Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ihnp4!homxb!mhuxt!mhuxm!mhuxo!ulysses!allegra!mit-eddie!husc6!think!ames!aurora!labrea!decwrl!decvax!ima!minya!jc
From: jc@minya.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: Aliasing text and data segments of a process
Keywords: Ultrix
Message-ID: <453@minya.UUCP>
Date: 23 Jan 88 15:00:40 GMT
References: <202@sdti.UUCP> <3106@super.upenn.edu>
Distribution: na
Organization: home
Lines: 42


You know, it occurred to me that the I-space vs D-space just might
be a problem, so I looked in the manuals for this machine (which
might as well remain incognito).  Sure enough, there is supposedly
separate address spaces for both, as well as for IO-space.  So I
modified the little program I posted earlier:

| #include <stdio.h>
| extern char *malloc();
| char *code;		/* This will point into D-space */
| int (*fct)();	/* So will this */
| 
| foobar(x,y)
| {	printf("foobar(%d,%d)\n",x,y);
| 	return x + y;
| }
| main()
| {	int i;
| 	char *p, *q;
| 
| 	code = (char*)malloc(1000);
| 	p = code;
| 	q = (char*)foobar; 
| 	while (p < code+1000)	/* Make copy of foobar() in code[] */
| 		*p++ = *q++;
| 	fct = (int(*)())code;	/* Point to the malloc()ed data area */
| 	i = (*fct)(7,9);	/* Call the copy */
| 	printf("(*fct)(7,9)=%d\n",i);
| 	exit(0);
| }

It compiled and linked without problems, so with a bit of trepidation
I told Unix (Sys/V) to run it, and guess what it said? Give up?  OK,
here's the output:

| foobar(7,9)
| (*fct)(7,9)=16

Seems like it worked, didn't it?  Clever people, those C compiler writers!

-- 
John Chambers <{adelie,ima,maynard,mit-eddie}!minya!{jc,root}> (617/484-6393)