Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!umd5!umbc3!alex
From: alex@umbc3.UMD.EDU (Alex S. Crain)
Newsgroups: comp.unix.wizards
Subject: Re: Aliasing text and data segments of a process
Message-ID: <730@umbc3.UMD.EDU>
Date: 21 Jan 88 05:36:23 GMT
References: <202@sdti.UUCP>
Reply-To: alex@umbc3.UMD.EDU (Alex S. Crain)
Organization: University of Maryland, Baltimore County
Lines: 29

In article <202@sdti.UUCP> mjy@sdti.UUCP (Michael J. Young) writes:
>
>Is there a way in Unix to create an "alias" between the text and data
>segments of a process?  More specifically, how does one go about executing a
>block of code that was generated in a data segment?

*** SYSTEM 5 ****

Koyto Common Lisp does this when it loads object code. The system builds
object files where the first symbol in the text segment is a function that
knows about all the other symbols in the file. There is an external loader
that makes a copy of the .o file and resolves all external symbols against
the lisp executable's symbol table. Lisp allocates space with brk(), and
loads the .o file as data, and then branches to the start of the text area
of the .o file, assuming that there is a function there that will put the 
rest of the symbols on the common obstack.

Boy, do things get weird when the .o file is corrupted :-).

But to answer the question, Nothing. That is, lisp doesn't do anything 
special to accomplish this, it just works. There is a short file that 
demonstrates this behavior, which I can send you if you like.



-- 
					:alex.

alex@umbc3.umd.edu