Hi, Im trying to use the BOOST_CLASS_EXPORT_GUID, but if i use it, i cant link my project. if got these error messages: /gcc-4.1.2/debug/threading-multi/crabd.o:/usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../include/c++/4.1.2/exception:57: multiple definition of `boost::archive::detail::guid_initializer<water::status>::instance' bin/gcc-4.1.2/debug/threading-multi/seal.o:/usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../include/c++/4.1.2/exception:57: first defined here bin/gcc-4.1.2/debug/threading-multi/water.o:/usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../include/c++/4.1.2/exception:57: multiple definition of `boost::archive::detail::guid_initializer<water::status>::instance' bin/gcc-4.1.2/debug/threading-multi/seal.o:/usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../include/c++/4.1.2/exception:57: first defined here The file with the macros is just one time defined. Is there an other possible reason than multiple definition or a more hidden way to define it more than one time? Kind Regards Manuel Jung