strnlen man page

strnlen — determine the length of a fixed-size string


#include <string.h>

size_t strnlen(const char *s, size_t maxlen);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


Since glibc 2.10:

_POSIX_C_SOURCE >= 200809L

Before glibc 2.10:



The strnlen() function returns the number of characters in the string pointed to by s, excluding the terminating null byte ('\0'), but at most maxlen. In doing this, strnlen() looks only at the first maxlen characters in the string pointed to by s and never beyond s+maxlen.

Return Value

The strnlen() function returns strlen(s), if that is less than maxlen, or maxlen if there is no null terminating ('\0') among the first maxlen characters pointed to by s.


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

Interface Attribute Value
strnlen() Thread safety MT-Safe

Conforming to


See Also



This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at

Referenced By

signal-safety(7), strlen(3), wcsnlen(3).

2016-03-15 GNU Linux Programmer's Manual