/* this function will return a sub-path name under directory. */
const char* dfs_subdir(const char *directory, const char *filename)
/* this function will return a sub-path name under directory. */
const char* dfs_subdir(const char *directory, const char *filename)
{
    const char *dir;
    if (strlen(directory) == strlen(filename))
        return NULL;
    dir = filename + strlen(directory);
    if ((*dir != '/') && (dir != filename))
    {
        dir --;
    }
    return dir;
}