[fpc-pascal]How to Lock a file?

[mili]

Hello, Every one

I opened a file with TFileStream, and I don't want any other program to access 
this file. So I try to lock it with FLock function in unit Linux as 
following:

>>>>>>
var
  fs : TFileStream;
  h  : integer;
begin
  fs:=TFileStream.Create('test.dat',fmOpenReadWrite);
  h:=fs.Handle;
  if LockF(h,LOCK_EX) then writeln('Lock OK!') else writeln('Lock Failed!');
  readln;  //  ----> Now, I test if I can still open this file with other 
program
  fs.Free;
end.
<<<<<<

But, Still, I can open the same file with other programs.
So, Can you tell me how to lock a file correctly with free pascal?

Thank you.
mili