strlen / wcslen / strnlen (3) - glibc

Split from the chapter glibc: strings, ctype and stdlib arithmetic into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <string.h>
#include <wchar.h>

size_t strlen(const char *s);            /* __attribute_pure__, __nonnull((1)) */
size_t strnlen(const char *s, size_t maxlen);  /* gated __USE_XOPEN2K8 in string.h */
size_t wcslen(const wchar_t *ws);        /* wchar.h */

strnlen in glibc headers is declared under __USE_XOPEN2K8 (POSIX.1-2008); the man page records the gate history as _POSIX_C_SOURCE >= 200809L since glibc 2.10, _GNU_SOURCE before that (strnlen(3) SYNOPSIS). The prototypes above are /usr/include/string.h lines 406-415 and wcslen(3).

Semantics

strlen returns the length of the string s in bytes, i.e. the offset of the terminating null byte within the array (glibc manual, String Length node of string.texi). strnlen returns strlen(s) if that is less than maxlen, otherwise maxlen; it never reads past s[maxlen-1] (strnlen(3)). wcslen is the wide-character equivalent: it counts wchar_t elements before the terminating L'\0' (wcslen(3)).

Gotchas and portability

• sizeof vs strlen: for char string[32] = "hello, world"; the manual’s own example gives sizeof(string) = 32 and strlen(string) = 12; on a pointer char *ptr = string;, sizeof(ptr) is the pointer size, not the string length (manual String Length). sizeof on a function parameter is always the pointer size.
• strlen on a buffer that may lack a terminator is undefined behavior; strlen(3) NOTES says to use strnlen in that case.
• Bytes, not characters: for multibyte-encoded strings the return value need not equal the character count; the manual suggests mbsrtowcs/wcslen for that (manual String Length).
• _FORTIFY_SOURCE: unlike the copy family, glibc defines no runtime __strlen_chk; bits/string_fortified.h (checked on this host) contains no strlen wrapper and nm -D libc.so.6 exports no __strlen_chk, so fortification of strlen is compile-time only.
• musl 1.2.5: strlen is a plain loop and strnlen is memchr-based (src/string/strlen.c, src/string/strnlen.c).

References

• glibc manual: string.texi, node String Length
• man3: strlen(3), strnlen(3), wcslen(3)
• headers: /usr/include/string.h, /usr/include/wchar.h
• symbols: nm -D --with-symbol-versions /lib/x86_64-linux-gnu/libc.so.6