Hanging of XShmPutImage event notification - c++

Question

I am using XShm extension to draw and manipulate images in Linux.
In order to not have screen flickering, I am passing send_event = TRUE to XShmPutImage and then waiting for the event with XIfEvent, immediately after the call to XScmPutImage.
This way, I am making the image drawing blocking in order to not changing the image until it is displayed on the window surface.
Usually everything works fine. But sometimes, when I have intensive image drawing, it seems that the event never comes and the drawing procedure hangs.
Where to see for the problem? Is using XIfEvent appropriate for this task? How can the event dissapear from the message queue?
Is it possible XShmPutImage to not send event (if send_event = TRUE) or to send event different than ShmCompletion on some circumstances? (for example on some internal error or something?)
EDIT:
After some more research, I found that such hangs happens only when the window manager intensively generate events to the window. For example when I resize the window by dragging its corners.
EDIT2:
I tried several ways to solve this problem, but without success. At the end I was forced to use some timeout and to cancel waiting after some time. But of course this is dirty hack and I want to fix it anyway.
So, what can be the reason XShmPutImage to not send event if send_event=TRUE or is it possible this event to disappear from the message queue?
EDIT3:
Here is the questionable code (FASM):
cinvoke XShmPutImage, ......, TRUE
.loop:
lea eax, [.event]
cinvoke XCheckTypedEvent, [Display], [ShmCompletionEvent], eax
test eax, eax
jz .loop ; there is no message
NB: XShmPutImage always return TRUE, regardless whether the event check hangs or not, so I didn't put error check after it.
EDIT4:
Because of request I am posting the whole code of the drawing function. The code uses some macro libraries of FASM, but at least the ideas are clear (I hope)
Notice that this code contains workaround code that limits the event waiting for only 20ms. Without this timeout the waiting loop simply hangs forever. The number of the XShm event is acquired by calling XShmGetEventBase as recommended in the Xshm documentation.
; Draws the image on a OS provided window surface.
proc DrawImageRect, .where, .pImage, .xDst, .yDst, .xSrc, .ySrc, .width, .height
.event XEvent
rb 256
begin
pushad
mov esi, [.pImage]
test esi, esi
jz .exit
mov ebx, [esi+TImage.ximage]
cinvoke XCreateGC, [hApplicationDisplay], [.where], 0, 0
mov edi, eax
cinvoke XShmPutImage, [hApplicationDisplay], [.where], edi, [esi+TImage.ximage], [.xSrc], [.ySrc], [.xDst], [.yDst], [.width], [.height], TRUE
stdcall GetTimestamp
lea esi, [eax+20] ; 20ms timeout
.loop:
lea eax, [.event]
cinvoke XCheckTypedEvent, [hApplicationDisplay], [ShmCompletionEvent], eax
test eax, eax
jnz .finish
stdcall GetTimestamp
cmp eax, esi
jb .loop
.finish:
cinvoke XFreeGC, [hApplicationDisplay], edi
.exit:
popad
return
endp
And here is the code of the main event loop of the application.
The procedure __ProcessOneSystemEvent simply dispatches the events to the GUI objects and ignores all events it does not use. It does not process ShmCompletionEvent at all.
All the windows created in the application have events mask of:
ExposureMask+FocusChangeMask+KeyPressMask+KeyReleaseMask+ButtonPressMask+ButtonReleaseMask+EnterWindowMask+LeaveWindowMask+PointerMotionMask+StructureNotifyMask
proc ProcessSystemEvents
.event XEvent
rb 256
begin
push ebx ecx edx
.event_loop:
; check for quit
get eax, [pApplication], TApplication:MainWindow
test eax, eax
jz .continue
cmp dword [eax], 0
jne .continue
cinvoke XFlush, [hApplicationDisplay]
xor eax, eax
mov [fGlobalTerminate], 1
stc
pop edx ecx ebx
return
.continue:
cinvoke XPending, [hApplicationDisplay]
test eax, eax
jz .noevents
push edi ecx
lea edi, [.event]
mov ecx, sizeof.XEvent/4
xor eax, eax
rep stosd
pop ecx edi
lea ebx, [.event]
cinvoke XNextEvent, [hApplicationDisplay], ebx
stdcall __ProcessOneSystemEvent, ebx
jmp .event_loop
.noevents:
clc
pop edx ecx ebx
return
endp
The full source code is available in the repository but it is a very big project not easy for navigation. The discussed source is in check-in 8453c99b1283def8.
