Releasing DCs! Kedaman, anyone?

[Fox]

What is the correct way to release a memory DC? I tried ReleaseDC and DeleteDC but both seem to leave a memory lack...

Here's my code:

Function LoadDC(iFileName As String) As Long
    Dim Temp As IPictureDisp
    Dim DC As Long
    
    'Create compatible DC
    DC = CreateCompatibleDC(MainWindow.hdc)
    
    'Load bitmap
    Set Temp = LoadPicture(iFileName)
    SelectObject DC, Temp
    
    'Apply values
    LoadDC = DC
    
    'Release memory
    DeleteObject Temp
    Set Temp = Nothing
End Function

Sub UnloadDC(iDC As Long)
    'Release DCs
    ReleaseDC MainWindow.hWnd, iDC
    DeleteDC iDC
End Sub

The mistake maybe elsewhere, but I assume it's somewhere around this functions...
I also noticed Windows does free the resources lost as soon as I close VB..

[The Hand]

Order is a very important thing when using the GDI32 API.

You don't need to call the "ReleaseDC" in the example you showed. You should just call "DeleteDC".

ReleaseDC is generally used when you grab a window's or control's DC using the "GetDC" function (hence the inclusion of the hWnd argument). When you call ReleaseDC, my understanding is that you are giving ownership of the device context back to its control.

Since you are creating a new one, you should just delete it. By releasing the DC first, you may make it impossible to delete the DC in the subsequent statement.

I could be wrong, but thats my 2 cents.
-Hand out

[Fox]

Nope. I just have both functions there because I hoped one will work..

I just made a sample project that loads 100 DCs from a file and both methods to release them (I runned a compiled exe). Heres a protocol of my tests: (text and system/user/gfx resources)

1) Started the program  (92/92/95)
2) Loaded 100 DCs       (86,92,86)
3) ReleasDC for all     (86,92,86)
4) Exit program         (92/92/95)

5) Started again        (92/92/94)
6) Loaded 100 DCs again (86/92/86)
7) DeleteDC for all     (86,92,86)
8) Exit program         (92,92,95)

9) Started last time    (92,92,94)
10) And exit again      (92,92,95)

As you see the resources aren't really lost, but as long as the program runs I have a problem! So what is the correct way to release the DCs?

[denniswrenn]

You're using graphics in VB? I'm ashamed of you I was going to reply to use the CS_OWNDC as the style in your window class, so you didn't have to create one, but I guess I'll just take it elsewhere..... *hummmf*

[Fox]

Sure I use graphics in VB Never seen my website? *hehe*

[denniswrenn]

Yeah, but after rumaging through my back up CDs, I found the LeMitsch game you sent me a while back(And the source).... and it was in C++, so I just assumed that's what you'd moved to for graphics..

[Jotaf98]

Try deleting the DC first, and then release it...

[Fox]

a) I don't have this problem in C/DirectX
b) I didn't say I make a game in VB, just graphics
c) @Jotaf: Nothing happens..

[denniswrenn]

Well, I'm still disapointed in you

[Fox]

*boohoooo*


*sighs*


if (you_make_games_in_cpp) Print( "I bet my VB games are still better than your C++ games *hihi* " ); else Print( "Well, do you know a solution for the problem???" );

[denniswrenn]

*kick* *kick*

I know your games are better... I'm still learning Dx and OGL...

The solution to your problem is to use C++


Where in C++ is the Print command, BTW?

[Fox]

Aaaahahhh!!

*runs away shouting*

[Jotaf98]

Well Fox, I guess that "Sam's teach yourself C++ in 24 hours" is not that good after all

Hum, btw, are you The Fox that made all those cool Graal levels?

[YoungBuck]

You need to select all the objects that were created with the DC back into it before using DeleteDC and then Delete all the objects that you created (the Temp variable in your example), SelectObject's return value is the handle to the object currently being replaced, here is something I put together to test this....

