Question

我正在写一个基于文本的Minecraft机器人，主要是（试图）惹恼一些玩家/管理员并查看Minecraft protocol的内部。

无论如何，我还处于设计阶段并编写发送和接收数据包的功能。声明现在看起来像这样：

int send_packet(int sock, con_state_t state, send_packet_t ptype, ...);
int recv_packet(int sock, con_state_t state, recv_packet_t ptype, ...);

_{（我计划为typedef s的错误和套接字类型添加int。）}

到目前为止，这么好。我知道变量参数是类型不安全的，但是我可以编写更好的代码而不是struct。我可以编写这样的代码（send_packet函数）：

/* BTW: I had to decide between structs passed as void* to imitate
 * inheritance and variable-length arguments.
 * Both are equally type-unsafe but variable-length arguments offer the
 * significant advantage of portably iterating over them. Therefore, I can
 * implement a packet-independent packet sending function without remarkable
 * struggle or packet-specific functions, which would be QUITE a hassle.
 * A drawback is that for every argument from the variable-length arguments,
 * data has to be pushed on the stack, so the stack frame grows linearly
 * to the argument count. Not so with a struct; only the address of it had to
 * be passed.
 * (Is variable-length argument the right term at all? I deduced it from
 * variable-length array...)
 */
int send_packet(int sock, send_packet_t ptype, ...) {
    va_list fields;
    uint8_t data[0];    /* TODO: replace 0 by something like PACK_MAX_LEN
                                          * or so! */

    /* TODO: as mentioned above, data should actually have the size of
     * PACK_MAX_LEN. because this might be very big, you could use some
     * dynamic memory allocation mechanism! same goes for string_t, chat_t,
     * and all similar data types. */

    uint8_t* data_it = data;

    /* TODO: ADD PREPENDING OF PACKET ID AND PACKET LENGTH HERE! */
    /* TODO: this function isn't finished. add some code, man! resuming the
     * above comment. */

    /* TODO: add support for legacy server list ping! (packet id = 0xfe) */

    /* TODO: for more complex data types: add code to convert from
     * simple data types to these complex ones like fix_varint_t to
     * varint_t! */

    va_start(fields, ptype);
    for (size_t i = 0; i < sp_structs[ptype].len; ++i) {
        /* Jeez, I hope my code is understandable... */
        switch (sp_structs[ptype].fields[i]) {
        case map_boolean:
            cpy_data_sp(boolean_t);
            break;
        case map_byte:
            cpy_data_sp(byte_t);
            break;
        case map_ubyte:
            cpy_data_sp(ubyte_t);
            break;
        case map_short:
            cpy_data_sp(short_t);
            break;
        case map_ushort:
            cpy_data_sp(ushort_t);
            break;
        case map_int:
            cpy_data_sp(int_t);
            break;
        case map_long:
            cpy_data_sp(long_t);
            break;
        case map_chat:
            cpy_data_sp(chat_t);
            break;
        case map_string:
            cpy_data_sp(string_t);
            break;
        case map_varint:
            /* TODO: maybe write a macro to generalize wr_varint
             * and wr_varlong. */

            fix_varint_t fix = va_arg(fields, fix_varint_t);
            data_it += wr_varint(data_it, fix);
            break;
        case map_varlong:
            fix_varlong_t fix = va_arg(fields, fix_varlong_t);
            data_it += wr_varlong(data_it, fix);
            break;
        /*case map_chunk:
            cpy_data_sp(chunk_t);
            break;*/
        }
    }

/*
    if (sock_send(sock, data, packet_len) == -1) {
        return -1;
    }
*/
    va_end(packet);

    return 0;
}

我只是想告诉你，所以你得到为什么变量参数在这里更好。
使用struct s，函数原型如下所示：

 int send_packet(int sock, con_state_t state, send_packet_t ptype, void* pack);

pack指向struct。现在我们可以使用ptype来推断数据包类型，但我们无法在字段上方便地进行迭代，如上面代码中第一条评论所述。

使用该函数的每个人都必须知道参数的确切顺序，否则可能会发生未定义的行为。由于指定的初始值设定项，struct在这里很不错，它可以启用与订单无关的成员初始化。
你能想出一些模仿变量参数的方法吗？
也许有些va_list魔法？但它应该严格遵守。

注意：使用C ++已经出现在我的脑海中，这可以解决我的所有问题，但我想用漂亮的标准C99和一些C11来做这件事。

Answer 1

您可以使用键/值对而不是定位值：

 int func(int argc, ...) {

 }

并将其称为

func(3, 
         P_NAME,    "John",
         P_SURNAME, "Myers", 
         P_AGE,     10
    );

或使用0值作为终结符以避免argc。

变量参数的指定初始值设定项

1 个答案: