---
name: "logf"
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/log/3"
---

# logf(3)

natural logarithmic function

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

## NAME

log, logf, logl - natural logarithmic function

## LIBRARY

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

## SYNOPSIS

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

**double log(double** *x***);** **float logf(float** *x***);** **long double logl(long double** *x***);**

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

**logf**(), **logl**():

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

## DESCRIPTION

These functions return the natural logarithm of *x*.

## RETURN VALUE

On success, these functions return the natural logarithm of *x*.

If *x* is a NaN, a NaN is returned.

If *x* is 1, the result is +0.

If *x* is positive infinity, positive infinity 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 (including negative infinity), then a domain error occurs, and a NaN (not a number) 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:

- Domain error: *x* is negative

  *errno* is set to **EDOM**. An invalid floating-point exception (**FE\_INVALID**) is raised.

- Pole error: *x* is zero

  *errno* is set to **ERANGE**. A divide-by-zero floating-point exception (**FE\_DIVBYZERO**) is raised.

## ATTRIBUTES

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

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **log**(), **logf**(), **logl**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

C99, POSIX.1-2001.

The variant returning *double* also conforms to SVr4, 4.3BSD, C89.

## BUGS

In glibc 2.5 and earlier, taking the **log**() of a NaN produces a bogus invalid floating-point (**FE\_INVALID**) exception.

## SEE ALSO

[**cbrt**(3)](/cbrt/3.md), [**clog**(3)](/clog/3.md), [**log10**(3)](/log10/3.md), [**log1p**(3)](/log1p/3.md), [**log2**(3)](/log2/3.md), [**sqrt**(3)](/sqrt/3.md)