Option Explicit


Private Declare Function DeleteDC Lib "gdi32" (ByVal hdc As Long) As Long
Private Declare Function CreateCompatibleDC Lib "gdi32" (ByVal hdc As Long) As Long
Private Declare Function GetDC Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function GetDesktopWindow Lib "user32" () As Long
Private Declare Function SelectObject Lib "gdi32" (ByVal hdc As Long, ByVal hObject As Long) As Long
Private Declare Function DeleteObject Lib "gdi32" (ByVal hObject As Long) As Long
Private Declare Function CreatePen Lib "gdi32" (ByVal nPenStyle As Long, ByVal nWidth As Long, ByVal crColor As Long) As Long
Private Declare Function ReleaseDC Lib "user32" (ByVal hwnd As Long, ByVal hdc As Long) As Long
Private Declare Sub GlobalMemoryStatus Lib "kernel32" (lpBuffer As MEMORYSTATUS)

Private Type MEMORYSTATUS
        dwLength As Long
        dwMemoryLoad As Long
        dwTotalPhys As Long
        dwAvailPhys As Long
        dwTotalPageFile As Long
        dwAvailPageFile As Long
        dwTotalVirtual As Long
        dwAvailVirtual As Long
End Type


Private Sub Form_Load()
 Dim hdc As Long
 Dim hPen As Long
 Dim hOrigPen As Long
 Dim hwndDesktop
 Dim hDesktop
 Dim iCt As Long
 
    MsgBox "PHYS MEM BEFORE DC OPERATIONS: " & GetAvailableMemory
    
 
    For iCt = 0 To 10000
        
        ' get the desktop's window handle
        hwndDesktop = GetDesktopWindow()
        ' get the handle to the desktop's DC
        hDesktop = GetDC(hwndDesktop)
        ' create a DC compatible with the Desktop
        hdc = CreateCompatibleDC(hDesktop)
        ' Create a pen
        hPen = CreatePen(0, 1, &H0)
        ' select the pen into the new DC and retrieve the handle _
          to the pen that was created with the DC
        hOrigPen = SelectObject(hdc, hPen)
        'before you use DeleteDC you need to put all objects back into _
         DC and remove the ones created yourself
        SelectObject hdc, hOrigPen
        ' delete the pen that we created
        DeleteObject hPen
        ' release the DC retrieved from the desktop
        ReleaseDC hwndDesktop, hDesktop
        ' delete the DC including the objects created with the DC
        DeleteDC hdc
        
    Next iCt
        
    MsgBox "PHYS MEM AFTER DC OPERATIONS: " & GetAvailableMemory
 
 
End Sub

Private Function GetAvailableMemory() As Long
 Dim m As MEMORYSTATUS
 
 GlobalMemoryStatus m
 
 GetAvailableMemory = m.dwAvailPhys

End Function

[Fox]

Hm, thanks.. but why do you need a pen here?

Anyways, if you look at my first code it does exactly the same as yours, first SelectObject, then DeleteObject, ReleaseDC and finally DeleteDC...?!

[kedaman]

Function LoadDC(iFileName As String) As Long
    Dim Temp As IPictureDisp
    Dim DC As Long
    
    'Create compatible DC
    DC = CreateCompatibleDC(MainWindow.hdc)
    
    'Load bitmap
    Set Temp = LoadPicture(iFileName)
    SelectObject DC, Temp
    
    'Apply values
    LoadDC = DC
    
    'Release memory
    DeleteObject Temp'<-- This will delete the selected bitmap fox! remove this line.
    Set Temp = Nothing'<-- this is meaningless, temp will run out of scope at the next line
End Function

Sub UnloadDC(iDC As Long)
    'Release DCs
    ReleaseDC MainWindow.hWnd, iDC'<-- no way!
    DeleteDC iDC'<-- the selected bitmap was not released, here's what to do:

    DeleteObject SelectObject(iDC,0)
    DeleteDC iDC  

End Sub

*Ahem* I thought you of all gameprogammers should know this

[Skitchen8]

Originally posted by Jotaf98
Well Fox, I guess that "Sam's teach yourself C++ in 24 hours" is not that good after all

Hum, btw, are you The Fox that made all those cool Graal levels?

oh cool... im not the only one that bought that worthless piece of sh*t of a book

[Fox]

Why doesn't it work anyways kedaman?


