Re: [Boost-users] [Serialization] Serializing classes defined inDLLs

From: Topi Mäenpää (topi.maenpaa_at_[hidden])
Date: 2005-09-22 04:22:34

I got the dynamic library stuff work for a while. But now I'm getting a 
segmentation fault with the following setup.

As before, I have class A defined in a dll, say A.dll. This is a library 
I link against in other dlls and in the main executable. I also have two 
other libraries, B.dll and C.dll that both include definitions for many 
classes derived from A. Let us call these B1, B2, ... and C1, C2 ..., 
respectively. I'm registering the classes in the dlls with 
BOOST_CLASS_EXPORT. B.dll and C.dll use the definition of A in A.dll so 
that only a single definition of A should be present. The main 
executable links against A.dll and loads B.dll and C.dll at run time.

With this setup, serialization goes fine if I only use classes defined 
in either of the dynamically loaded libraries:

// create_in_dynamic_library_{b,c} are function pointers to dll
// functions

A* b1 = (A*)create_in_dynamic_library_b("B1");
A* b2 = (A*)create_in_dynamic_library_b("B2");
A* c1 = (A*)create_in_dynamic_library_c("C1");
A* c2 = (A*)create_in_dynamic_library_c("C2");

ar & b1 & b2; //works fine
ar & c1 & c2; //works fine

ar & b1 & c1; //crashes
ar & c2 & b2; //crashes

In other words, I cannot serialize classes defined and exported in 
different dlls. Only the first one works. The segmentation fault is 
caused by line 213 in extended_type_info.cpp.

extended_type_info::type_info_key_cmp(const extended_type_info & rhs) 
const {
   if(type_info_key == rhs.type_info_key)  // <---- this == 0!
     return 0;

My debugger reports that "this" is zero. This happens just after 
boost::serialization::base_object<A>(*this) is called in the serialize 
member of class {B,C}{1...N}.

I'm stuck. Any suggestions?

-Topi-

Robert Ramey wrote:
> This problem has been reported by Martin Ecker.  The easy fix is to just 
> permit multiple definitions of the same virtual functions in different DLLs. 
> This has eliminated the symptom for this user.

I didn't quite get this. What do you mean by permitting multiple 
definitions? How is it done?

> I'm concerned that this won't really work in the general case.
> 
> I would be prefer to:
> a) trap attempts at multiple registrations of the same class as errors.
> b) explain how to avoid these situtations in the first place.
> 
> I'm kind of stuck on b).  It seems to me that if A (base) is in a DLL and 
> B(derived) is
> in an executable, there should be no multiple definitions of B.  But the 
> brief
> description of this user's case seems to suggest that this isn't the case - 
> though
> without more information one can't be sure.
> 
> Robert Ramey