Firebird UDF
lazarus / 16-Databases

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simeonb · 2016-04-09 20:59

Hello,

I have a problem with "so" library with user defined function written
in Free Pascal. I use it with 32 bit firebird 2.5.4 on 64 bit OpenSuse
13.2 linux kernel 3.16.6.
I want to store a value in threadvar variable and use it later, but
after a while the threadvar variable is zeroed! How this is possible ?
What can I do to debug the issue ?

My test code is:

library TestUDF;

uses
  cthreads;

threadvar
  TheThreadVar: Integer;

function InitThreadVar_FB(var a: Integer): Integer;
begin
  TheThreadVar := a;
  Result := a;
end;

function GetThreadVar_FB(var a: Integer): Integer;
begin
  Result := TheThreadVar;
end;

function GetCurrentThreadID_FB(var a: Integer): Integer;
begin
  Result := Integer(GetCurrentThreadID);
end;

exports
  InitThreadVar_FB,
  GetThreadVar_FB,
  GetCurrentThreadID_FB;

begin
  IsMultiThread := True;
end.

Definition of functions:
DECLARE EXTERNAL FUNCTION InitThreadVar
  INTEGER
  RETURNS INTEGER BY VALUE
  ENTRY_POINT 'InitThreadVar_FB' MODULE_NAME 'TestUDF';

DECLARE EXTERNAL FUNCTION GetThreadVar
  INTEGER
  RETURNS INTEGER BY VALUE
  ENTRY_POINT 'GetThreadVar_FB' MODULE_NAME 'TestUDF';

DECLARE EXTERNAL FUNCTION GetCurrentThreadID
  INTEGER
  RETURNS INTEGER BY VALUE
  ENTRY_POINT 'GetCurrentThreadID_FB' MODULE_NAME 'TestUDF';


I set the threadvar variable with
SELECT InitThreadVar(123456)
FROM RDB$DATABASE

I read the threadvar variable with
SELECT GETTHREADVAR(1), GETCURRENTTHREADID(1)
FROM RDB$DATABASE

At first I receive the correct value 123456.

But after 1 minute, when I try to read the value again, it returns
ZERO, but the value of GetCurrentThreadID is the same. Thread is NOT
changed, but the value of threadvar variable is lost. How this is
possible ?

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balazsszekely · 2016-04-10 07:17

I did a little experiment. You can see the result in the
picture(attachment). The image is self explanatory and answer to your
question: "The value of GetCurrentThreadID is the same".
Apparenly, there is no mistery at all, your so is unloaded after a
while. You should check firebird ducumentation, maybe there is a way
to prevent udfs to be unloaded.

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simeonb · 2016-04-10 11:53

To check if the so library is unloaded I had made the following
experiment:

I added a global variable

var
  TheVar: Integer;

threadvar
  TheThreadVar: Integer;

function InitThreadVar_FB(var a: Integer): Integer;
begin
  TheThreadVar := a;
  TheVar := a;
  Result := a;
end;

....

and function to read it:
function GetVar_FB(var a: Integer): Integer;
begin
  Result := TheVar;
end;

with declaration:
DECLARE EXTERNAL FUNCTION GetVar
  INTEGER
  RETURNS INTEGER BY VALUE
  ENTRY_POINT 'GetVar_FB' MODULE_NAME 'TestUDF';

I have set the value with
SELECT InitThreadVar(123456)
FROM RDB$DATABASE

To be sure that so library cannot be loaded again, I deleted the so
file.

Then I checked with
SELECT GETTHREADVAR(1), GetVar(1), GETCURRENTTHREADID(1)
FROM RDB$DATABASE

First result:
GETTHREADVAR   GETVAR   GETCURRENTTHREADID
123456   123456   -154993856

After a minute:
GETTHREADVAR   GETVAR   GETCURRENTTHREADID
0   123456   -154993856

Only the theadvar was lost   And I think that the library is not
unloaded.

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balazsszekely · 2016-04-10 14:28

Quote@simeonb
Only the theadvar was lost   And I think that the library is not
unloaded.Ok, then please read this: http://wiki.freepascal.org/Multith
readed_Application_Tutorial#Initialization_and_Finalization
Search for "threadvar". Apparently the threadvar system needs to be
initialized, otherwise thredvars won't work correctly. You should give
it a try.

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simeonb · 2016-04-12 14:46

I have compiled that code with CrossKylix. And the same happened.
After a minute Threadvar was zeroed. So this is may be firebird issue
and is not connected with Free Pascal.