The files: "freshlib/graphics/images.asm" and "freshlib/graphics/Linux/images.asm" are about the image drawing.
The files: "freshlib/gui/Main.asm" and "freshlib/gui/Linux/Main.asm" are about the general events handling in the application.

Answer 1

What is the X server doing?
The X server can and will suppress a ShmCompletionEvent if the parameters passed to XShmPutImage exceed the geometry of the shared memory area attached to the XImage in the call. The server checks X/Y and width/height against the previously stored limits for the given shared area and if the call parameters are out-of-bounds, the server will return BadValue, suppress the drawing operation, and suppress the completion event.
The above is exactly what is happening in your library. Here's how:
The main event dispatcher routine is ProcessSystemEvents. It performs an XEventNext, and based on event type, using a jump table .jump_table dispatches to an event specific handler function.
The event specific function for an Expose event is .expose
The .expose function will, in turn, call DrawImageRect using X/Y and width/height values from the XExposeEvent struct. This is wrong and is the true source of the bug, as we shall see momentarily.
DrawImageRect will pass these values along in a call to XShmPutImage
The handler for XShmPutImage in the X server will examine these parameters and reject if they're out of bounds.
The parameters are being rejected because they come from an exposure event and are related to the geometry of the window and not the geometry of the shared memory attached to the XImage used in the XShmPutImage call.
Specifically, if the window has just be resized (e.g. by the window manager) and has been enlarged, and there has been a prior ConfigureNotify event for resize. Now, with a new Expose event, it will have larger width/height that will exceed the width/height of the shared memory area that the server knows about.
It is the responsibility of the client to field the window resize events [etc.] and teardown/recreate the shared memory area with the enlarged size. This is not being done and is the source of the bug.
NOTE: Just to be completely clear on this, the server can only report on the error and not do anything about it for a few reasons:
The server knows about the window [and its resize].
It knows about the XImage, its shared memory area and size
But they're only associated during the XShmPutImage call [AFAIK]
Even if the server could associate them, it couldn't adjust the shmarea
That's because it has no way to relink the shmarea to the client side
Only the client can do that via XShmDetach/XShmAttach
Below are redacted versions of the relevant source files from the c5c765bc7e commit. They have been cleaned up a bit, so only the most germane parts remain. Some lines have been truncated or wrapped to eliminate horizontal scrolling.
The files have been annotated with NOTE and NOTE/BUG which I did while analyzing them.
gui/Main.asm The top level generic main loop. Nothing to see much to see here.
; FILE: gui/Main.asm
; _____________________________________________________________________________
;| |
;| ..::FreshLib::.. Free, open source. Licensed under "BSD 2-clause" license." |
;|_____________________________________________________________________________|
;
; Description: Main procedure of GUI application library.
;
; Target OS: Any
;
; Dependencies:
;
; Notes: Organize the main message/event loop needed by every GUI engine.
; This file contains only OS independent part and includes OS dependent
; files.
;______________________________________________________________________________
module "Main library"
proc Run
begin
.mainloop:
stdcall ProcessSystemEvents
jc .terminate
mov eax, [pApplication]
test eax, eax
jz .eventok
get ecx, eax, TApplication:OnIdle
jecxz .eventok
stdcall ecx, eax
.eventok:
stdcall WaitForSystemEvent
jmp .mainloop
.terminate:
DebugMsg "Terminate GUI application!"
return
endp
include '%TargetOS%/Main.asm'
endmodule
gui/Linux/Main.asm The event handlers
; FILE: gui/Linux/Main.asm
; _____________________________________________________________________________
;| |
;| ..::FreshLib::.. Free, open source. Licensed under "BSD 2-clause" license." |
;|_____________________________________________________________________________|
;
; Description: Main procedure of GUI application library.
;
; Target OS: Linux
;
; Dependencies:
;
; Notes: Organize the main message/event loop needed by every GUI engine.
