scandir

    struct dirent *file;
    unsigned long long ans = 0;

    while ((file = readdir(d))) {
        if (!strcmp(file->d_name, ".") || !strcmp(file->d_name, "..")) {
            continue;
        }
        struct stat buf;
        full_name[dir_name_len] = '/';
        strcpy(full_name + dir_name_len + 1, file->d_name);
        
        if (!stat(full_name, &buf) && check(file->d_name, &buf)) {
            ans += buf.st_size;
        }
    }