On 02/09/2016 11:37 PM, Giuliano Colla wrote:
Hi Lazarus guru's,
I have stumbled into a problem with a Lazarus application. At random
times, from 5 minutes to several hours, it crashes suddenly, and silently.
The only way to get a clue has been to run it with gdb.
It turns out that the crash occurs because of a Segmentation Fault in
QTextEngine, triggered by a PostMessage, but apparently the cause was an
endless loop in a previous PostMessage, which was executed some 1723
times. Attached the gdb stack trace (with 1722 identical items suppressed).
The scenario is a main thread using LCL, which is used, among other
things not relevant here, to display information to the user, and
another thread which communicates with a real-time process, and uses a
PostMessage to wake up the main thread and make it show what required.
It appears that in some circumstances, the Application.ProcessMessages
invoked in the main thread restarts the handling of the same message,
thus creating and endless loop.
Is Application.ProcessMessages the vilain, or PostMessage or what?
Snippet of auxiliary thread code:
procedure TSendRec.DoDisplayError;
var
Box: integer;
begin
ErrList[ErrPut] := ErrMsg;
inc(ErrPut);
if ErrPut > High(ErrList) then ErrPut := 0;
{$IFDEF USE_MESSAGES}
PostMessage(Form1.Handle,LM_ERROR,Integer(ErrPut),0);
{$ENDIF}
end;
Snippet of main thread code:
procedure TForm1.HandleError(var Msg: TLMessage);
var
.....
begin
inc(ErrorCnt);
if ErrGet <> ErrPut then begin
ErrMsg := ErrList[ErrGet];
.......
AnswerString := Format('UNIT %XH device %s ERROR
%XH',[ErrMsg.yunit,Localize(NamUn[ErrMsg.device]),ErrMsg.ytype]);
ShowError(Self,Answerstring,BoxError[Box],Box);
inc(ErrGet);
if ErrGet > High(ErrList) then ErrGet := 0;
end;
end;
1.Your ErrList access does not look thread safe.
2.You are casting postMessage param as integer, IMO it's wrong,
shouldn't it be PtrInt or LPARAM ?
PostMessage(Form1.Handle,LM_ERROR, LPARAM(ErrPut),0);
3.Application.ProcessMessages probably recurse.
I'm using PostMessage() with QtLCL inside one app which heavily uses
threads for gui controls updates and it works w/o problems, atm, one of
our users have that app running, uptime is cca 90 days (since last
update). I don't use such lists, but creating record pointer and free it
in main thread message queue....
zeljko
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