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From: bzs@bu-cs.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: Dynamic function loading
Message-ID: <6448@bu-cs.BU.EDU>
Date: Sun, 12-Apr-87 16:34:48 EST
Article-I.D.: bu-cs.6448
Posted: Sun Apr 12 16:34:48 1987
Date-Received: Mon, 13-Apr-87 23:48:44 EST
References: <600@rdin.UUCP>
Organization: Boston U. Comp. Sci.
Lines: 55
In-reply-to: perl@rdin.UUCP's message of 10 Apr 87 18:35:08 GMT
Posting-Front-End: GNU Emacs 18.41.4 of Mon Mar 23 1987 on bu-cs (berkeley-unix)


>I can dig that you use "ld -A -T" to create a file
>that can be loaded into a specific location within a running process,
>but what do you do to do the actual loading?  Exec? What strange
>arguments or calling sequences do I have to know about?

No, not exec, wrong thought, bzzzt.

You'll need to know the symbol you wish to link to (eg. the function
in the file to be loaded.)

Then grovel the sizes out of the header (see a.out.h) from the object
deck and the offsets.

Allocate the memory for the mess, toss the object into this (after
having been relocated with ld of course.)

Then make the "link". You will need to associate the code with a
function pointer, depends on how your program plans to notice that it
is going to use this stuff.

For example, say you had a jump table:

struct jtable {
	char *fnname;
	int (*fnptr)();
} jtable[MAXFNS];

then called via funcall("_dynsubr"), funcall would look something like:

	funcall(name) char *name;
	{
		struct jtable *jp;

		for(jp = jtable; jp < &jtable[MAXFNS]; jp++)
			if(strcmp(jp->fnname,name) == 0) { /* found it */
				if(jp->fnptr == NULL) /* not loaded yet */
				    if((jp->fnptr = funload(name)) == NULL)
					return(CANTLOADERROR);
				return((*jp->fnptr)());
		return(NOTFOUNDERROR);
	}

Obviously you would adjust the code above to allow for args to the function
and return values but it should give you the flavor (also, a linear search
is unnecessary if you need something faster.)

The code for funload() is too hairy to throw in off the top of my head
but groveling the stuff I mentioned above should steer you in the right
direction.

Or, I could send you my consulting schedules :-) It's not that hard, but
it takes some attention to detail.

	-Barry Shein, Boston University