www.LinuxHowtos.org

strlen

Section: C Library Functions (3)
Updated: 2026-02-08
Index Return to Main Contents

strlen - calculate the length of a string

Standard C library (libc,~-lc)

#include <string.h>
size_t strlen(const char *s);

The strlen() function calculates the length of the string pointed to by s, excluding the terminating null byte ([aq][rs]0[aq]).

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

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
strlen()	Thread safety	MT-Safe

C11, POSIX.1-2008.

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

In cases where the input buffer may not contain a terminating null byte, strnlen(3) should be used instead.