;______________________________________________________________________________
body ProcessSystemEvents
; NOTE: this is the storage for the dequeued event -- all dispatch routines
; should use it and process it
.event XEvent
rb 256
begin
push ebx ecx edx
.event_loop:
; check for quit
get eax, [pApplication], TApplication:MainWindow
test eax, eax
jz .continue ; ???????????
cmp dword [eax], 0
jne .continue
cinvoke XFlush, [hApplicationDisplay]
xor eax, eax
mov [fGlobalTerminate], 1
stc
pop edx ecx ebx
return
; NOTE: it is wasteful for the main loop to call WaitForSystemEvent, then call
; us and we do XPending on the first loop -- we already know we have at least
; one event waiting in the queue
.continue:
cinvoke XPending, [hApplicationDisplay]
test eax, eax
jz .noevents
push edi ecx
lea edi, [.event]
mov ecx, sizeof.XEvent/4
xor eax, eax
rep stosd
pop ecx edi
lea ebx, [.event]
cinvoke XNextEvent, [hApplicationDisplay], ebx
stdcall __ProcessOneSystemEvent, ebx
jmp .event_loop
.noevents:
clc
pop edx ecx ebx
return
endp
body WaitForSystemEvent
.event XEvent
begin
push eax ecx edx
lea eax, [.event]
cinvoke XPeekEvent, [hApplicationDisplay], eax
pop edx ecx eax
return
endp
proc __ProcessOneSystemEvent, .linux_event
begin
pushad
mov ebx, [.linux_event]
; mov eax, [ebx+XEvent.type]
; cmp eax, [ShmCompletionEvent]
; je .shm_completion
stdcall _GetWindowStruct, [ebx+XEvent.window]
jc .notprocessed
test eax, eax
jz .notprocessed
mov esi, eax
mov ecx, [ebx+XEvent.type]
cmp ecx, LASTEvent
jae .notprocessed
mov ecx, [.jump_table+4*ecx]
jecxz .notprocessed
jmp ecx
.notprocessed:
popad
stc
return
.finish:
popad
clc
return
;.shm_completion:
; DebugMsg "Put back completion event!"
;
; int3
; cinvoke XPutBackEvent, [hApplicationDisplay], ebx
; jmp .finish
;.........................................................................
; seMove and seResize events.
;-------------------------------------------------------------------------
.moveresize:
; NOTE/BUG!!!!: we must not only process a resize/move request, but we must also
; adjust the size of the shmarea attached to the XImage -- that is _not_ being
; done. (e.g.) if the window is enlarged, the shmarea must be enlarged
cinvoke XCheckTypedWindowEvent, [hApplicationDisplay],
[ebx+XConfigureEvent.window], ConfigureNotify, ebx
test eax, eax
jnz .moveresize
; resize event...
mov eax, [esi+TWindow._width]
mov edx, [esi+TWindow._height]
cmp eax, [ebx+XConfigureEvent.width]
jne .resize
cmp edx, [ebx+XConfigureEvent.height]
je .is_move
.resize:
exec esi, TWindow:EventResize, [ebx+XConfigureEvent.width],
[ebx+XConfigureEvent.height]
; move event...
.is_move:
mov eax, [esi+TWindow._x]
mov edx, [esi+TWindow._y]
cmp eax, [ebx+XConfigureEvent.x]
jne .move
cmp eax, [ebx+XConfigureEvent.y]
je .finish
.move:
exec esi, TWindow:EventMove,
[ebx+XConfigureEvent.x], [ebx+XConfigureEvent.y]
jmp .finish
;.........................................................................
; DestroyNotify handler it invalidates the handle in TWindow structure and
; then destroys TWindow.
.destroy:
test esi, esi
jz .finish
mov [esi+TWindow.handle], 0
destroy esi
jmp .finish
;.........................................................................
; Window paint event
.expose:
get edi, esi, TWindow:ImgScreen
; NOTE:BUG!!!!!
;
; if the window has been resized (e.g. enlarged), these values are wrong!
; they relate to the _window_ but _not_ the shmarea that is attached to the
; XImage
;
; however, DrawImageRect will call XShmPutImage with these values, they
; will exceed the geometry of what the X server knows about the shmarea and
; it will return BadValue and _suppress_ the completion event for XShmPutImage
stdcall DrawImageRect, [esi+TWindow.handle], edi,
[ebx+XExposeEvent.x],[ebx+XExposeEvent.y],
[ebx+XExposeEvent.x], [ebx+XExposeEvent.y],
[ebx+XExposeEvent.width], [ebx+XExposeEvent.height]
jmp .finish
;.........................................................................
