strlen(3) Library Functions Manual strlen(3)

strlen - string length

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 ('\0').

It is equivalent to


strnul(s) - s

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.

string(3), strnul(3), wcslen(3)

2026-08-10 Linux man-pages 6.19