filter i/o in C

[utzoo!decvax!goutal]

Most implementations of C that I know of
prohibit me from having the same file open for reading and writing
at the same time.  Most operating systems that I use, however,
think this is okay, under various interpretations.
The most obvious example of why I might want to do this is a simple
filter or, more clearly, an editor of some sort;
I want to read from a file and write the result back into the same
file (or a new version of it, more likely).
Anybody know how I should do this?
Just mail to me, I guess.
-- Kenn (decvax!)goutal