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Path: utzoo!mnetor!seismo!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.unix.wizards,comp.unix.questions
Subject: Re: ld and the -A option
Message-ID: <4688@mimsy.UUCP>
Date: Mon, 8-Dec-86 04:41:39 EST
Article-I.D.: mimsy.4688
Posted: Mon Dec  8 04:41:39 1986
Date-Received: Mon, 8-Dec-86 05:41:31 EST
References: <2223@bu-cs.bu-cs.BU.EDU> <591@rdin.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 39
Xref: mnetor comp.unix.wizards:287 comp.unix.questions:284

In article <591@rdin.UUCP> perl@rdin.UUCP (Robert Perlberg) writes:
>...  Just how do you load the code into a running process and how do
>you know where to load it and where the various functions within it
>start?

Every one of those questions has an answer.  The answer is different
on every machine.

On a Vax running 4BSD, load the code by reading it into a data area
that is aligned on a `page' (1K byte, or getpagesize()) boundary.
The function entry points may be found by using nlist(3) on the
ld- generated symbol table.  `ld' should be invoked with the -T
option to set the text address for the new code to start at the
address into which you will read it.  There is one small problem
here: to run ld, you need the address; to get the address, you need
the size of the code before you can call valloc(3).  One solution
is to use sbrk() directly:

		int page_size, page_offset;
		long addr;
		char txtaddr[30];
		...
		page_size = getpagesize();
		page_offset = page_size - 1;
		addr = (long) sbrk(0);
		/* align to a page boundary */
		(void) sbrk((page_size - addr) & page_offset);
		addr = (long) sbrk(0);
		sprintf(txtaddr, "%x", addr);
		...			/* run ld "-T" txtaddr ... */
		/* read a.out header, obtaining `size' */
		if (sbrk(size) != addr)
			/* trouble */
		/* seek to code */
		if (read(fd, (char *) addr, size) != size)
			/* more trouble */
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris@mimsy.umd.edu