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(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.

       An implementation of this function might be:

           char *
           strncat(char *restrict dst, const char *restrict src, size_t ssize)
           {
               #define strnul(s)  (s + strlen(s))

               stpcpy(mempcpy(strnul(dst), src, strnlen(src, ssize)), "");
               return 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 <err.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>

       #define nitems(arr)  (sizeof((arr)) / sizeof((arr)[0]))

       int
       main(void)
       {
           size_t  n;

           // Null-padded fixed-size character sequences
           char    pre[4] = "pre.";
           char    new_post[50] = ".foo.bar";

           // Strings
           char    post[] = ".post";
           char    src[] = "some_long_body.post";
           char    *dest;

           n = nitems(pre) + strlen(src) - strlen(post) + nitems(new_post) + 1;
           dest = malloc(sizeof(*dest) * n);
           if (dest == NULL)
               err(EXIT_FAILURE, "malloc()");

           dest[0] = '\0';  // There's no 'cpy' function to this 'cat'.
           strncat(dest, pre, nitems(pre));
           strncat(dest, src, strlen(src) - strlen(post));
           strncat(dest, new_post, nitems(new_post));

           puts(dest);  // "pre.some_long_body.foo.bar"
           free(dest);
           exit(EXIT_SUCCESS);
       }

SEE ALSO
       string(3), string_copying(7)

Linux man-pages 6.7               2023-12-05                        strncat(3)