I tried this code:

   dim a as long
   dim dc as long
   dim temp as ipicturedisp

   for a=0 to 100
      'Load
      dc = createcompatibledc(me.hdc)
      set temp=loadpicture("c:\test.bmp")
      selectobject dc, temp
      set temp=nothing

      'Release
      deleteobject selectobject(dc, 0)
      deletedc dc
   next

And if I right-understood your corrections the function above shouldn't waste any memory..? Well it does..

[YoungBuck]

The only reason I put the code to add the Pen was to just illustrate the fact that you have to put back all the objects originally created with the DC before you use DeleteDC, when you create a DC it comes stocked with a set of objects (a Pen, a Brush, a Bitmap, etc.) and you have to Select these objects back into the DC before you can use DeleteDC or you will have memory leaks as you are experiencing right now. So to fix your code you need to do this...

dim a as long
   dim dc as long
   dim temp as ipicturedisp
   dim orig as Long

   for a=0 to 100
      'Load
      dc = createcompatibledc(me.hdc)
      set temp=loadpicture("c:\test.bmp")
      ' get handle to original bitmap created with DC
      orig = selectobject(dc, temp)

      'Release
      ' put the original bitmap back into the DC
      selectobject dc, orig
      ' delete the bitmap that was created
      deleteobject temp
      ' free the DC's resources
      deletedc dc

   next

Hope that is clearer.

[Fox]

Ok, this seems to work now. Thanks!

[Jotaf98]

Hey Fox, don't forget to update the BitBlt tutorial in your site

[kedaman]

You crazy blitters

[Fox]

Ya I will..

Blit, blit, hurray!

[YoungBuck]

Don't forget to show your gratitude towards your fellow VB-World brethren either.

[denniswrenn]

lpDDSPrimary->BltFast, lpDDSPrimary->BltFast Hurray!

[Jotaf98]

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[kedaman]

You really crazy blitters
bohooo! Bltfast won't work with clippers

[DarkMoose]

bah, my RPG will own all of yourz! KYAHAHAHA....

**sees everyone grab a torch / pitchfork**

....................AHHHH

[Jotaf98]

(Suddenly, everyone that has a torch sticks it to the dungeon's walls and everyone else makes a circle, and then undeads start to appear and...)

Ok, time for me to shut up

Anyway, if your RPG is sooo good, could you please send us a screenshot of it?

[DarkMoose]

Ugh, bad question. Sure the programming is good, but ..... I'm drawing the graphics myself X[ I just can't find tilesets anywhere so...

Well I'll be sure to tell all of you when my RPG is done (10 years from now)

[The Hand]

'Tis the crux of the problem, isn't it?

I've got mad programming & GDI32 skills too, but no time and no artistic ability...

**boo hoo**

-Hand out

[DarkMoose]

I'm telling you, there's not a programmer in the world that can draw. It's the law of life :((((

[Sastraxi]

I'm okay at it... a lot of people say I'm really good, but I wouldn't know by your standards...

[Nirces]

I nicked my tileset from the UO CD using a program called Inside UO.

1000's of tiles (non-rotated) ready to go, all 64x64

[kedaman]

You should worry about your programming skills more than anything. They will fail you

[Fox]

*hehe* not sure.. I cant work on a game too long without nice graphics to create test levels

[Jotaf98]

Well I'm not THAT bad at graphics, I just don't have the right tools, and hand-drawn graphics usually don't look very good in a game

Look at this small image I drew in my eraser during a boring History class