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GetMessage return values

Richard2000
Richard2000 asked
on
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Last Modified: 2012-06-27
Hi,

I have written a message loop below.  However, the compiler gives me an incompatible types error when trying to interpret the return value.

var
  ExitLoop: Boolean;
  B: BOOL;
  lpMsg: TMsg;

ExitLoop := False;
repeat
  B := GetMessage(lpMsg, 0, 0, 0);
  if B = 0 then ExitLoop := True // Incompatible types
  else if B <> -1 then // Incompatible types
  begin
    TranslateMessage(lpMsg);
    DispatchMessage(lpMsg);
  end;
until ExitLoop = True;

According to the GetMessage documentation that comes with Delphi it says:

"If the function retrieves a message other than WM_QUIT, the return value is nonzero.
If the function retrieves the WM_QUIT message, the return value is zero.
If there is an error, the return value is -1. For example, the function fails if hWnd is an invalid window handle."

Also:

"Note that the function return value can be TRUE, FALSE, or -1. Thus, you should avoid code like this:

while (GetMessage(lpMsg, hWnd, 0, 0)) ...

The possibility of a -1 return value means that such code can lead to fatal application errors."

Therefore, how do I reliably discriminate between the three different return value possibilities of GetMessage in Delphi?

Thanks in Advance,

Richard
Comment
Watch Question

the -1 and WM_QUIT sort of mean the same thing because if it crashes its going to close anyways.
any nonzero value is false. So the function will return false if there is an invalid handle and if there is a WM_QUIT message.



SO, the code now would be like

var
  ExitLoop: Boolean;
  B:bool;
  lpMsg: TMsg;
begin

ExitLoop := False;
repeat
  B := GetMessage(lpMsg, 0, 0, 0);
  if B = false then  ExitLoop := True
  else
  begin
    TranslateMessage(lpMsg);
    DispatchMessage(lpMsg);
  end;
until ExitLoop = True;


hope this helps, if not let me know

Author

Commented:
Jase-Coder,

Thank you for your comment.

I'm not sure that this would work though.  I assume that Delphi's BOOL would use FALSE for zero and TRUE for any nonzero value.  Therefore, if -1 for an error is returned, this is a nonzero value, so it will be seen as TRUE.  This would then inadvertently cause TranslateMessage and DispatchMessage to be executed after the error, which should be avoided because according to the documentation, this can lead to fatal application errors.

Any ideas?

Richard
Commented:
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Author

Commented:
Slick812,

Many thanks for your help.  After reading your comment, I have decided to just use GetMessage in a while loop and ignore the -1 error situation.

Richard

Commented:
you might find some info about this at

http://www.angelfire.com/hi5/delphizeus/first1.html

Author

Commented:
Slick812,

Thanks for the link.  I've read the article and it uses GetMessage in a while loop (no mention of -1 for error).  So it looks as though this is the standard way of using GetMessage.

Richard
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