Since I observed some strange behavior of global variables in my dynamically loaded libraries, I wrote the following test.

At first we need a statically linked library: The header test.hpp

#ifndef __BASE_HPP
#define __BASE_HPP

#include <iostream>

class test {
private:
  int value;
public:
  test(int value) : value(value) {
    std::cout << "test::test(int) : value = " << value << std::endl;
  }

  ~test() {
    std::cout << "test::~test() : value = " << value << std::endl;
  }

  int get_value() const { return value; }
  void set_value(int new_value) { value = new_value; }
};

extern test global_test;

#endif // __BASE_HPP

and the source test.cpp

#include "base.hpp"

test global_test = test(1);

Then I wrote a dynamically loaded library: library.cpp

#include "base.hpp"

extern "C" {
  test* get_global_test() { return &global_test; }
}

and a client program loading this library: client.cpp

#include <iostream>
#include <dlfcn.h>
#include "base.hpp"

typedef test* get_global_test_t();

int main() {
  global_test.set_value(2); // global_test from libbase.a
  std::cout << "client:        " << global_test.get_value() << std::endl;

  void* handle = dlopen("./liblibrary.so", RTLD_LAZY);
  if (handle == NULL) {
    std::cout << dlerror() << std::endl;
    return 1;
  }

  get_global_test_t* get_global_test = NULL;
  void* func = dlsym(handle, "get_global_test");
  if (func == NULL) {
    std::cout << dlerror() << std::endl;
    return 1;
  } else get_global_test = reinterpret_cast<get_global_test_t*>(func);

  test* t = get_global_test(); // global_test from liblibrary.so
  std::cout << "liblibrary.so: " << t->get_value() << std::endl;
  std::cout << "client:        " << global_test.get_value() << std::endl;

  dlclose(handle);
  return 0;
}

Now I compile the statically loaded library with

g++ -Wall -g -c base.cpp
ar rcs libbase.a base.o

the dynamically loaded library

g++ -Wall -g -fPIC -shared library.cpp libbase.a -o liblibrary.so

and the client

g++ -Wall -g -ldl client.cpp libbase.a -o client 

Now I observe: The client and the dynamically loaded library possess a different version of the variable global_test. But in my project I'm using cmake. The build script looks like this:

CMAKE_MINIMUM_REQUIRED(VERSION 2.6)
PROJECT(globaltest)

ADD_LIBRARY(base STATIC base.cpp)

ADD_LIBRARY(library MODULE library.cpp)
TARGET_LINK_LIBRARIES(library base)

ADD_EXECUTABLE(client client.cpp)
TARGET_LINK_LIBRARIES(client base dl)

analyzing the created makefiles I found that cmake builds the client with

g++ -Wall -g -ldl -rdynamic client.cpp libbase.a -o client

This ends up in a slightly different but fatal behavior: The global_test of the client and the dynamically loaded library are the same but will be destroyed two times at the end of the program.

Am I using cmake in a wrong way? Is it possible that the client and the dynamically loaded library use the same global_test but without this double destruction problem?