From owner-freebsd-hackers  Tue Nov 17 08:39:04 1998
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Received: (from majordom@localhost)
          by hub.freebsd.org (8.8.8/8.8.8) id IAA02737
          for freebsd-hackers-outgoing; Tue, 17 Nov 1998 08:39:04 -0800 (PST)
          (envelope-from owner-freebsd-hackers@FreeBSD.ORG)
Received: from ns.mt.sri.com (sri-gw.MT.net [206.127.105.141])
          by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id IAA02732
          for <hackers@FreeBSD.ORG>; Tue, 17 Nov 1998 08:39:03 -0800 (PST)
          (envelope-from nate@mt.sri.com)
Received: from mt.sri.com (rocky.mt.sri.com [206.127.76.100])
	by ns.mt.sri.com (8.8.8/8.8.8) with SMTP id JAA22434;
	Tue, 17 Nov 1998 09:38:38 -0700 (MST)
	(envelope-from nate@rocky.mt.sri.com)
Received: by mt.sri.com (SMI-8.6/SMI-SVR4)
	id JAA24053; Tue, 17 Nov 1998 09:38:37 -0700
Date: Tue, 17 Nov 1998 09:38:37 -0700
Message-Id: <199811171638.JAA24053@mt.sri.com>
From: Nate Williams <nate@mt.sri.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
To: Thomas David Rivers <rivers@dignus.com>
Cc: hackers@FreeBSD.ORG, nate@mt.sri.com
Subject: Re: Wrapping a function
In-Reply-To: <199811171348.IAA11670@lakes.dignus.com>
References: <199811170517.WAA22627@mt.sri.com>
	<199811171348.IAA11670@lakes.dignus.com>
X-Mailer: VM 6.34 under 19.16 "Lille" XEmacs Lucid
Sender: owner-freebsd-hackers@FreeBSD.ORG
Precedence: bulk
X-Loop: FreeBSD.ORG

> > Does anyone have an easy way of 'wrapping' an already existing library
> > function so that any programs linked against your .o will call your
> > function, but so your function can call the 'real' library function?
> > 
> > Example:
> > 
> > my_malloc.c:
> > 
> > void *malloc(size_t size)
> > {
> >     void *ret;
> > 
> >     printf("Calling malloc\n");
> >     ret = REALMALLOC(size);
> >     printf("Leaving malloc\n");
> >     return ret;
> > }
> > 
> 
>  I don't know about our linker (gnu-ld) - but many UNIX linkers (and other
> systems as well) have a way to re-name identifiers.
> 
>  The standard trick would be to pre-link the malloc code from the library;
> renaming the malloc entry there - to say, MALLOC.  (ld -r is how you 
> prelink.)
> 
>  Then, you can link your code with that.

Unfortunately, the symbol exists in a shlib, so it must use the same
symbol name as the library since sometimes the shlib exists, and other
times it doesn't. :(

(Don't ask, it's not my code but the JDK.  They get around this problem
in Solaris by being aware that dlsym happens to be a simple C function
that calls _dlsym, so in the JDK they call _dlsym directly and hope
no-one else is aware of this feature.)

Sounds like I'll just not be able to wrap those functions. :(


Nate

To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message