HELP ? How to protect a process in a C/Sr-apps ??

Hi everybody,

I have several processes in my application with the following rule :
when a proces X is launched by a user A,  then no other user can launch
process X till user A has finished with this process (example : a
process prints
automatically bills for the client with a field = TOPRINT : when I
creates this bills,
I don't want that another user start this process also,
when then it's possible that the bill is printed out by me and by
another user.

Must I work with status-flags (record-level),  can I lock this tables
(table-level), or
can I protect this processes (process-level) ...

Any suggestions are welcome

Thanks in advance

System : Oracle 8,CPP 3,Delphi, NT 4.0 SP3.