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From: scs@adam.pika.mit.edu (Steve Summit)
Newsgroups: comp.lang.c
Subject: Re: %p and different pointer representations
Message-ID: <9571@bloom-beacon.MIT.EDU>
Date: 3 Mar 89 05:43:10 GMT
References: <11915@haddock.ima.isc.com> <9765@smoke.BRL.MIL>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: scs@adam.pika.mit.edu (Steve Summit)
Lines: 55

In article <11915@haddock.ima.isc.com> karl@haddock.ima.isc.com (Karl Heuer) writes:
>Therefore, a completely generic pointer
>type (to function or data) is `union {void (*)(void); void *}', though I can't
>think of any practical use for such a thing.

In article <9765@smoke.BRL.MIL> gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes:
>I don't understand why so many people seem to be concerned about
>packing both object AND function pointers into the same type of
>variable.  (By the way, it CAN be done; use a union.)  What sort
>of program calls for such weirdness, anyway?  The only genuine
>"problem" I've seen mentioned is the inability to portably print
>out a function pointer, something I don't think is all that useful.

I will have to use a variation on this union in the symbol table
and the rvalue structure of a C interpreter I wrote once, if I
ever want it to pass ANSI muster.  (I'm not sure I'll bother.)

So far, the interpreter's symbol table has assumed that there is
one type which can contain any pointer, to code or data.  There
is also an "nm" ("namelist") command, which prints the symbol
table; this command has been using %p but will require revision
if %p will not portably print a function pointer.

The same difficulties would hold for any assemblers, linkers, or
other object file manipulation programs (nm, etc.) written in C
on a system with incompatible code and data pointers.  Those
programs don't have to be portable, and can use whatever system-
dependent kludges are required.  It's more troublesome for the
interpreter, most of which (outside of its dynamic linker) is
quite portable.

Admittedly, a C interpreter is a rather extreme, one-of-a-kind
program, that pushes the edges of the envelope in several ways.
(The dynamic linker reads object code into malloc'ed memory and
then jumps to it, a maneuver which some architectures reject
outright.  Others require a special call to create new text
segments, which the interpreter would have to use.)

                                            Steve Summit
                                            scs@adam.pika.mit.edu

P.S. The astute reader may wonder why an interpreter needs to
handle code pointers at all, since the "code" being interpreted
by an interpreter is really data.  However, an interpreter that
allows intermixing of interpreted and compiled code must keep
"code" pointers, whether to interpreble or compiled code, as true
code pointers, partly to keep function calls made from
interpreted code consistent, and more importantly to allow calls
from compiled code back to interpreted code to be made at all.
(When you link and relocate object code with undefined external
references, you've got to have a real function pointer to fill
in.  Arranging that calls to interpreted routines from compiled
code work is a challenging problem, and is left as an exercise
for the reader :-).  A correct solution allows installing
interpreted routines as signal handlers, a fascinating concept.)