dynamic_cast fails when used with dlopen/dlsym

Question

Intro

Let me apologise upfront for the long question. It is as short as I could make it, which is, unfortunately, not very short.

Setup

I have defined two interfaces, A and B:

class A // An interface
{
public:
  virtual ~A() {}

  virtual void whatever_A()=0;
};

class B // Another interface
{
public:
  virtual ~B() {}

  virtual void whatever_B()=0;
};

Then, I have a shared library "testc" constructing objects of class C, implementing both A and B, and then passing out pointers to their A-interface:

class C: public A, public B
{
public:
  C();
  ~C();

  virtual void whatever_A();
  virtual void whatever_B();
};

A* create()
{
  return new C();
}

Finally, I have a second shared library "testd", which takes a A* as input, and tries to cast it to a B*, using dynamic_cast

void process(A* a)
{
  B* b = dynamic_cast<B*>(a);
  if(b)
    b->whatever_B();
  else
    printf("Failed!\n");
}

Finally, I have main application, passing A*'s between the libraries:

A* a = create();
process(a);

Question

If I build my main application, linking against the 'testc' and 'testd' libraries, everything works as expected. If, however, I modify the main application to not link against 'testc' and 'testd', but instead load them at runtime using dlopen/dlsym, then the dynamic_cast fails.

I do not understand why. Any clues?

Additional information

• Tested with gcc 4.4.1, libc6 2.10.1 (Ubuntu 9.10)
• Example code available

Answer 1

In general, gcc does not support RTTI across dlopen boundaries. I have personal experience with this messing up try/catch, but your problem looks like more of the same. Sadly, I'm afraid that you need to stick to simple stuff across dlopen.

Answer 2

I found the answer to my question here. As I understand it, I need to make the typeinfo available in 'testc' available to the library 'testd'. To do this when using dlopen(), two extra things need to be done:

• When linking the library, pass the linker the -E option, to make sure it exports all symbols to the executable, not just the ones that are unresolved in it (because there are none)
• When loading the library with dlopen(), add the RTLD_GLOBAL option, to make sure symbols exported by testc are also available to testd

Answer 3

I have to add to this question since I encountered this problem as well.

Even when providing -Wl,-E and using RTLD_GLOBAL, the dynamic_casts still failed. However, passing -Wl,-E in the actual application's linkage as well and not only in the library seem to have fixed it.