; Mouse event handlers
.mousemove:
cinvoke XCheckTypedWindowEvent, [hApplicationDisplay],
[ebx+XConfigureEvent.window], MotionNotify, ebx
test eax, eax
jnz .mousemove
stdcall ServeMenuMouseMove, [ebx+XMotionEvent.window],
[ebx+XMotionEvent.x], [ebx+XMotionEvent.y],
[ebx+XMotionEvent.state]
jc .finish
cinvoke XCheckTypedWindowEvent, [hApplicationDisplay],
[ebx+XMotionEvent.window], MotionNotify, ebx
test eax, eax
jnz .mousemove
mov edi, [__MouseTarget]
test edi, edi
jz .search_target_move
stdcall __GetRelativeXY, edi, [ebx+XMotionEvent.x], [ebx+XMotionEvent.y]
jmp .target_move
.search_target_move:
exec esi, TWindow:ChildByXY, [ebx+XMotionEvent.x],
[ebx+XMotionEvent.y], TRUE
mov edi, eax
.target_move:
cmp edi, [__LastPointedWindow]
je .move_event
cmp [__LastPointedWindow], 0
je .leave_ok
exec [__LastPointedWindow], TWindow:EventMouseLeave
.leave_ok:
mov [__LastPointedWindow], edi
exec edi, TWindow:EventMouseEnter
.move_event:
exec edi, TWindow:EventMouseMove, ecx, edx, [ebx+XMotionEvent.state]
jmp .finish
;.........................................................................
; event jump table
.jump_table dd 0 ; event 0
dd 0 ; event 1
dd .key_press ; KeyPress = 2
dd .key_release ; KeyRelease = 3
dd .mouse_btn_press ; ButtonPress = 4
dd .mouse_btn_release ; ButtonRelease = 5
dd .mousemove ; MotionNotify = 6
dd 0 ; EnterNotify = 7
dd 0 ; LeaveNotify = 8
dd .focusin ; FocusIn = 9
dd .focusout ; FocusOut = 10
dd 0 ; KeymapNotify = 11
dd .expose ; Expose = 12
dd 0 ; GraphicsExpose = 13
dd 0 ; NoExpose = 14
dd 0 ; VisibilityNotify = 15
dd 0 ; CreateNotify = 16
dd .destroy ; DestroyNotify = 17
dd 0 ; UnmapNotify = 18
dd 0 ; MapNotify = 19
dd 0 ; MapRequest = 20
dd 0 ; ReparentNotify = 21
dd .moveresize ; ConfigureNotify = 22
dd 0 ; ConfigureRequest = 23
dd 0 ; GravityNotify = 24
dd 0 ; ResizeRequest = 25
dd 0 ; CirculateNotify = 26
dd 0 ; CirculateRequest = 27
dd 0 ; PropertyNotify = 28
dd 0 ; SelectionClear = 29
dd 0 ; SelectionRequest = 30
dd 0 ; SelectionNotify = 31
dd 0 ; ColormapNotify = 32
dd .clientmessage ; ClientMessage = 33
dd .mapping_notify ; MappingNotify = 34
graphics/Linux/images.asm The image drawing code [including the DrawImageRect function] and the shared memory create/destroy code.
; FILE: graphics/Linux/images.asm
; _____________________________________________________________________________
;| |
;| ..::FreshLib::.. Free, open source. Licensed under "BSD 2-clause" license." |
;|_____________________________________________________________________________|
;
; Description: Memory based images manipulation library.
;
; Target OS: Linux
;
; Dependencies: memory.asm
;
; Notes:
;______________________________________________________________________________
uses libX11, xshm
struct TImage
.width dd ? ; width in pixels.
.height dd ? ; height in pixels.
.pPixels dd ? ; pointer to the pixel memory.
; os dependent data
.ximage dd ?
