Using TObject.Dispatch

[VTwin]

You should try to override the TForm.DefaultHandler and do some logging in order to figure out which forms causes the SIGSEGV and under what conditions. This could maybe disclose a bug in your code you have not spottet yet

True, perhaps I have a lurking bug that I should squish. I'll try that.

[VTwin]

You may have inadvertently checked "Use Heaptrc unit (check for mem-leaks) (-gh)" in the Project option dialog. (See attached image)

Remove the check and see if the message remains.

Ah, that is possible! I will try that. Thanks!

[VTwin]

I'm trying to make some progress. I added a TForm.DefaultHandler to all the Forms, and am checking the messages coming in. One of my Forms is getting message ids like:

$B02C
$B015

even when I don't use TObject.Dispatch to send any. I thought user messages were free to use any id greater than:

$400 (LM_USER)

Does anyone have an idea where these might come from? If I do send messages using Dispatch they show up along with these.

Cheers,
VTwin

[VTwin]

Maybe I answered that myself, a word > 32767 is a negative smallint. But message id is 4 bytes...

[Fungus]

I'm getting confused.. The TObject.Dispatch is described as a "integer message handler" in FPC, but in Delphi it is 2-byte integers. The message structure in this subject:

Code: Pascal  [Select][+][-]

1. TVMessage = packed record
2.   Id : dword; //This might be a wrong data type..
3.   Param : word;
4.   Flag  : word;
5.   Text  : string;
6. end;

May be wrong all along since the message's id should be a word (or smallint if signed - both 2 bytes) and not a dword (4 bytes). You might want to experiment with this to check if that is the problem.

[VTwin]

I'm fairly sure that a 64 bit unsigned integer (dword, longword, or cardinal) is correct:

http://freepascal.org/docs-html/ref/refsu5.html

http://www.freepascal.org/docs-html/ref/refsu32.html

"The TObject.Dispatch method can be used to call a message handler. It is declared in the system unit and will accept a var parameter which must have at the first position a cardinal with the message ID that should be called."

I was however confused as the message id seemed to be a 2 bit signed integer. I thought I read somewhere that negative values are reserved by the system, and that seems to be what I was seeing.

I did change the string to a shortstring just in case, but did not see a difference.

Cheers,
VTwin

[VTwin]

At this point I am no longer able to reproduce the error. The messages are all getting through, and the program does not crash. I am using the Dispatch method I described.

I added a DefaultHandler to each form that responded to a message and monitored the messages. Note the peculiar (w <= 32767) to screen system messages.

Code: Pascal  [Select][+][-]

1. procedure TGraphForm.DefaultHandler(var msg);
2. begin
3.   UnhandledMessageDlg('GraphForm', msg); // debug
4. end;      
5.  

Code: Pascal  [Select][+][-]

1. procedure UnhandledMessageDlg(form: string; var msg);
2. var
3.   w: longword;
4.   s: string;
5. begin
6.   w := TVMessage(msg).Id;
7.   if (w > LM_User) and (w <= 32767) then begin
8.     s := 'Unknown';
9.     case w of
10.       OM_FileChange        : s := 'FileChange';
11.       OM_DataChange        : s := 'DataChange';
12.  
13.       ...
14.  
15.     end;
16.     s := 'Unhandled Message in ' + form + ': ' + s + ' ($' + IntToHex(w, 4) + ')';
17.     ShowMessage(s);
18.   end;
19. end;        
20.  

Then I commented out all the message directives, and turned them back on one by one. No error, and all is working, but I am not clear why.

So... I guess this is closed unless the behavior reappears. 

I appreciate the input!

Cheers,
VTwin

[VTwin]

By the way, howardpc and rvk, yes I had turned on the Heaptrc unit!

Thanks!

Cheers,
VTwin

[Fungus]

Fair enough! But then there may be a difference in Delphi and FPC which should be considered when designing code that may be compiled by different compilers:

http://docs.embarcadero.com/products/rad_studio/delphiAndcpp2009/HelpUpdate2/EN/html/delphivclwin32/System__TObject__Dispatch.html