PureBasic Forums - English

Posted: **Mon Sep 08, 2025 9:32 pm**

I should first say that I am using PB 5.73 cause I want the original backend for now.

Anyway, I am making a game in PB that uses an OpenWindowedScreen(). Now for some really strange reason I can't figure out (nor can ChatGPT - it gives me random irrelevant stuff) why the call StartDrawing(ScreenOutput()) is failing randomly as if the window is no longer in existence, but I do not close the window or anything. The exact error is "StartDrawing(): The specified output is NULL (0 value)." It happens at random times throughout gameplay. It tends to happen after about 3 minutes, but it varies a bit. It's one of those scary intermittent problems that seems to have no rhyme or rhythm. Has anyone encountered anything like this before?

I will post the source of the game if I have to, but I wasn't planning on making the game open source. It's a pretty simple game, so I don't see why such a thing is happening.

Posted: **Mon Sep 08, 2025 10:09 pm**

Without code, just speculation is possible:

ScreenOutput() must be called inside StartDrawing(). You can't call ScreenOutput() alone nor use the retured value.
Have you enabled the Purifier of the Pure Basic debugger? The Purifier checks for invalid memory access which could happen at a different point of the source but affect the StartDrawing() call.
Do you use Threads and do you call ScreenOutput() in a different thread than the screen was opened?

Posted: **Mon Sep 08, 2025 10:13 pm**

Solved it!

The problem was that I was loading a font in the main drawing loop of the game. It was my mistake. I didn't know that would cause such strange random crashes. It kind of makes sense though cause loading a font might cause some type of delay or maybe asynchronous code execution. So the problem wasn't even on the line it was failing on. It was just happening there cause it was the line right after the font load call.

Here's the function so nobody makes the same mistake:

Code: Select all

Procedure DrawScene()
  Protected x, y, c, px, py
  
  ; Draw blocks (no grid lines)
  For y = 0 To #Rows-1
    For x = 0 To #Cols-1
      c = Grid(x, y)
      If c >= 0
        px = x * #Size
        ; py = y * #Size
        ; apply falling offset in pixels
        Protected offY.i = 0
				If AnimActive
				  offY = Int(FallOffset(x, y))
				EndIf
				py = y * #Size + offY
        ; px, py already include falling offset
				Select Grid(x, y)
				  Case 0 : DisplayTransparentSprite(#SprRed,    px, py)
				  Case 1 : DisplayTransparentSprite(#SprGreen,  px, py)
				  Case 2 : DisplayTransparentSprite(#SprBlue,   px, py)
				  Case 3 : DisplayTransparentSprite(#SprYellow, px, py)
				EndSelect
      EndIf
    Next
  Next
  
  ; HUD
  ; <----- WAS LOADING THE FONT RIGHT HERE. FIXED.
  
  StartDrawing(ScreenOutput())
  	DrawingMode(#PB_2DDrawing_Transparent)
  	DrawingFont(FontID(MainFont))
  	DrawText(6, 10, "Score: " + Str(Score), #Black)
	  DrawText(8, 8, "Score: " + Str(Score), #White)
	  
	  If AnimActive
	  	DrawText(6, 50, #StrFalling, #Black)
	  	DrawText(8, 48, #StrFalling, RGB(255,60,60))
	  Else
	  	DrawText(6, 50, #StrNewQuit, #Black)
	  	DrawText(8, 48, #StrNewQuit, #White)
		EndIf
	  
		If GameOver
			DrawText(6, 30, #StrGameOver, #Black)
	    DrawText(8, 28, #StrGameOver, #White)
	  Else
	  	DrawText(6, 30, #StrRules, #Black)
	    DrawText(8, 28, #StrRules, #White)
	  EndIf
	StopDrawing()
	
EndProcedure

PureBasic Forums - English

StartDrawing(ScreenOutput()) the specified output is NULL

StartDrawing(ScreenOutput()) the specified output is NULL

Re: StartDrawing(ScreenOutput()) the specified output is NULL

Re: StartDrawing(ScreenOutput()) the specified output is NULL