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

#include <string.h>
char *strncat(char *restrict dst, const char src[restrict .ssize],
 size_t ssize);

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;
}

strncat() returns dst.

#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);
}

string(3), string_copying(7)