strnlen(3) — Linux manual page

strnlen(3)               Library Functions Manual              strnlen(3)

NAME         top

       strnlen - determine the length of a fixed-size string

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <string.h>

       size_t strnlen(size_t maxlen;
                      const char s[maxlen], size_t maxlen);

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

       strnlen():
           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _GNU_SOURCE

DESCRIPTION         top

       The strnlen() function returns the number of bytes 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-1].

RETURN VALUE         top

       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.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).
       ┌──────────────────────────────────────┬───────────────┬─────────┐
       │ Interface                            │ Attribute     │ Value   │
       ├──────────────────────────────────────┼───────────────┼─────────┤
       │ strnlen()                            │ Thread safety │ MT-Safe │
       └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS         top

       POSIX.1-2008.

HISTORY         top

       POSIX.1-2008.

SEE ALSO         top

       strlen(3)

COLOPHON         top

       This page is part of the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
       for this manual page, see
       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
       This page was obtained from the tarball man-pages-6.15.tar.gz
       fetched from
       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
       2025-08-11.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to
       man-pages@man7.org

Linux man-pages 6.15            2025-06-28                     strnlen(3)

Pages that refer to this page: strlen(3),  wcsnlen(3),  signal-safety(7)

HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH.