How does X Server assign Window ID?

[Sheng Yu]

Thanks for the detailed explanation.

I tried generating two windows and draw on the second window in
displays ':2.0' and ':2.1'.
This makes the window id used for rendering different from the windows
in displays ':1.0'
and ':1.1'. And it works! The function glXMakeCurrent() does not
crash, and the rendering
also seems to be correct.

I looked into the source code of GLX, and I cannot find any clue that
the same window id
will cause this function to crash with a segmentation fault. Could it
be possible that it is a bug
in the graphics driver?

Yu Sheng

On Wed, Mar 4, 2009 at 11:04 PM, Glynn Clements
<glynn at gclements.plus.com> wrote:
>
> Sheng Yu wrote:
>
>> I have been tracking for a bug that makes my program crashing when
>> calling function glXMakeCurrent.
>> My program runs with multi-displays, ':0.0', ':1.0', ':1.1', ':2.0',
>> ':2.1'. When glXMakeCurrent() is called
>> to switch rendering context from ':0.0' to either ':1.0', ':1.1' or
>> ':2.0', ':2.1', the program runs fine.
>> However, if the rendering context is switch from ':1.1' to ':2.0',
>> the program crashes with with a
>> segmentation fault.
>>
>> The interesting thing  I found when debugging is the window ID and
>> root window ID for each display:
>> display:':0.0',screen: 0, root window id: 315, window id: 71303171
>> display:':1.0',screen: 0, root window id: 595, window id: 2097156
>> display:':1.1',screen: 1, root window id: 597, window id: 4194308
>> display:':2.0',screen: 0, root window id: 595, window id: 2097156
>> display:':2.1',screen: 1, root window id: 597, window id: 4194308
>>
>> It seems that X Server 1 and 2 assign window ID with the same
>> strategy, which is different from X Server 0.
>> I am wondering whether it is causing the crashing, which indicates
>> problems in X Server settings. I have
>> been digging into the source of X Server and I cannot find how the
>> window ID is assigned. Could someone
>> tell me which file I should look into? Thanks a lot!
>
> The X server doesn't assign individual XIDs (except for resources
> which are created internally by the server, e.g. the root window).
> Each client is assigned a block of XIDs upon connection, and the
> client (i.e. Xlib) allocates XIDs from this block.
>
> It would be more clear if you print the XIDs in hexadecimal:
>
> display:':0.0',screen: 0, root window id: 13b, window id: 4400003
> display:':1.0',screen: 0, root window id: 253, window id:  200004
> display:':1.1',screen: 1, root window id: 255, window id:  400004
> display:':2.0',screen: 0, root window id: 253, window id:  200004
> display:':2.1',screen: 1, root window id: 255, window id:  400004
>
> The most significant 11 bits of the XID indicate the client, leaving
> 21 bits for each client, giving each client 2^21 (= 2,097,152) XIDs.
>
> In any case, I doubt that this has anything to do with your problems
> with OpenGL.
>
> --
> Glynn Clements <glynn at gclements.plus.com>
>