Thanks for anybody who replied and

again my apology for the easiness i charged firebird for the problem...

The problem's reason and the solution for any who may concern:

As some of repliers said the problem was in memory management of delphi under concurrent threads.

After  15 years using delphi,  i discovered by googling the variable IsMultiThread

Adding in initialization section the following line

IsMultiThread :=true;

everything is ok, also using superclassic (at least in my machine).

PS1: <IsMultiThread :=true> is silently done by Delphi itself, if threads exist in the uses list. Bad for me, my functions where pretty simple and so threads unit was not declared in uses.

PS2:  I think that this could be a major trap for many delphi programmers and that a relative note in documentation about UDF with Delphi in http://www.firebirdfaq.org/faq83/ should be added

Best Regards

A.Kyriakos

Athens, Greece