[Xcb] ListHosts entry lengths and padding to 4-byte alignment

[Nigel Tao]

Hi. I maintain x-go-binding, a port of xcb to the Go programming language.

I'm hoping someone can clarify the rules about padding list replies so
that their elements' lengths are a multiple of 4.

For example, the X11 spec at
ftp://ftp.x.org/pub/X11R7.0/doc/PDF/proto.pdf says that ListHosts
(opcode 110, page 148) is marked "n always a multiple of 4" but
ListExtensions (opcode 99, page 145) is not similarly marked.

IIUC the relevent parts of xcb's xproto.xml are:

-----------
  <request name="ListExtensions" opcode="99">
    <reply>
      <field type="CARD8" name="names_len" />
      <pad bytes="24" />
      <list type="STR" name="names">
        <fieldref>names_len</fieldref>
      </list>
    </reply>
  </request>

<!-- other elements elided -->

  <struct name="HOST">
    <field type="CARD8" name="family" enum="Family" />
    <pad bytes="1" />
    <field type="CARD16" name="address_len" />
    <list type="BYTE" name="address">
      <fieldref>address_len</fieldref>
    </list>
  </struct>

  <request name="ListHosts" opcode="110">
    <reply>
      <field type="BYTE" name="mode" enum="AccessControl" />
      <field type="CARD16" name="hosts_len" />
      <pad bytes="22" />
      <list type="HOST" name="hosts">
        <fieldref>hosts_len</fieldref>
      </list>
    </reply>
  </request>
-----------

How am I supposed to tell, from the XML file, that the elements of a
<list type="HOST"> should be 4-padded (and not 1-padded or 2-padded),
but a <list type="STR"> is 1-padded?

I can make up a number of rules that each seem to match what I see for
the core protocol, but I'd like to know the actual rule so that I can
also handle any extensions.


Apologies if this is documented somewhere and I've simply missed it,
Nigel.