Xref: utzoo alt.msdos.programmer:1582 comp.sys.ibm.pc.programmer:1307
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!matrix!venkat
From: venkat@matrix.UUCP (D Venkatrangan)
Newsgroups: alt.msdos.programmer,comp.sys.ibm.pc.programmer
Subject: termination handler address
Keywords: int 22h/23h
Message-ID: <141@matrix.UUCP>
Date: 3 May 90 19:40:09 GMT
Distribution: na
Organization: Matrix Computer Systems, Nashua, NH
Lines: 22


I am attempting to trap the exit of a program so I can clean up things
that DOS would not clean up (such as interrupt vectors that were changed).
As per well-documented sources, I am replacing int 1bh, int 22h and int 23h
vector entries.

After initializations, I do see that these entries are changed to entry
points within the code.  When the program exits (through an exit() call from
a C function), I see that my handlers are not being called.  I can trace
up to the int 21 (function 4ch) instruction without my handlers being called.
Are they called as part of the 4ch function processing?  In my handler,
I do the cleanup that I need to do and then jump to the handler value that
I saved before installing my entry.  (this old handler I guess is supposed to
copy the old values for the int 22h/23h etc. from the PSP of the program).

If instead the program does a terminate and stay resident (function 31h), will
my termination handler be called as well?  If so, is there a way to distinguish
whether the call to an exit handler is from function 31h as opposed to
function 4ch?  If it is the stay resident case, I do not want to do the
cleanups.

Is there an established way to do this correctly?