Hi, The thread_data_base constructor is defined in a header file. Function make_external_thread_data() is defined in a .cpp file. This can lead to a situation when different libraries contain the definitions of these functions. The example below explains how this can happen and why this causes problems. The new version of struct thread_data_base contains an std::map member. The size of an std::map object depends on if libboost_thread.so was build with or w/o _GLIBCXX_DEBUG. libboost_thread.so contains make_external_thread_data(). This function allocates an instance of thread_data_base. Due to an std::map member the size of the allocated object depends on _GLIBCXX_DEBUG. There is a library (e.g. libuser.so) which uses the Boost.Thread library. Since the thread_data_base constructor is defined in a header file libuser.so can contain its definition. Again, the size of thread_data_base object, that the thread_data_base constructor from libuser.so expects, depends on if libuser.so was built with _GLIBCXX_DEBUG defined or not. An application cannot link agains both libboost_thread.so and libuser.so if these libraries were built with different _GLIBCXX_DEBUG definitions (or different alignment, etc). While loading the application the dynamic linker will use the definition of thread_data_base constructor either from libuser.so be or from libboost_thread.so. Whichever it finds first. Can both the thread_data_base constructor and make_external_thread_data() reside either in a .cpp file or in a header? BR, Dmitry