.shminfo XShmSegmentInfo
ends
body CreateImage
begin
pushad
stdcall GetMem, sizeof.TImage
jc .finish
mov esi, eax
xor eax, eax
inc eax
mov ecx, [.width]
mov edx, [.height]
cmp ecx, 0
cmovle ecx, eax
cmp edx, 0
cmovle edx, eax
mov [esi+TImage.width], ecx
mov [esi+TImage.height], edx
lea eax, [4*ecx]
imul eax, edx
cinvoke shmget, IPC_PRIVATE, eax, IPC_CREAT or 777o
test eax, eax
js .error
mov [esi+TImage.shminfo.ShmID], eax
cinvoke shmat, eax, 0, 0
cmp eax, -1
je .error_free
mov [esi+TImage.shminfo.Addr], eax
mov [esi+TImage.pPixels], eax
mov [esi+TImage.shminfo.fReadOnly], 1
lea ebx, [esi+TImage.shminfo]
cinvoke XShmCreateImage, [hApplicationDisplay], 0, $20, ZPixmap, eax,
ebx, [esi+TImage.width], [esi+TImage.height]
mov [esi+TImage.ximage], eax
cinvoke XShmAttach, [hApplicationDisplay], ebx
clc
mov [esp+4*regEAX], esi
.finish:
popad
return
.error_free:
cinvoke shmctl, [ebx+XShmSegmentInfo.ShmID], IPC_RMID, 0
.error:
stdcall FreeMem, esi
stc
jmp .finish
endp
body DestroyImage
begin
pushad
mov esi, [.ptrImage]
test esi, esi
jz .finish
lea eax, [esi+TImage.shminfo]
cinvoke XShmDetach, [hApplicationDisplay], eax
cinvoke XDestroyImage, [esi+TImage.ximage]
cinvoke shmdt, [esi+TImage.shminfo.Addr]
cinvoke shmctl, [esi+TImage.shminfo.ShmID], IPC_RMID, 0
stdcall FreeMem, esi
.finish:
popad
return
endp
;if used ___CheckCompletionEvent
;___CheckCompletionEvent:
;
;virtual at esp+4
; .display dd ?
; .pEvent dd ?
; .user dd ?
;end virtual
;
;; timeout
; stdcall GetTimestamp
; cmp eax, [.user]
; jbe #f
;
; DebugMsg "Timeout!"
;
; mov eax, 1
; retn
;
;##:
; mov eax, [.pEvent] ;.pEvent
; mov eax, [eax+XEvent.type]
;
; cmp eax, [ShmCompletionEvent]
; sete al
; movzx eax, al
; retn
;end if
body DrawImageRect
.event XEvent
rb 256
begin
pushad
mov esi, [.pImage]
test esi, esi
jz .exit
mov ebx, [esi+TImage.ximage]
; NOTE: is this necessary? it seems wasteful to create and destroy a GC
; repeatedly. Dunno, does this _have_ to be done here, _every_ time?
cinvoke XCreateGC, [hApplicationDisplay], [.where], 0, 0
mov edi, eax
; NOTE/BUG: The return ShmCompletionEvent will be suppressed due to a BadValue
; if the X/Y and width/height parameters given to us by caller exceed the
; geometry/range of the shmarea attached to .ximage
;
; the routine that calls us is .expose and it _is_ giving us bad values. it is
; passing us X/Y width/height related to an exposure event of the .where
; _window_ which we put in the call. The X server will compare these against
; the size of the shmarea of TImage.xmage and complain if we exceed the bounds
cinvoke XShmPutImage, [hApplicationDisplay], [.where], edi,
[esi+TImage.ximage], [.xSrc], [.ySrc], [.xDst], [.yDst],
[.width], [.height], TRUE
; NOTE/BUG: this code should _not_ be looping on XCheckTypedEvent because it
; disrupts the normal event processing. if we want to be "synchronous" on this
; we should loop on the main event dispatcher (ProcessSystemEvents) and let it
; dispatch to a callback we create. we can set a "pending" flag that our [not
; yet existent] dispatch routine can clear
; THIS CODE SOMETIMES CAUSES HANGS!
stdcall GetTimestamp
lea esi, [eax+20]
.loop:
lea eax, [.event]
cinvoke XCheckTypedEvent, [hApplicationDisplay], [ShmCompletionEvent],
eax
test eax, eax
jnz .finish
stdcall GetTimestamp
cmp eax, esi
jb .loop
.finish:
cinvoke XFreeGC, [hApplicationDisplay], edi
.exit:
popad
return
endp
Xext/shm.c The X server code that checks and processes the XShmPutImage call.
