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From: dix@clinet.fi (Risto Kaivola)
Newsgroups: comp.sys.amiga.programmer
Subject: Creating a 'Front-End' For Library Functions
Keywords: system integrity, libraries, SetFunction
Message-ID: <1991Jun25.084622.13642@clinet.fi>
Date: 25 Jun 91 08:46:22 GMT
Organization: City Lines Oy, Helsinki, Finland
Lines: 40

In a recent project I've had to create a front-end for certain dos.library
functions, meaning that first I patch a piece of code so that it will jump
to the library routine in question, and then change the library jump vector
so that it will jump to my code. So far, no problem (provided, of course,
that I remember to call the appropriate cache-handling library routines).
But when I'm finished, and want to remove the front-ends, and thus return
the system to the state in which it was earlier on, problems arise.
Obviously, I can't just return the library jump vector to its earlier state,
and then free the memory allocated for my code. If I did that, I would
take the chance that the system crashes when some task is executing off
non-allocated memory, and some other task allocates this memory block then
placing its data in this block. The possible solutions I've thought of are
as follows:

   (a) Create a front-end for the OpenLibrary function, and implement my
       own version of resource tracking. The front-ends for the dos.library
       functions will not be freed until all processes that have opened
       the dos.library after I inserted the front-end have closed the
       dos.library. To prevent the same kind of system malfunction as
       with the dos.library functions in the first-mentioned scenario,
       leave this front-end there when we're finished. Not very clean,
       but there isn't so much memory lost.

   (b) Do not fiddle with any other routines (like OpenLibrary), but rather,
       when finished, first make the library jump vector point to the
       real library routine, and then Forbid task switching. Check each
       processes task structure to determine whether it's executing my
       code. If there is a process that is doing so, wait for a while,
       then do the same check again. When there no longer are processes
       executing my code, free the memory block allocated for the code.

   (c) Do not worry about the memory loss caused by leaving the front-end
       or front-ends hanging around. This is no real solution.
 
Please tell me your suggestions and opinions on the issue. I apologize
for the lengthy posting.
 
Risto
-- 
Risto Kaivola (dix@clinet.fi) or (Risto.Kaivola@f123.n220.z2.FIDONET.ORG)