get an unexpected Exception External:SIGSGEV

[krull]

Sorry for another Newbish qustion.

For practice I wanted to write an upward moving Spacefighter.
I know I'm only at the beginning.

Here's the problem. When my shot hit the upper edge the compiler means External:SIGSGEV.
Perhapse this has something to do with the 'shot.free';

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Windows ,Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, ExtCtrls, BGRABitmap, BGRABitmapTypes;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     playerPosTimer : TTimer;
16.     shotTimer: TTimer;
17.     procedure FormClick(Sender: TObject);
18.     procedure FormCreate(Sender: TObject);
19.     procedure playerPosTimerTimer(Sender: TObject);
20.     procedure playerPaint(Sender:Tobject);
21.     procedure shotTimerTimer(Sender: TObject);
22.     procedure shotPaint(Sender:TObject);
23.   private
24.     { private declarations }
25.   public
26.     { public declarations }
27.  
28.    playerShip : TBGRABitmap;
29.    shot : TBGRABitmap;
30.    pPlayer : Point;
31.    pFormer: Point;
32.    shotSpeed : Integer;
33.    pShot : Point;
34.    shooting : Integer;
35.    leftClick : boolean;
36.   end;
37.  
38. var
39.   Form1: TForm1;
40.  
41. implementation
42.  
43. {$R *.lfm}
44.  
45. { TForm1 }
46.  
47. procedure TForm1.playerPosTimerTimer(Sender: TObject);
48. begin
49.   pFormer:= pPlayer;
50.   pPlayer:= Mouse.CursorPos;
51.   pPlayer:= Form1.ScreenToClient(pPlayer);
52.   playerPaint(Sender);
53. end;
54.  
55. procedure TForm1.FormCreate(Sender: TObject);
56. begin
57.   playerShip := TBGRABitmap.Create('1.png');
58.   pPlayer:= Mouse.CursorPos;
59.   pPlayer:= Form1.ScreenToClient(pPlayer);
60.   shooting:=0;
61.   shotSpeed:=10;
62.   leftClick:= false;
63.   pShot.y:=1;
64. end;
65.  
66. procedure TForm1.FormClick(Sender: TObject);
67. begin
68.   leftClick:=true;
69. end;
70.  
71. procedure TForm1.playerPaint(Sender:Tobject);
72. begin
73.   Form1.Canvas.FillRect(pFormer.x-round(0.5*playerShip.Width),pFormer.y-round(0.5*playerShip.Height),pformer.x+playerShip.Width-round(0.5*playerShip.Width),pFormer.y+playerShip.Height-round(0.5*playerShip.Height));
74.   playerShip.Draw(Canvas, pPlayer.x-round(0.5*playerShip.Width),pPlayer.y-round(0.5*playerShip.Height),false);
75. end;
76.  
77. procedure TForm1.shotTimerTimer(Sender: TObject);
78. begin
79.   if (leftClick = true) and (shooting = 0) then
80.   begin
81.     try shot:= TBGRABitmap.Create('Beam1.png'); except end;
82.     pShot.x:=pPlayer.x- round(0.5*shot.width);
83.     pShot.y:=pPlayer.y- round(0.5*playerShip.Height)-shot.Height;
84.     shot.draw(Canvas,pShot.x,pShot.y,true);
85.     shooting:= shooting+1;
86.   end;
87.   leftClick:=false;
88.   if pShot.y <= 0 then
89.   begin
90.     shot.Free;
91.     shooting:=shooting-1;
92.   end;
93.   if shooting >=1 then
94.   begin
95.     pShot.y:= pShot.y-shotSpeed;
96.     shotPaint(Sender);
97.   end;
98. end;
99.  
100. procedure TForm1.shotPaint(Sender:TObject);
101. begin
102.  Form1.canvas.FillRect(pShot.x, pShot.y+shotSpeed, pShot.x+shot.Width, pShot.y+shot.Height+shotSpeed);
103.  shot.draw(Canvas,pShot.x,pShot.y,true);
104. end;
105.  
106. end.

[jmm72]

It's just a simple logic check which is flawed. Don't worry, it happens to everyone, me included (dozens of times).

After you destroy the shot object with shot.Free, the next time the shotTimer triggers, 'if pShot.y <= 0 then' still executes and it's still true, thus tries to free the object again, which causes the sigsev.

Just replace everything after the 'leftclick := false;' (keep that one) with this, it's just your code with the check moved and merged with another part of the code. With this change it works.

Code: Pascal  [Select][+][-]

1. if shooting >=1 then
2.   begin
3.     if pShot.y <= 0 then
4.     begin
5.       shot.Free;
6.       shooting:=shooting-1;
7.     end
8.     else
9.     begin
10.       pShot.y:= pShot.y-shotSpeed;
11.       shotPaint(Sender);
12.     end;
13.   end;

With this change, the check for shot being at the top only happens if you're shooting; if true, destroys the object and stops shooting, otherwise moves the shot.