strncat(3)

#include <string.h>

char *strncat(char *restrict \ndst\n, const char \nsrc\n[restrict .\nssize\n],\n
\n              size_t \nssize\n);

char *
strncat(char *restrict dst, const char *restrict src, size_t ssize)
{
\n
    #define strnul(s)  (s + strlen(s))
\n
    stpcpy(mempcpy(strnul(dst), src, strnlen(src, ssize)), "");
\n
    return dst;
}

#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define nitems(arr)  (sizeof((arr)) / sizeof((arr)[0]))
int
main(void)
{
\n
    size_t  n;
\n
    // Null-padded fixed-size character sequences
\n
    char    pre[4] = "pre.";
\n
    char    new_post[50] = ".foo.bar";
\n
    // Strings
\n
    char    post[] = ".post";
\n
    char    src[] = "some_long_body.post";
\n
    char    *dest;
\n
    n = nitems(pre) + strlen(src) - strlen(post) + nitems(new_post) + 1;
\n
    dest = malloc(sizeof(*dest) * n);
\n
    if (dest == NULL)
\n
        err(EXIT_FAILURE, "malloc()");
\n
    dest[0] = '\0';  // There's no 'cpy' function to this 'cat'.
\n
    strncat(dest, pre, nitems(pre));
\n
    strncat(dest, src, strlen(src) - strlen(post));
\n
    strncat(dest, new_post, nitems(new_post));
\n
    puts(dest);  // "pre.some_long_body.foo.bar"
\n
    free(dest);
\n
    exit(EXIT_SUCCESS);
}