Re: Stupid question: linking with a symbol under a different name

[Sasha Unknown <sasha2048 at gmail dot com>]

Thank you for informative answer.

I know about "objcopy --redefine-sym", it can't  be applied here.

The fact is that in general main.o may need to be linked with several 
*.so files. And these *.so files may have name conflicts (export 
different symbols under the same name). That's actually the main reason 
why linking with a symbols under different names seems to be must-have 
feature.

For example:

    I'm given main.o (with unresolved symbols: symbolA, symbolB, 
symbolC1, symbolC2, symbolD, symbolE).

    I'm given libused.o (with exported symbols: symbolA, symbolB, 
symbolC).

    I'm given libsecond.o (with exported symbols: symbolC, symbolD, 
symbolE).

    I'm given recipe:
      - map symbolA (from main.o) to symbolA (from libused.so) [names 
coincide];
      - map symbolB (from main.o) to symbolB (from libused.so) [names 
coincide];
      - map symbolC1 (from main.o) to symbolC (from libused.so) [names 
differ];
      - map symbolC2 (from main.o) to symbolC (but from libsecond.so) 
[names differ];
      - map symbolD (from main.o) to symbolD (from libsecond.so) [names 
coincide];
      - map symbolE (from main.o) to symbolE (from libsecond.so) [names 
coincide].
      [Note that libused.so and libsecond.so both export symbolC.]

So, applying "objcopy --redefine-sym" to main.o is possible, but it 
doesn't solve name conflict and thus doesn't help here. While applying 
"objcopy --redefine-sym" to *.so files isn't possible at all, because: 
(1) it would be different shared library, while linking to original is 
required; (2) objcopy doesn't work with dynamic symbols (it affects 
output of "objdump -t", not "objdump -T").

Anyway, thanks for your answer. It could be very helpful if libused and 
libsecond were static libraries (*.a), not shared (*.so). That's 
actually the way I use to solve the same problem with static libraries.

[Sorry for duplicate, first reply was accidentally sent to wrong address.]

On 08.10.16 17:23, Rayson Ho wrote:
> Sasha ,
>
> Did you try the objcopy command's "--redefine-sym" option??
>
> --redefine-sym old=new
>     Change the name of a symbol old, to new. This can be useful when
> one is trying link two things together for which you have no source,
> and there are name collisions
>
> See: https://sourceware.org/binutils/docs/binutils/objcopy.html
>
> Rayson
>
> ==================================================
> Open Grid Scheduler - The Official Open Source Grid Engine
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>
> On Sat, Oct 8, 2016 at 7:06 AM, Sasha Unknown <sasha2048@gmail.com> 
> wrote:
> > It's even more complex. I actually don't have any info about what that
> > symbol is and what convention it follows (except output of objdump, of
> > course).
> >
> > The full task is probably like that:
> >
> >      I'm given main.o (with some unresolved symbols).
> >
> >      I'm given libused.so (with some exported symbols).
> >
> >      I'm given recipe, like:
> >        - map symbol1 (from main.o) to symbol1 (from libused.so) [names
> > coincide];
> >        - map symbol2 (from main.o) to symbol2 (from libused.so) [names
> > coincide];
> >        - map symbol3 (from main.o) to symbol862 (from libused.so) [names
> > differ];
> >        - map symbol4 (from main.o) to symbol5837 (from libused.so) 
> > [names
> > differ];
> >        - map symbol5 (from main.o) to symbol5 (from libused.so) [names
> > coincide];
> >        - ...
> >
> > How?
> >
> > [Sorry for duplicate, first reply was accidentally sent to wrong 
> > address.]
> >
> > On 06.10.16 23:00, Sasha Unknown wrote:
> > > Hello.
> > >
> > > 1. This is possible only with function. If it's a variable, I can't 
> > > create
> > > "wrapper variable" for it. (E.g. if symbol2 is "int" variable, I of 
> > > course
> > > can create foo.c with "int * const symbol1 = &symbol2", but this 
> > > won't match
> > > symbol1 convention from main.o, which expects symbol1 to be "int", 
> > > not "int
> > > *".)
> > >
> > > 2. Even with function this gives some overhead (one additional call;
> > > AFAIK, wrapper function won't be inlined in this case).
> > >
> > > Thanks.
> > >
> > > On 06.10.16 15:58, Nick Clifton wrote:
> > > > Hi Sasha,
> > > >
> > > > > Suppose I have:
> > > > > - compiled object file "main.o" with unresolved symbol "symbol1";
> > > > > - build shared library "libused.so" with exported symbol "symbol2".
> > > > >
> > > > > Is there way to somehow link program "main" from "main.o", telling 
> > > > > it to
> > > > > use "symbol2" from "libused.so" as "symbol1"?
> > > > Why not create a foo.c file that defines symbol1 and then passes any
> > > > reference on to symbol2 ?
> > > >
> > > > Cheers
> > > >     Nick