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Bug 3383 - event queue
Summary: event queue
Status: WAITING
Alias: None
Product: SDL
Classification: Unclassified
Component: events (show other bugs)
Version: 2.0.4
Hardware: x86_64 Windows 7
: P2 normal
Assignee: Sam Lantinga
QA Contact: Sam Lantinga
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2016-07-05 03:10 UTC by sage
Modified: 2017-11-09 16:42 UTC (History)
2 users (show)

See Also:

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Description sage 2016-07-05 03:10:31 UTC 
1,in the main thread process events, get the mouse motion event data, look like thats 
local position is  666 --- 494 and time is 44547
local position is  619 --- 506 and time is 44616
local position is  601 --- 514 and time is 44667
local position is  581 --- 524 and time is 44734
local position is  565 --- 532 and time is 44784
local position is  544 --- 538 and time is 44834
local position is  527 --- 545 and time is 44882
local position is  502 --- 557 and time is 44954
local position is  468 --- 570 and time is 45005
local position is  437 --- 584 and time is 45067
local position is  416 --- 594 and time is 45124
local position is  391 --- 607 and time is 45187
local position is  373 --- 616 and time is 45243
local position is  355 --- 627 and time is 45302

local position is  663 --- 495 and time is 45418 (that one is wrong, it should be in the second)

local position is  352 --- 627 and time is 45699

2,i have a threadpool ,the thread of threadpool will use SDL_PushEvent().

3,if i use SDL_AddEventWatch set a callback to process events, the event queue will be ok.
Comment 1 Sam Lantinga 2017-08-12 01:16:02 UTC 
Can you attach your test program so we can replicate this here?

Thanks!
Comment 2 Sylvain 2017-11-09 16:42:22 UTC 
Just looking at the bug report, it seems because SDL_PushEvent() is doing:

First:
 getting the timestamp: event->common.timestamp = SDL_GetTicks();
Then:
 locking the queue and pushing the event.

If there is several threads pushing events, the issue can occur. For instance:

thread-1: getting the timestamp1
thread-2: getting the timestamp2
thread-2: pushing the event with timestamp2
thread-1: pushing the event with timestamp1


several solutions: 
- put the timestamp once you have locked the queue, but it may be older then.
or
- insert in the middle of queue. But user might have already read the queueu ...
or
- discard the event ...
or
- implement double timestamp: native and when pushing. see bug 2011
or
- just don't expect event be timestamp order in that case ...