ldd, shared libs, overriding functions, and undefined symbols

[Duane Ellis]

Short question:

   How do I get a shared library to resolve to symbols - that are
   defined in the application at run time - that did not need to
   resolve at link time and visa-versa.

   This works under SunOS 4.1.3 - but not under Linux.

   It does not work under RH Linux 7.3 intel using the
   default compiler linker and libraries.

* I have duplicated this problem with a couple of trivial files - see
  below for an explination.

* If somebody would like a more complete example I can forward a small
  makefile and short c files that demonstrate the problem.

-Duane.

I have a simple program - the main application it includes a call back
that the shared library *MIGHT* but not always need.

>> ----- main.c
>>        #include <stdio.h>
>>        int 
>>        main( int argc, char **argv )
>>        {
>>              custom_lib_init();
>>              printf("This is my program\n");
>>        }
>> 
>>        void
>>        some_call_back_function( void )
>>        {
>>              printf("The library called the call back\n");
>>        }
>> ----

I create a STUB library - that is used to link against for normal
operation of the program.

>> ---- sublib.c
>>    #include <stdio.h>
>> 
>>     void
>>     custom_lib_init(void)
>>     {
>>        printf("This is a stub - nothing to do\n");
>>        /* Stubs don't use the call back function */
>>     }
>> ----

When I normally build the application - I link against the STUB shared
library.  Everything works.

NOW - I build a full-featured shared library that does something more
in this example - i just call the call back function.

>> ---- fulllib.c
>>    #include <stdio.h>
>> 
>>     void
>>     custom_lib_init(void)
>>     {
>>        printf("This is *NOT* the stub, call the callback. \n");
>>        some_call_back_function();
>>     }
>> ----


OPTION 0:
       LINK with the stub library
       RUN  with the stub library.
       
       -- It works.

OPTION 1:
       LINK with the full library.
       RUN  with the full library.

       -- It works.

OPTION 2:
       LINK with the stub library
       RUN  with the full library.

or
OPTION 3:
       LINK with the full library
       RUN  with the stub library.

       Both *FAIL* and this is my problem.

I get a message like this:

>> ./test_prog: relocation error: ./libQA.so.1.0: undefined symbol: 
>> some_call_back_function

I am not expecting this. 

The main program only requires the "custom_lib_init()" function in the
library - it is defined at run time and already exists.

ld.so should be able to resolve the call back to the main application.
but for some reason it cannot.

I have tried things like LD_PRELOAD and LD_LIBRARY_PATH from
the ld.so man page and I'm getting no where.

I can also duplicate the problem with 'ldd -r test_prog'
in the same manner as running the program.

Again - this basic functionality works under SunOS 4.1 - using gcc and
the standard sun linker.

I'm at a loss to figure out what I have to do here.

Switching to a 100% dlopen(), dlsym(), dlclose() method - where I look
up the symbols is very *NON* trivial - I need access to hundreds of
functions and variables.

Any suggestions or pointers would be very helpful.

-Duane.