How to make the symbol to be resolved within the shared library ?

Question

I have two definitions of a symbol, one is in the main code, the other in a shared library. I want the symbol to be resolved within the shared library if it needed in the shared library.
From the 'C++ User�s Guide', use the option '-Bsymbolic' to build the shared library is a choice. But it also said 'Never use -Bsymbolic with programs containing C++ code, use linker map files instead. So I have to try to use the map file, but I did not get more useful information from 'Linker and Libraries Guide'.
I want to know what should be included in the map file and how to generate the map file?
Additionally, from the 'Linker and Libraries Guide', the 'Direct Binding' also could resolve this problem. And I tried to build the shared library by the '-B direct' option, it works.
Could anybody tell me what's the difference between the '-Bsymbolic' and '-B direct'?
Any comments are appreciated.

Answer 1

You didn't provide fun.h, and you didn't show all the commands that you must have used -- there are no commands that compile the source code into .o files. So I can't tell why the -xldscope option didn't work for you.
Problems and errors that I see in your example:
Functions func and swap are referenced from C code, but swap and one of the func functions are compiled by the C++ compiler and will get mangled names. If you compile a function with the C++ compiler that is to be called from C code, you need to declare the function as extern "C". But the extern "C" in a shared header needs to be guarded by +#ifdef __cplusplus+, since a C compiler won't understand the declaration.
When you build a shared library, you must explicitly list the libraries it depends on, since no dependencies are created by default. For a C++ library, you need at least -lCrun -lc in that order. Otherwise, in a real-world program you can have program failures due to incorrect library initialization order. To be sure you have listed all needed libraries, use the -zdefs option, which causes the linker to complain about missing definitions.
To be sure the shared library is picked up at run time, provide a RUNPATH via the -R option when you build the executable.
The -xldscope option must appear on compile commands for the files where you want it to take effect. It has no effect on a link command, because the option affects how code is generated.
Here is my rewrite:
% cat fun.h
#ifdef __cplusplus
extern "C"
#endif
void func();
#ifdef __cplusplus
extern "C"
#endif
void swap();
% cat fun.c
#include <stdio.h>
#include "fun.h"
extern "C" void func()
    printf("func in the shared lib\n");
extern "C" void swap()
    func();
% cat fun_in.c
#include <stdio.h>
#include "fun.h"
void func()
    printf("func in the main code\n");
% cat main.c
#include <stdio.h>
#include "fun.h"
int main()
    func();
    printf("invoke the swap:\n");
    swap();
% CC -G -xldscope=symbolic fun.c -Kpic -o liboutside.so -lCrun -lc -zdefs
% cc -o run main.c fun_in.c -L. -loutside -R .
% ./run
func in the main code
invoke the swap:
func in the shared libAs you can see, it works as advertised.
Please note that the -xldscope option affects all functions in the module. If you need more fine-grained control, you must add declarations to the source code, or use a mapfile instead of -xldscope .