Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!datapg!com50!jhereg!mark
From: mark@jhereg.Jhereg.MN.ORG (Mark H. Colburn)
Newsgroups: comp.lang.c
Subject: Re: %p and different pointer representations
Message-ID: <592@jhereg.Jhereg.MN.ORG>
Date: 25 Feb 89 22:23:50 GMT
References: <9382@bloom-beacon.MIT.EDU> <234@mstan.Morgan.COM> <16112@mimsy.UUCP>
Reply-To: mark@jhereg.MN.ORG (Mark H. Colburn)
Organization: Minnetech Consulting, Inc., St. Paul, MN
Lines: 67

In article <16112@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
>Yes.  Moreover, since sizeof(void *)==sizeof(char *)==2, when you
>print a pointer-to-function using %p and a (void *) cast (on that
>machine in that model), you will lose some information.

Wait a minute, Chris.  The pANS says (ss 3.2.2.3, pp 37 Oct 31, 88):

"A pointer to void may be converted to or from a pointer to any 
 incomplete or other type.  A pointer to any incomplete or object 
 type may be converted to a pointer to void and back again; the 
 result shall compare equal to the original pointer."

What you are saying would seem to contradict this.

Also, I don't beleive that there is any requirement that (void *) has to be
2 bytes long.  In my opinion void should be defined to be at least as
large as the largest pointer available in the current compiler model.

Therefore, on the IBM-PC, sizeof (void *) should be 4 in every model except
for small or tiny.  This can be checked using the following program:

	#include <stdio.h>

	int main()
	{
	    printf("sizeof(char *) = %d\n", sizeof(char *));
	    printf("sizeof(void *) = %d\n", sizeof(void *));
	    printf("sizeof(int (*)()) = %d\n", sizeof(int (*)()));
	    return (0);
	}

Using Turbo C, I got the following results:

   +-------------------------------------------------------------------+
   | Model       | sizeof(char *) | sizeof(void *) | sizeof(int (*)()) |
   +=============+================+================+===================|
   | Huge        |        4       |        4       |         4         |
   +-------------+----------------+----------------+-------------------|
   | Large       |        4       |        4       |         4         |
   +-------------+----------------+----------------+-------------------|
>> | Medium      |        2       |        2       |         4         | <<
   +-------------+----------------+----------------+-------------------|
   | Small       |        2       |        2       |         2         |
   +-------------+----------------+----------------+-------------------|
   | Tiny        |        2       |        2       |         2         |
   +-------------+----------------+----------------+-------------------+


If you take a look at the medium model, you will notice the annomally.

However, on the IBM-PC, the compiler may be able to tell the difference
between a code and data pointer by checking which segment it is in.  I am
not sure exactly how Turbo C does this, although I will check.  This 
assumption would break down if the application was attempting to
dynamically load functions and execute them (which would tend to put them
into the data segment).  Taking a function pointer in this case, may,
indeed, case problems.  There are several applications that I know of that
do this.

I will point out, however, the the %p notation does appear to work in all
of the memory models correctly.  I did not try dynamic function loading,
but I suppose that I could.

-- 
Mark H. Colburn                  "Look into a child's eye;
Minnetech Consulting, Inc.        there's no hate and there's no lie;
mark@jhereg.mn.org                there's no black and there's no white."