Access main program from shared library in C - c

Question

After some googling, I got shared libraries working quite well. I can open my module with dlopen and load functions with dlsym. However, this is always running functions from the library in the program, how do I use functions from my program in the library?
My simplified code:
main.c
#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
int twice(int a){
return a*2;
}
int main(int argc, char ** argv){
char *plugin_name;
void *plugin;
int (*init)();
if(argc<2){
printf("give lib name\n");
return 0;
}
plugin = dlopen (argv[1], RTLD_NOW);
if (!plugin)
{
printf("Cannot load %s: %s\n",argv[1], dlerror ());
return 0;
}
init = dlsym (plugin, "init");
char * result = dlerror ();
if (result)
{
printf("Cannot find init in %s: %s\n", plugin_name, result);
return 0;
}
int res = init();
printf("result: %i\n",res);
return 0;
}
plugin.c
int init()
{
return twice(4);
}
Makefile:
all: main lib
main: main.c
gcc -ldl -o main main.c
lib: plugin.c
gcc -shared -o plugin.so plugin.c
And I run it with:
./main ./plugin.so

Answer 1

With Linux, glibc, and GNU ld you can run -rdynamic to your executable link flags. This will make all symbols in your executable visible to loadable modules, as if it were a shared library.
Note that this is not necessarily portable to other Unix-like systems.