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From: marick@ccvaxa.UUCP
Newsgroups: net.emacs
Subject: Re: gnu design question
Message-ID: <11600009@ccvaxa>
Date: Sun, 5-Jan-86 21:36:00 EST
Article-I.D.: ccvaxa.11600009
Posted: Sun Jan  5 21:36:00 1986
Date-Received: Tue, 7-Jan-86 03:48:06 EST
References: <21@cornell.UUCP>
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Nf-ID: #R:cornell.UUCP:21:ccvaxa:11600009:000:2017
Nf-From: ccvaxa.UUCP!marick    Jan  5 20:36:00 1986


Setjmp/longjmp work just fine on Gould machines; only alloca() is missing.

I haven't looked at unexec in gnumacs, but I've written a rather large program
that does something similar for much the same reason.  It might be nice for 
unexec to be a library routine, but in my experience those people who use it 
all need to do something slightly different -- you need an example, not a 
library routine.

I can't provide a full example, but I can tell you how to write an unexec-like
thing for ZMAGIC files:

1.  Find the old executable -- you'll need it for the symbol table.
    The Franz Lisp gstab() routine is an example of what you need to do.

2.  Unlink the destination file (so creat() doesn't reuse the modes).

3.  Fetch the old header (out of the old executable or starting at the first
    word of this process's text space).

4.  Fill in the new header -- note that both header.a_data and header.a_nbdata
    must be updated if you've used sbrk().

5.  Write the new header out.  Its size in bytes is ZTXTOFF (from a.out.h).

6.  Write the text out, starting at header.a_txbase + ZTXTOFF and going for
    header.a_text-ZTXTOFF bytes.

7.  Write the data out.  header.a_text is rounded up to a multiple of pages,
    so you don't have to.

8.  Copy the symbol and symbol tables from the old executable 
    to the new one.  The symbol table starts at a_text + a_data in the old
    executable. 

The only problem I know of is a bug in signal handling.  In (at least some
distributions of) UTX, sigvec breaks when called "again" from a saved process
image.  See sigvec.s -- you will need to do a setsigc system call to get
around the problem.  This bug will be (has been?) fixed in later versions of
UTX.

Hope this helps someone.  Usual disclaimers about opinions apply; further, 
I've been known to make mistakes both writing and reading code, so anything
I wrote above could be wrong.


Brian Marick, Wombat Consort
Gould Computer Systems -- Urbana
...ihnp4!uiucdcs!ccvaxa!marick
ARPA:  Marick@GSWD-VMS