strnlen(3) — Linux manual page

strnlen(3)               Library Functions Manual              strnlen(3)

NAME         top

       strnlen - nonstring length

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <string.h>  // see memory.h(3head)

       size_t strnlen(const char strn[], size_t n);

   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

       This function determines the length of the character sequence
       contained in the array pointed to by strn.

       If strn contains a string, the array can have less than n
       elements.

       It is equivalent to

           (memchr(strn, '\0', n) ?: strn + n) - strn

RETURN VALUE         top

       The strnlen() function returns the number of nonnull characters in
       strn.

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

       memory.h(3head)

COLOPHON         top

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Linux man-pages 6.19            2026-08-10                     strnlen(3)

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

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