2.2.21 size_t strlcat

size_t strlcat(char *, const char *, size_t)

Appends src to string dst of size siz (unlike strncat(), siz is the full size of dst, not space left). At most siz-1 characters will be copied. Always NULL terminates (unless siz <= strlen(dst)).

Remember:

The strlcat() function returns strlen(src) + MIN(siz, strlen(initial dst)). If retval >= siz, truncation occurred.

Appends src to string dst of size siz (unlike strncat(), siz is the full size of dst, not space left). At most siz-1 characters will be copied. Always NULL terminates (unless siz <= strlen(dst)).
Remember:

The strlcat() function returns strlen(src) + MIN(siz, strlen(initial dst)). If retval >= siz, truncation occurred.