SYNC extension: misalignment of system counter structs?

[Daniel Neugebauer]

Hi,

I've recently started using XCB for screen capture and was looking for a 
good way to wait for events with a timeout. I'm aware of the hack to 
monitor the socket/file descriptor but stumbled upon SYNC alarms based 
on SERVERTIME [1] which actually looks like the clean solution to 
eventually get unblocked if no other events arrive.

Following that article, I tried to iterate over all system counters as 
returned by xcb_sync_list_system_counters but couldn't find SERVERTIME. 
This is odd, also "VICEIDLETIME" and empty counter names made no sense 
so I started looking into memory and noticed that XCB appears to have a 
misalignment when accessing/iterating over xcb_sync_systemcounter_t: 
Struct xcb_sync_systemcounter_t gets aligned to a length of 16 bytes but 
actually ends after 14 bytes, followed by the counter's name (explaining 
the missing 2 characters on [DE]VICEIDLETIME). This misalignment makes 
both xcb_sync_systemcounter_next and xcb_sync_systemcounter_name unusable.

As I am compiling XCB myself for static linking, I wasn't sure if that's 
a local (self-induced) issue or a bug, so I tried to link dynamically 
against system libraries and also tried it on another system [2] but the 
issue seems to be present in all cases.

Accessing the SYNC extension via XCB appears to be pretty rare, at least 
I couldn't find any prior implementations for reference when I googled it.

I saw that events use a define XCB_PACKED and tried to apply that to the 
xcb_sync_systemcounter_t struct but the iterator then (still?) ends up 
miscalculating paddings (I end up 2 bytes past the struct).

It would be great if someone could confirm the issue. See [3] for a test 
implementation incl. a local workaround. Is there anything I'm doing 
wrong on my end?

Thanks,
Daniel


[1] https://nrk.neocities.org/articles/x11-timeout-with-xsyncalarm
[2] Gentoo       on Ryzen 7800X3D | xorg 21.1.15, xcb 1.17.0
     Ubuntu 24.04 on i5-1235U      | xorg 21.1.12, xcb 1.15
[3] https://gist.github.com/dneuge/79795ea67a730d0a893239b92c220373