[Xcb] Question: Why are structs not serialized elementwise in the autogenerated C cocde?

[Christoph Reimann]

Hi again,
in order to parse xkb.xml and implement the <switch> tag I need to fix
a few things in the C code generator:
- currently requests with variable-sized fields that are not lists are
not supported
- neither are requests with lists of variable-sized element.
- and neither are variable-sized fields followed by fixed-sized fields

In order to adress these issues, I would like to change the way how
the request's fields are serialized.
At the moment a struct is filled with all fixed-sized fields collected
together in xcb_out, and it always gets serialized like this:

    xcb_parts[2].iov_base = (char *) &xcb_out;
    xcb_parts[2].iov_len = sizeof(xcb_out);
    xcb_parts[3].iov_base = 0;
    xcb_parts[3].iov_len = -xcb_parts[2].iov_len & 3;
    (... variable-sized fields ...)

Now I would like to do the serializing element-wise, e.g. in case of
the xcb_create_window request

    unsigned int xcb_pad_bytes = 0;
    xcb_parts[2].iov_base = (char *) &depth;
    xcb_parts[2].iov_len  = sizeof(uint8_t);
    xcb_pad_bytes += sizeof(uint8_t);
    xcb_parts[3].iov_base = (char *) &wid;
    xcb_parts[3].iov_len  = sizeof(xcb_window_t);
    xcb_pad_bytes += sizeof(xcb_window_t);
    xcb_parts[4].iov_base = (char *) &parent;
    xcb_parts[4].iov_len  = sizeof(xcb_window_t);
    xcb_pad_bytes += sizeof(xcb_window_t);
    xcb_parts[5].iov_base = (char *) &x;
    xcb_parts[5].iov_len  = sizeof(int16_t);
    xcb_pad_bytes += sizeof(int16_t);
    xcb_parts[6].iov_base = (char *) &y;
    xcb_parts[6].iov_len  = sizeof(int16_t);
    ...

When everything gets serialized like this, there should be no problem
in handling variable-sized structs etc. as the serializer code can
always be generated recursively in c_client.py.
One more thing: What is the rule for padding with respect to xcb_parts?
At the moment element no. 3 is always

    xcb_parts[3].iov_base = 0;
    xcb_parts[3].iov_len = -xcb_parts[2].iov_len & 3;

and also code like this gets inserted between fixed and variable sized
data. When is it necessary to insert padding?

Thanks in advance for answers

Christoph