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

# strlen(3)

string length

## NAME

strlen - string length

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <string.h>
```

**size\_t strlen(const char \****s***);**

## DESCRIPTION

The **strlen**() function calculates the length of the string pointed to by *s*, excluding the terminating null byte ('\\0').

It is equivalent to

```
strnul(s) - s
```

## RETURN VALUE

The **strlen**() function returns the number of bytes in the string pointed to by *s*.

## ATTRIBUTES

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

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **strlen**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

POSIX.1-2001, C89, SVr4, 4.3BSD.

## SEE ALSO

[**string**(3)](/string/3.md), [**strnul**(3)](/strnul/3.md), [**wcslen**(3)](/wcslen/3.md)