// FILE: Xext/shm.c
static int
ProcShmPutImage(ClientPtr client)
{
GCPtr pGC;
DrawablePtr pDraw;
long length;
ShmDescPtr shmdesc;
REQUEST(xShmPutImageReq);
REQUEST_SIZE_MATCH(xShmPutImageReq);
VALIDATE_DRAWABLE_AND_GC(stuff->drawable, pDraw, DixWriteAccess);
VERIFY_SHMPTR(stuff->shmseg, stuff->offset, FALSE, shmdesc, client);
// NOTE: value must be _exactly_ 0/1
if ((stuff->sendEvent != xTrue) && (stuff->sendEvent != xFalse))
return BadValue;
if (stuff->format == XYBitmap) {
if (stuff->depth != 1)
return BadMatch;
length = PixmapBytePad(stuff->totalWidth, 1);
}
else if (stuff->format == XYPixmap) {
if (pDraw->depth != stuff->depth)
return BadMatch;
length = PixmapBytePad(stuff->totalWidth, 1);
length *= stuff->depth;
}
else if (stuff->format == ZPixmap) {
if (pDraw->depth != stuff->depth)
return BadMatch;
length = PixmapBytePad(stuff->totalWidth, stuff->depth);
}
else {
client->errorValue = stuff->format;
return BadValue;
}
// NOTE/BUG: The following block is the "check parameters" code. If the
// given drawing parameters of the request (e.g. X, Y, width, height) [or
// combinations thereof] exceed the geometry/size of the shmarea, the
// BadValue error is being returned here and the code to send a return
// event will _not_ be executed. The bug isn't really here, it's on the
// client side, but it's the client side bug that causes the event to be
// suppressed
/*
* There's a potential integer overflow in this check:
* VERIFY_SHMSIZE(shmdesc, stuff->offset, length * stuff->totalHeight,
* client);
* the version below ought to avoid it
*/
if (stuff->totalHeight != 0 &&
length > (shmdesc->size - stuff->offset) / stuff->totalHeight) {
client->errorValue = stuff->totalWidth;
return BadValue;
}
if (stuff->srcX > stuff->totalWidth) {
client->errorValue = stuff->srcX;
return BadValue;
}
if (stuff->srcY > stuff->totalHeight) {
client->errorValue = stuff->srcY;
return BadValue;
}
if ((stuff->srcX + stuff->srcWidth) > stuff->totalWidth) {
client->errorValue = stuff->srcWidth;
return BadValue;
}
if ((stuff->srcY + stuff->srcHeight) > stuff->totalHeight) {
client->errorValue = stuff->srcHeight;
return BadValue;
}
// NOTE: this is where the drawing takes place
if ((((stuff->format == ZPixmap) && (stuff->srcX == 0)) ||
((stuff->format != ZPixmap) &&
(stuff->srcX < screenInfo.bitmapScanlinePad) &&
((stuff->format == XYBitmap) ||
((stuff->srcY == 0) &&
(stuff->srcHeight == stuff->totalHeight))))) &&
((stuff->srcX + stuff->srcWidth) == stuff->totalWidth))
(*pGC->ops->PutImage) (pDraw, pGC, stuff->depth,
stuff->dstX, stuff->dstY,
stuff->totalWidth, stuff->srcHeight,
stuff->srcX, stuff->format,
shmdesc->addr + stuff->offset +
(stuff->srcY * length));
else
doShmPutImage(pDraw, pGC, stuff->depth, stuff->format,
stuff->totalWidth, stuff->totalHeight,
stuff->srcX, stuff->srcY,
stuff->srcWidth, stuff->srcHeight,
stuff->dstX, stuff->dstY, shmdesc->addr + stuff->offset);
// NOTE: this is where the return event gets sent
if (stuff->sendEvent) {
xShmCompletionEvent ev = {
.type = ShmCompletionCode,
.drawable = stuff->drawable,
.minorEvent = X_ShmPutImage,
.majorEvent = ShmReqCode,
.shmseg = stuff->shmseg,
.offset = stuff->offset
};
WriteEventsToClient(client, 1, (xEvent *) &ev);
}
return Success;
}

Answer 2

Your source code will be the ultimate piece which we can analyse but since I understand Assembly very less, I will give you an answer on macro level. Exact answer is still unknown to me.
I case of too many events only it is creating this issue but not with normal occurrence of events which means you framework is running out of virtual memory or another frame of event is triggered before the previous one releases its memory. In this kind of situations you can do few things
Try to check if there is any memory leak or not.. After one frame of event is over, try to clean up the memory or end that frame object properly before triggering the new one.
You can also develop a mechanism to make the second frame wait for the first frame to get over. In C/C++ we do it using so many synchronization methods like Mutex or select system calls. If you design follows that kind of pattern, then you can do these
If anywhere you have the authority to change the allocated memory given to your window, try to increase it. Because one thing for sure (according to your explanation ) that this is some memory issue.
Replying on Edit 3 It looks like you are calling some method cinvoke. How it is internally handling the even is unknown to me. Why don't you implement it directly in C. I am sure for whatever target that you are working, you will get some Cross-compiler.