Programming

I load some dynamic libraries from main module (with dlopen). These libraries use 1 function from main module, therefore in Makefile I must use

gcc -g -Wl,--export-dynamic,-rpath,./lib -o not not.o db.o -ldl -ldb -lpcap

Note option --export-dynamic that is passed to the ELF linker.

The question is how can I determine which symbols must be put to the dynamic symbol table. I do not want to put all symbols, but putting only used once the function called by dynamically loaded libraries is not visible for these libraries.

If I understand your question:
dynamic loading means you know the symbol names and the libraries names ahead of time - for the all the entrypoints you are going to map dynamically by calling dlopen yourself...

Right?

Code:

nm myexcutablefile

will list all of the external symbols your program uses. There will be a lot of them that are part of __start and the general C support runtime. Your special symbols need to be mapped by a call to dlopen.

Am I on track so far?

This is not what I meant
Suppose I have 2 files: main.c, lib.c
Function func1() is defined in main.c, and lib.so is loaded from main.c: dlopen("lib.so", RTLD_LAZY)
Then func1() is called from lib.c. Becouse of this I need to put all symbols to the dynamic symbol table:
gcc -g -Wl,--export-dynamic,-rpath,. -o main main.o -ldl
How can put only func1() to the dynamic symbol table (omitting other symbols)?

Sorry. I did not understand.

Since you are in a GNU development environment do you know about libltdl?
This allows you to handle what you are doing. See:
http://www.gnu.org/software/libtool/...#Using-libltdl

If you don't like that - what you have to do in code: do not directly reference the module you want by name, this is how you "hide" it from ld.

Example:

Code:

#include <stdlib.h>
#include <stdio.h>
#include <dlfcn.h>

int main(int argc, char **argv) {
    void *libhandle=NULL;
    double (*myfunc)(double);
    char *errormsg=NULL;

    libhandle = dlopen ("/lib/libwhatever.so.1", RTLD_LAZY);
    if (!libhandle) {
        fputs (dlerror(), stderr);
        exit(EXIT_FAILURE);
    }

    myfunc = dlsym(libhandle, "myfunction");
    if ((errormsg = dlerror()) != NULL)  {
        fputs(errormsg, stderr);
        exit(EXIT_FAILURE);
    }

    printf ("%f\n", (*myfunc)(1.0));
    dlclose(libhandle);
    return 0;
}

You cannot hide fully qualified symbols from ld - it will try to link against each one, or generate an error.

Again, this is what I think you are saying: you have not "hidden" all the symbols. Correct me if I'm wrong. You can't play mix-n-match, some hidden, some not.

Tanks a lot for a reply
I've solve my problem by moving the 'visible' functions (func1() in above example) from main module to another library that is linked at runtime