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Note for Windows users: It is highly recommended to declare the Windows OS version you are targetting when building the library as well as when using it. You can do so thanks to the well known Microsoft macros WINVER, _WIN32_WINDOWS or _WIN32_WINNT, see your platform SDK documentation for a description of those macros and how to use them. To define it when building the library, use the configure script --extra-cxxflag option. Here is the configuration to build STLport using Visual Studio 2005 and targetting Windows XP: configure -c msvc8 --extra-cxxflag "/D_WIN32_WINNT=0x0501" If you do not declare it at build time STLport will adapt to the PSDK in use, windows.h gives a default value to WINVER. However, when using the library, windows.h is not necessarily included, at least not by STLport internally, so none of the macros are defined which will result in an inconsistency in the build process which most of time will generate undefined behavior at runtime. Here is the main reason for following this advise, the Windows 95 compatibility issue: Because of a modification in the behavior of the Win32 API functions InterlockedIncrement and InterlockedDecrement after Windows 95, STLport libraries built for Windows 95 cannot be used to generate an application built for Windows XP for instance. So, if you build STLport with a Windows 95 PSDK, STLport will be ready for Windows 95. If you then use it without defining _WIN32_WINDOWS to signal Windows 95 compatibility, STLport will consider that it can use latest Windows OS features like the new InterlockedIncrement and InterlockedDecrement functions which change the memory footprint of some internal STLport objects making it incompatible with the libraries built for Windows 95. Normally, doing so wouldn't generate any compilation or link error, you would only experiment undefined behavior at runtime. In order to make this problem more obvious STLport forces a link time error in debug mode (_DEBUG macro defined). Unresolved symbol will be: - 'building_for_at_least_windows98_but_library_built_for_windows95' if you are trying to use STLport libraries built for Windows 98 or later to generate an application targetting the Windows 95 platform. - 'building_for_windows95_but_library_built_for_at_least_windows98' if you are trying to use STLport libraries built for Windows 95 to generate an appliation targetting the Windows XP platform for instance. Of course, targetting the latest Windows OS versions will give you the best performance from STLport. This is why when none of the platform macros are defined STLport consider that there is no minimum OS requirement and will use the latest API functions. There is only one exception to this behavior, the SwitchToThread function that is used only if you define _WIN32_WINNT to a value higher or equal to 0X0400.