[Xcb] reason for xcb_parts being 2 elements larger?

[Barton C Massey]

IIRC positions 0 and 1 are reserved for various kinds of
header information that also have to go out, to avoid having
to allocate a new iovec and copy.  Somebody correct me if
I'm wrong...

	Bart

In message <200709032246.40331.hto at arcor.de> you wrote:
> Hello, 
> 
> I wonder what the reason is to use e.g.
> 
>     struct iovec xcb_parts[6];
> 
> then start filling with position 2:
> 
>     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;
>     [...]
> 
> and then only submit the parts starting from 2:
> 
> xcb_ret.sequence = xcb_send_request(c, 0, xcb_parts + 2, &xcb_req);
> 
> ??
> 
> It seems to me that I could create a proper sized array and submit 
> just that without adding two?
> 
> Tom
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