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From: Arne.Venstad@elab-runit.sintef.no
Newsgroups: comp.sys.mac.hypercard
Subject: XFCN/XCMD
Message-ID: <90Oct22.134718*Arne.Venstad@elab-runit.sintef.no>
Date: 22 Oct 90 12:47:18 GMT
Sender: news@idt.unit.no (Usenet news admin)
Distribution: comp.sys.mac.hypercard
Organization: ELAB-RUNIT, SINTEF, Trondheim, Norway.
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I have just started experimenting with writing XFCNs for HyperCard (1.2.x), using
Think C 4.0 as environment. My XFCN is a kind of filter procedure for the contents
of a HC field. The function thus always returns a field contents which is a (not
necessarily true) subset of the original field contents. In order to save space
and prevent heap fragmentation, I thougth it was a good idea to use the parameter
handle HC delivers for the field parameter as the result handle for the function.
(Of course taking into account the problem of overwriting data before they are 
fetched).
I have tried it and it works in my test. But is this a safe solution, i.e. will
HC alway copy the contents of the returnValue handle before it disposes the
params handles? 
I have no other documentation for writing XCMD/XFCN other than the HyperGlue 
document(s) by Dan Winkler and what is said about in Think C manuals. Is there
any free documnetation on this subject available, both for HC 1.2.x and HC 2.0?
I know there is a book: "XCMD's for HyperCard" by Dan Winkler. Is there or will
it be a new revision of this book in connection with the HC 2.0 release?

Any help or hints are appreciated, thank you!

Arne Venstad            E-mail:Arne.Venstad@elab-runit.sintef.no
ELAB-RUNIT
SINTEF
N-7034 Trondheim, Norway.