Hi,

I'm wondering if shared libraries (on Linux) support call exported function from multiple threads. I made small demo. Threads send JSON PChar into shared lib function which parse it and log in syslog. No global vars, everything locally in function. But even that I'm getting random errors like:

29.03.2017 16:33 kernel [26260.212884] traps: test_libmjrende[12177] general protection ip:7fe823eed2f1 sp:7fe8238571e0 error:0 in libmjrender.so[7fe823de0000+284000]
29.03.2017 16:36 kernel [26443.509924] test_libmjrende[12247]: segfault at 7f5e02d179f0 ip 00007f5e02d1797c sp 00007f5e02d179f0 error 7 in libmjrender.so[7f5e02c02000+286000]
29.03.2017 16:42 kernel [26819.543353] test_libmjrende[12350]: segfault at 7f1f1d65b0a0 ip 00007f1f1d65b0a0 sp 00007f1f0c000fd8 error 15 in libmjrender.so[7f1f1d62e000+112000]
29.03.2017 16:39 kernel [26627.190568] test_libmjrende[12297]: segfault at 8 ip 00007fe4c6851c2c sp 00007fe4c0766c90 error 4 in libc-2.23.so[7fe4c67d0000+1bf000]

I guess that just not all RTL are thread safe and this is not supported. Even critical sections don't work here.
This is function:

procedure Test(ACounter: Integer; AJson: PChar); cdecl;
var
  js: TJSONObject=nil;
  jp: TJSONParser=nil;
  i: Integer;
  s: String;
begin
  syslog(log_info, 'Enter: %d', [ACounter]);
  try try
    jp := TJSONParser.Create(AJson);
    js := TJSONObject(jp.Parse);
    FreeAndNil(jp);
    Sleep(500);
    i := js.Get('1', -1);
    Sleep(500);
    s := js.Get('2', '');
    syslog(log_info, 'Got %d: %d %s', [ACounter, i, pchar(s)]);
  except on e: Exception do
    begin
      syslog(log_info, 'Error: %s', [PChar(e.Message)]);
      raise;
    end;
  end;
  finally
    FreeAndNil(jp);
    FreeAndNil(js);
    syslog(log_info, 'Leave: %d', [ACounter]);
  end;
end;

Attached also full source with library and client.

Regards

It should be supported but must have threads support compiled-in.

You didn't add cthreads to the uses list of the library, so your library will not function when called from multiple threads.

Of course, you need to keep your code thread-safe. And indeed, not all RTL calls are thread safe.
If any initialization must happen, it must happen the first time the library is loaded (or it must be done in critical sections).

Michael.

Yes, I'm aware of cthreads and tested it too (without it also critical sections don't work) but it doesn't help with attached examples. Still same errors in logs

Krzysztof wrote

Yes, I'm aware of cthreads and tested it too (without it also critical sections don't work) but it doesn't help with attached examples. Still same errors in logs

Nevertheless, that should work.

I've used it in multi-threaded apache modules.

Michael.

I should add that WITHOUT using cthreads, it certainly will NOT work.

Michael.

2017-03-30 15:19 GMT+02:00 Michael Van Canneyt

Well it doesn't work or fpjson module is not thread safe in this case. You can test it by yourself with attached demo (first post). Try it few times because sometimes if you are lucky then there is no error

If I find a moment, I will test.

Michael.

I can reproduce the problem.

At least partially it is related to the threads mechanism not being initialized correctly.

To fix that, you can do the following.
In the library startup code, create a dummy thread. This will initialize the threads mechanism:

library testlib;
{$mode objfpc}{$H+}
uses
   cthreads, cmem, Classes, testlibu;
exports
   InitTest, FinalizeTest, Test;
begin
   with TThread.Create(False) do
end.

After this, I no longer get the problem (which doesn't mean there are no other possible problems :) )

Michael.

Thanks a lot! Seems to working fine now

Hello.

Michael Van Canneyt wrote

I use this for initialize my libraries:

 With TThread.Create(False) do Terminate;

It works ok but there is that warning after compiling:

Z:\home\fred\uos\examples\uos.pas(7438,29) Warning: (4046) Constructing a class "TThread" with abstract method "Execute"

Huh, is it Is it serious doctor?

Fre;D

-----
Many thanks ;-)

Best is probably:

Type
   TDummyThread = Class(TThread)
   public
     procedure execute; override;
   end;
procedure TDummyThread.Execute;
begin
   FreeOnTerminate:=True;
   Terminate;
end;
begin
   TDummyThread.Create(True)
end.

Michael.

fredvs wrote

> Z:\home\fred\uos\examples\uos.pas(7438,29) Warning: (4046) Constructing a > class "TThread" with abstract method "Execute" Huh, is it Is it serious doctor?

I use this:

function DummyThread(param: pointer): ptrint;
begin
  Result := 0;
  EndThread(Result);
end;
begin
  BeginThread(@DummyThread);
  ...
  ...
end.

Michael Van Canneyt wrote

OK, perfect, no more warning, thanks doctor.

Fre;D

-----
Many thanks ;-)

What about calling TM directly? Something like this:

uses cthreads;
var
  TM: TThreadManager;
begin
  TM := Default(TThreadManager); // just hiding hint 'variable TM doesn't
seem to be initialized'
  GetThreadManager(TM);
  IsMultiThread := TM.InitManager;
...
end.

I can't test it now (I would like to debug it asap), but I think FPC offer some option to start the TM without starting a new -- dummy -- thread. o.O

TDummyThread.Create(True)

Huh, I have memory leak with this.

Should it not be: --> TDummyThread.Create(false) ? (because with this, no memory leak)

Fre;D

-----
Many thanks ;-)

Hello.

Sorry to come back with this but without answer I am still full of doubt.

Is TDummyThread.Create(True) wanted and so the dummy thread must exist during all the application and freed at end of application ?

Or is TDummyThread.Create(false) ok ?

Thanks.

Fre;D

-----
Many thanks ;-)