 2.16, the following bugs existed in the glibc implementation of these functions:

- •

  The domain error case where *x* is 0 or a NaN did not cause *errno* to be set or (on some architectures) raise a floating-point exception.

- •

  The domain error case where *x* is an infinity did not cause *errno* to be set or raise a floating-point exception.

## SEE ALSO

[**log**(3)](/log/3.md), [**logb**(3)](/logb/3.md), [**significand**(3)](/significand/3.md)