strncat(3)                 Library Functions Manual                 strncat(3)

NAME
       strncat - append non-null bytes from a source array to a string, and
       null-terminate the result

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #include <string.h>

       char *strncat(size_t ssize;
                     char *restrict dst, const char src[restrict ssize],
                     size_t ssize);

DESCRIPTION
       This function appends at most ssize non-null bytes from the array
       pointed to by src, followed by a null character, to the end of the
       string pointed to by dst.  dst must point to a string contained in a
       buffer that is large enough, that is, the buffer size must be at least
       strlen(dst) + strnlen(src, ssize) + 1.

       It is equivalent to

           stpcpy(mempcpy(strnul(dst), src, strnlen(src, ssize)), ""), dst

RETURN VALUE
       strncat() returns dst.

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

       +--------------------------------------------+---------------+---------+
       |Interface                                   | Attribute     | Value   |
       +--------------------------------------------+---------------+---------+
       |strncat ()                                  | Thread safety | MT-Safe |
       +--------------------------------------------+---------------+---------+
STANDARDS
       C11, POSIX.1-2008.

HISTORY
       POSIX.1-2001, C89, SVr4, 4.3BSD.

CAVEATS
       The name of this function is confusing; it has no relation to
       strncpy(3).

       If the destination buffer does not already contain a string, or is not
       large enough, the behavior is undefined.  See _FORTIFY_SOURCE in
       feature_test_macros(7).

BUGS
       This function can be very inefficient.  Read about Shlemiel the painter
       <https://www.joelonsoftware.com/2001/12/11/back-to-basics/>.

EXAMPLES
       #include <stdcountof.h>
       #include <stdio.h>
       #include <string.h>
       #include <utmp.h>

       void print_ut_user(struct utmp *ut);

       void
       print_ut_user(struct utmp *ut)
       {
            char  buf[countof(ut->ut_user) + 1];

            strcpy(buf, "");
            strncat(buf, ut->ut_user, countof(ut->ut_user));
            puts(buf);
       }

SEE ALSO
       string(3), string_copying(7)

Linux man-pages 6.18              2026-02-25                        strncat(3)