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List:       freebsd-hackers
Subject:    Re: dlopen: resolving external library symbols to calling program
From:       Kostik Belousov <kostikbel () gmail ! com>
Date:       2007-11-30 20:19:48
Message-ID: 20071130201948.GL83121 () deviant ! kiev ! zoral ! com ! ua
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On Fri, Nov 30, 2007 at 04:40:33PM -0300, Alejandro Pulver wrote:
> On Fri, 30 Nov 2007 19:02:01 +0200
> Kostik Belousov <kostikbel@gmail.com> wrote:
> 
> > On Fri, Nov 30, 2007 at 01:28:58PM -0300, Alejandro Pulver wrote:
> > > Hello.
> > > 
> > > When I was updating the games/deng port, I found it failed at runtime
> > > with the following error:
> > > 
> > > % doomsday
> > > While opening dynamic library
> > > /usr/local/lib/libdropengl.so:
> > >   /usr/local/lib/libdropengl.so: Undefined symbol "ArgExists"
> > > DD_InitDGL: Loading of libdropengl.so failed.
> > >   (null).
> > > 
> > > The function is defined in m_args.c which is included in both
> > > "doomsday" and "libdropengl.so". But nm(1) reports it as undefined for
> > > "libdropengl.so". Also, it is loaded with RTLD_NOW.
> > > 
> > > % nm `which doomsday` | grep ArgExists
> > > 080d9ef0 T ArgExists
> > You are looking at the wrong symbol table. ELF objects have the dynamic
> > symbol table that is used during run-time linking, and symbol table used
> > by the static linker ld. The former table is shown by nm -D.
> > 
> > I suspect that you need to link the doomsday binary with the
> > --export-dynamic flag. See the info ld for details.
> > > 
> 
> It worked, thank you very much. I am reading some books that explain
> the basics of COFF/ELF formats (like Write Great Code Volume 2:
> Thinking Low-Level, Writing High-Level), but didn't know about the
> dynamic symbol table.
> 
> I found the following article which briefly describes it (though it's
> for Solaris):
> http://blogs.sun.com/ali/entry/inside_elf_symbol_tables
The ELF specification is freely available, and contains at least the
neccessary minimum of information. The object format has evolved
since then.

Sun' Linkers and Loaders Guide also contain a lot of useful information,
applicable to any ELF platform.
> 
> Now that I remember, the games/quakeforge port had the same problem.
> But someone fixed it by referencing the symbol (it was only one
> function) with a function pointer so it got exported in the dynamic
I remember it was me.

> table. In this case, could that be done with "-u symbol" when linking
> the executable, or it isn't possible to export a symbol with linker
> parameters?

I do not know of any such facility in GNU ld. You may limit the exported
symbols by using the version script. But, by default, no symbols are
exported for executable (for shared object, reverse is true, all symbols
are exported).

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