having object file symbols become dynamic symbols in executable

Question

I have several object files (from C++) that are spat out of a build system. They have several extern "C"-linkage symbols I want to use in a program and have accessible via dlopen/dlsym from elsewhere.

When compiled into an executable with gcc, these symbols are not listed using nm -D <executable-here> (i.e. afaik they're not dynamic symbols).

How do I get them to become exposed as dynamic symbols within the compiled executable?

I can change the build flags for the object files and for the executable, but changing how the C++ files end up in the final executable (i.e. not making them into object files first) is difficult.

(GCC 4.8, ld 2.24)

EDIT: I came across this question, which may or may not be what I'm asking, but I'm not entirely sure. Use dlsym on a static binary

Answer 1

You may like to take a look at --export-dynamic ld option:

   -E
   --export-dynamic
   --no-export-dynamic
       When creating a dynamically linked executable, using the -E option
       or the --export-dynamic option causes the linker to add all symbols
       to the dynamic symbol table.  The dynamic symbol table is the set
       of symbols which are visible from dynamic objects at run time.

       If you do not use either of these options (or use the
       --no-export-dynamic option to restore the default behavior), the
       dynamic symbol table will normally contain only those symbols which
       are referenced by some dynamic object mentioned in the link.

       If you use "dlopen" to load a dynamic object which needs to refer
       back to the symbols defined by the program, rather than some other
       dynamic object, then you will probably need to use this option when
       linking the program itself.

       You can also use the dynamic list to control what symbols should be
       added to the dynamic symbol table if the output format supports it.
       See the description of --dynamic-list.

       Note that this option is specific to ELF targeted ports.  PE
       targets support a similar function to export all symbols from a DLL
       or EXE; see the description of --export-all-symbols below.

Also, if none of the objects during link refers to your extern symbols, you may like to put them into --dynamic-list to make sure they are exported.

---

Example:

$ cat test.cc
#include <stdio.h>

int main() {
    printf("Hello, world\n");
}

extern "C" void export_this() {
    printf("Hello, world from export_this\n");
}

$ g++ -o test -W{all,extra} -Wl,--export-dynamic test.cc

$ ./test
Hello, world

$ nm --dynamic test | grep export_this
00000000004007f5 T export_this # <---- here you go