c - 变量参数的指定初始值设定项

Question

我正在编写一个基于文本的 Minecraft 机器人，主要是为了(尝试)惹恼一些玩家/管理员并检查 Minecraft protocol 的内部结构。 .

无论如何，我仍处于设计阶段并编写发送和接收数据包的函数。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, ...);

_{(我计划为错误和套接字类型添加 typedef，均为 int。)}

到目前为止，一切都很好。我知道变量参数是类型不安全的，但使我能够编写比 struct 更好的代码。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;
}

我只是想向您展示这一点，以便您了解为什么变量参数在这里更好。
对于struct，函数原型(prototype)将如下所示:

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

pack 指向这里的struct。现在我们可以使用 ptype 推断数据包类型，但我们无法方便地迭代字段，如上面代码中第一条注释中所述。

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。 struct在这里很好，因为指定的初始化器可以实现与顺序无关的成员初始化。
你能想出一些方法来模拟变量参数吗？
也许有一些 va_list 魔法？但它应该严格遵守。

注意:我已经想到使用 C++，这可以解决我所有的问题，但我想在漂亮的标准 C99 和一些 C11 中做到这一点。

Answer 1

您可以使用键/值对而不是定位值:

 int func(int argc, ...) {

 }

并这样调用它

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

或使用 0 值作为终止符以避免 argc。