I don't personally know, but the one example I came across online is using +1.
Search the page for the function os_get_runtime_executable_path. Let's see what @emaste has to say.
error = vn_fullpath(req->td, vp, &retbuf, &freebuf); vrele(vp); if (error) return (error); error = SYSCTL_OUT(req, retbuf, strlen(retbuf) + 1);
and in vn_fullpath:
buf = malloc(MAXPATHLEN, M_TEMP, M_WAITOK);
So the pathname, including the '\0' fits in MAXPATHLEN/PATH_MAX.