Hi, Trying to port the code to V2.1, a number of redefinition errors get generated by the compiler. The first instance is the extern InterLockedIncrement variable at line 73 of the interlocked.hpp file. The code is segmented for different platforms using #if #elif and so on directives. The project is at the same time ported from Visual Studio 2008 to 2010 RC1, and the redefinition error is caused by compiler not excluding parts of that file. The define by the Visual Studio is the _WIN32_WINNT base on the use of the 32 bit development PC. This definition is not listed by the #elif chain within the interlocked.hpp code and it seems that BOOST_USE_WINDOWS_H is used instead. Knowing the segments are coded to be mutually exclusive, still the code generates errors? Regards, Reza Bemanian