The Cygwin environment is special in that it supports both the native Windows API's and Linux/POSIX emulation API's. The default for Boost.Filesystem builds is to treat Cygwin environment as native Windows. Some Cygwin users would prefer the Linux/POSIX emulation. Boost.Filesystem has now been tested in both Cygwin environments. No changes were required in the 1.31.0 or current CVS head implementation code. Two of the test programs have been changed in CVS to make testing both Cygwin environments easier. The docs in CVS have been updated with the following "Notes for Cygwin users": The library's implementation code automatically detects the current platform, and compiles the POSIX or Windows implementation code accordingly. Automatic platform detection during object library compilation can be overridden by defining BOOST_POSIX or BOOST_WINDOWS macros. With the exception of the Cygwin environment, there is usually no reason to define one of the macros, as the software development kits supplied with most compilers only support a single platform. The Cygwin package of tools supports traditional Windows usage, but also provides an emulation layer and other tools which can be used to make Windows act as Linux (and thus POSIX), and provide the Linux look-and-feel. GCC is usually the compiler of choice in this environment, as it can be installed via the Cygwin install process. Other compilers can also use the Cygwin emulation of POSIX, at least in theory. Those wishing to use the Cygwin POSIX emulation layer should define the BOOST_POSIX macro when compiling the Boost Filesystem Library's object-library. The macro does not need to be defined (and will have no effect if defined) for Boost Filesystem Library user programs. --Beman