c++ - XShmPutImage 事件通知挂起

Question

我正在使用 XShm 扩展在 Linux 中绘制和操作图像。

为了不出现屏幕闪烁，我将 send_event = TRUE 传递给 XShmPutImage，然后在调用 XScmPutImage 之后立即使用 XIfEvent 等待事件。

这样，我正在阻止图像绘制，以便在图像显示在窗口表面之前不改变图像。

通常一切正常。但有时，当我进行密集的图像绘制时，似乎事件永远不会发生，绘制过程会挂起。

在哪里查看问题？使用 XIfEvent 是否适合此任务？事件如何从消息队列中消失？

在某些情况下 XShmPutImage 是否可能不发送事件(如果 send_event = TRUE)或发送不同于 ShmCompletion 的事件？ (例如一些内部错误或其他什么？)

编辑:

经过更多研究，我发现只有当窗口管理器密集地向窗口生成事件时才会发生这种挂起。例如，当我通过拖动窗口的角来调整窗口大小时。

EDIT2:

我尝试了几种方法来解决这个问题，但都没有成功。最后，我被迫使用一些超时并在一段时间后取消等待。但当然这是肮脏的黑客攻击，无论如何我都想修复它。

那么，如果 send_event=TRUE，XShmPutImage 不发送事件的原因是什么，或者该事件是否可能从消息队列中消失？

EDIT3:

这是有问题的代码(FASM):

        cinvoke XShmPutImage, ......, TRUE

    .loop:
        lea     eax, [.event]
        cinvoke XCheckTypedEvent, [Display], [ShmCompletionEvent], eax

        test    eax, eax
        jz      .loop      ; there is no message

注意:无论事件检查是否挂起，XShmPutImage 总是返回 TRUE，所以我没有在它后面加上错误检查。

EDIT4:

由于请求，我发布了绘图功能的整个代码。代码使用了FASM的一些宏库，但至少思路清晰(希望)

请注意，此代码包含限制事件仅等待 20 毫秒的解决方法代码。如果没有这个超时，等待循环就会永远挂起。 XShm 事件的编号是按照 Xshm 文档中的建议通过调用 XShmGetEventBase 来获取的。

; 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

这是应用程序的主事件循环的代码。

__ProcessOneSystemEvent 过程只是将事件分派(dispatch)给 GUI 对象并忽略它不使用的所有事件。它根本不处理 ShmCompletionEvent。

在应用程序中创建的所有窗口都有以下事件掩码: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

完整的源代码可在 repository 中找到。但这是一个非常大的项目，不容易导航。讨论的来源在 checkin 8453c99b1283def8 中。

文件:"freshlib/graphics/images.asm" 和 "freshlib/graphics/Linux/images.asm" 是关于图像绘制的。

文件:"freshlib/gui/Main.asm" 和 "freshlib/gui/Linux/Main.asm" 是关于应用程序中的一般事件处理的.

Answer 1

X 服务器在做什么？

如果传递给 ShmCompletionEvent 的参数超过调用中附加到 XShmPutImage 的共享内存区域的几何形状，X 服务器可以并且将抑制 XImage。服务器根据先前存储的给定共享区域的限制检查 X/Y 和宽度/高度，如果调用参数超出范围，服务器将返回 BadValue，抑制绘图操作，并抑制完成事件。

以上内容正是在您的图书馆中发生的事情。方法如下:

1. 主要的事件调度程序例程是 ProcessSystemEvents 。它执行 XEventNext ，并根据事件类型，使用跳转表 .jump_table 调度到特定于事件的处理函数。
2. Expose 事件的事件特定函数是 .expose
3. .expose 函数将依次使用 DrawImageRect 结构体中的 X/Y 和宽度/高度值调用 XExposeEvent。这是错误，是错误的真正来源，我们马上就会看到。
4. DrawImageRect 将在调用 XShmPutImage 时传递这些值
5. X 服务器中 XShmPutImage 的处理程序将检查这些参数，如果超出范围，则拒绝。

参数被拒绝，因为它们来自曝光事件并且与 窗口 的几何形状有关，而 不是 附加到 XImage 的共享内存的几何形状在 XShmPutImage 调用中使用。

具体来说，如果窗口刚刚被调整大小(例如通过窗口管理器)并且已经被放大，并且之前有一个 ConfigureNotify 事件用于调整大小。现在，有了一个新的 Expose 事件，它的宽度/高度将超过服务器知道的共享内存区域的宽度/高度。

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.

注意:为了完全清楚这一点，服务器只能报告错误并且不能对它做任何事情几个原因:

1. 服务器知道窗口[及其调整大小]。
2. 它知道 XImage、它的共享内存区域和大小
3. 但它们仅在 XShmPutImage 调用 [AFAIK] 期间关联
4. 即使服务器可以关联它们，它也无法调整 shmarea
5. 那是因为它无法将 shmarea 重新链接到客户端
6. 只有客户端可以通过 XShmDetach/XShmAttach 做到这一点

---

以下是来自 c5c765bc7e 提交的相关源文件的修订版本。它们已经被清理了一点，所以只剩下最密切的部分。某些行已被截断或换行以消除水平滚动。

这些文件已用 NOTE 和 NOTE/BUG 注释，我在分析它们时这样做了。

---

gui/Main.asm 顶级通用主循环。这里没什么可看的。

; 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 事件处理程序

; 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图像绘制代码[包括DrawImageRect函数]和共享内存创建/销毁代码。

; 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 检查和处理 XShmPutImage 调用的 X 服务器代码。

// 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;
}