Delphi call a function in anonymous thread?

Question

I have to call a function in a anonymous thread in a while

my sample function is like this, just for print output:

function processPureTmFrame(rowFrame : string;tmDataGroupRef:string ):string;
    TThread.Synchronize(nil,
      procedure
      begin
         form2.Memo1.Lines.Add( tmSlitFrame );
      end
    );      
end;

when i call the function like this :

code1

while tmBody.Length>0 do
begin
    tmBodyFrameLength := ((hextodec( copy(tmBody,11,2) )+6)*2)+2;
    tmSplitFrame :=  copy(tmBody , 1 , tmBodyFrameLength );
    delete( tmBody, 1, tmBodyFrameLength );

    myThread := TThread.CreateAnonymousThread(
    procedure
    begin
        processPureTmFrame( tmSplitFrame , tmDataGroupRef );
    end);
    myThread.Start;
end;

in the first cycle of loop the output is missing

but when i call my code without thread every thing is ok!!

code2

while tmBody.Length>0 do
begin
    tmBodyFrameLength := ((hextodec( copy(tmBody,11,2) )+6)*2)+2;
    tmSplitFrame :=  copy(tmBody , 1 , tmBodyFrameLength );
    delete( tmBody, 1, tmBodyFrameLength );
    processPureTmFrame( tmSplitFrame , tmDataGroupRef );
end;

the correct output must be like this

0851C007000C010100000007581850C001F116
0836C0BE001003627169DCA200000000000090D72AACAF
0814C0B6001C03197169DCA31901E2041211131D001F00001F1E1C1F1F1E1E1E0077AA
0814C0B7001E03197169DCA31902FE00540F0000000000000000000000000000000000E238
0814C0B8000B03197169DCA31903FE01384E
0817C0B9000D05017169DCA3E6010190B03F042D
0852C000000B036200000000FAFFFFBF16A3
0852C001000B036200000001F4FF00000000

but when call in thread(code 1) its is like

0836C0BE001003627169DCA200000000000090D72AACAF
0814C0B6001C03197169DCA31901E2041211131D001F00001F1E1C1F1F1E1E1E0077AA
0814C0B7001E03197169DCA31902FE00540F0000000000000000000000000000000000E238
0814C0B8000B03197169DCA31903FE01384E
0817C0B9000D05017169DCA3E6010190B03F042D
0852C000000B036200000000FAFFFFBF16A3
0852C001000B036200000001F4FF00000000

without thread(code 2) output is ok

Note #1: I don't get any error like:

System Error. Code:1400. Invalid window handle or any thing else

Note #2: as I said just first cycle of while not sending to the new threads. Other lines are sending and processing just fine!

@david Heffernan i added some more data,this is not the same as you marked! — peiman F., May 05 '19 at 10:38
Dude, you changed the question after I marked it as a dupe. The edit history is quite clear. — David Heffernan, May 05 '19 at 10:57
i just added some notes!! you must read better , i wrote the function was a sample! my question was about calling the function not get the output! — peiman F., May 05 '19 at 11:00
This is not your real code (has syntax errors, can't compile). Please do not post fake code, because that makes it impossible for others to see what is **really** wrong. — Rudy Velthuis, May 05 '19 at 17:05

David Heffernan · Accepted Answer · 2019-05-05T11:11:44.633

4

The problem is that the anonymous method captures variables. Because the variable is captured, its value changes during the main loop. Essentially all the threads share the same variable. The threads run in parallel with the main loop and there are no ordering constraints. So it's perfectly possible that the main loop modifies the captured variable before one of your threads has a chance to use the value.

Your code would work with value capture (as opposed to variable capture). Value capture is not supported directly but the same effect is easy to simulate. See Anonymous methods - variable capture versus value capture.

I would comment though that this threading code is going to be slower than the serial code. What are you hoping to achieve?

edited May 05 '19 at 11:11

answered May 05 '19 at 11:06

David Heffernan

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Thank you for your answer.the next part is decode frame and save it to a database over the internet. so to avoid that lag in ui i try to use threads... – peiman F. May 05 '19 at 11:11
You probably don't want to create one thread per frame. That's liable to mean far more threads than can usefully be serviced. Also, please don't be rude. Telling the people whose knowledge you want to tap into that they can't read is pretty uncool. – David Heffernan May 05 '19 at 11:13
you mean before my anonymous thread use the variable main thread change it!? i must have a local variable in thread to copy the value in first line !? – peiman F. May 05 '19 at 11:16
No. That will still be too late. Please spend some time reading the post I linked and others on the subject of variable vs value capture. Not enough time has elapsed since I answered for you to have fully understood the issue. Learning concepts like this takes time. No shortcuts. – David Heffernan May 05 '19 at 11:18
im really appreciated for your helps.i dont understood how to resolve the problem yet, i must read more, but for a shortcut i added `sleep(1);` in my while :))) – peiman F. May 05 '19 at 11:32

Delphi call a function in anonymous thread?

1 Answers1