strlen(3)                  Library Functions Manual                  strlen(3)

NAME
       strlen - calculate the length of a string

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).

       +--------------------------------------------+---------------+---------+
       |Interface                                   | Attribute     | Value   |
       +--------------------------------------------+---------------+---------+
       |strlen ()                                   | Thread safety | MT-Safe |
       +--------------------------------------------+---------------+---------+
STANDARDS
       C11, POSIX.1-2008.

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

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

SEE ALSO
       string(3), strnlen(3), strnul(3), wcslen(3), wcsnlen(3)

Linux man-pages 6.18              2026-02-25                         strlen(3)