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Re: dlopen from dlopend library question


From: Gleb Natapov
Subject: Re: dlopen from dlopend library question
Date: Sun, 7 Aug 2005 12:31:54 +0300

On Tue, Aug 02, 2005 at 03:44:05PM +0200, Ralf Wildenhues wrote:
> * Gleb Natapov wrote on Mon, Aug 01, 2005 at 04:58:03PM CEST:
> > On Mon, Aug 01, 2005 at 03:42:17PM +0200, Ralf Wildenhues wrote:
> > > * Gleb Natapov wrote on Sun, Jul 17, 2005 at 11:39:34AM CEST:
> > > > 
> > > > I have library (lets say liba.so) that on startup checks available 
> > > > hardware and loads appropriate plugin (say plugin.so) to drive the 
> > > > hardware.
> > > > plugin.so uses symbols from liba.so. 
> > > > 
> > > > Everything works OK if liba.so linked with application, but if
> > > > application is modular by itself and use module (say module.so) that
> > > > is linked with liba.so then dlopen of plugin.so from liba.so fails with
> > > > unresolved symbols.
> > > > 
> > > > How can I solve this issue?
> > > 
> > > If you use Libtool/libltdl for portable modules (which you do not seem
> > > to do), look at Libtool branch-2-0 or CVS HEAD, which allows ltdlopen'ing
> > > from libraries.  Make sure to read the corresponding documentation
> > > pertaining to libltdl, make use of `-module' and `-export-dynamic'.
> > > 
> > The project I saw this problem with actually use libtool (not 2.0
> > though). And both flags are used.
> 
> OK.  Well, for portable use of modules from within a library you should
> use either branch-2-0 or HEAD.
> 
> > After investigation of the problem I found that liba.so tries to load
> > plugin.so from library constructor. Even if liba.so is dlopen with 
> > RTLD_GLOBAL flag set, plugin.so can't resolve symbols from liba.so
> 
> Just so we don't misunderstand each other: what exactly do you mean by
> library constructor?  Are you overwriting DT_INIT?  (If so, you are a far
> way from portable shared libraries.)
I am talking about function marked __attribute__((constructor)).
Portability is not an issue for me.

> 
> > Let me explain this one more time.
> > 
> > Program 'prog' calls dlopen(liba.so, RTLD_GLOBAL), in constructor liba.so
> > calls dlopen(plugin.so) and this call fails with unresolved symbols
> > from liba.so.
> > 
> > If we add function init_a() to liba.so and move dlopen(plugin.so) from 
> > constructor to the function and we call this function from 'prog' after
> > dlopen (liba.so, RTLD_GLOBAL) then everything works as expected.
> 
> Best would be if you showed short reproducible code.  Not that I know
> whether I could help you then.
>
Attached. To compile run following:
$ gcc liba.c -shared -o liba.so
$ gcc plugin.c -shared -o plugin.so
$ gcc prog.c -ldl -o prog

Run:
$ ./prog
Load of plugin.so failed

Notice that second dlopen of plugin.so succeeds.

--
                        Gleb.

Attachment: liba.c
Description: Text Data

Attachment: plugin.c
Description: Text Data

Attachment: prog.c
Description: Text Data


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