According to http://www.boost.org/doc/libs/1_36_0/libs/filesystem/doc/index.htm#Cgywin Boost.Filesystem seems to be supported on Cygwin. However with the latest version 1.36.0 (which I've built with BOOST_POSIX_API defined) I don't seem to get any Boost.Filesystem function to work on Cygwin. I'm currently running some test cases and had to modify them for Cygwin like this: #if defined(__CYGWIN__) BOOST_CHECK(a == b); #else BOOST_CHECK(bfs::equivalent(a, b)); #endif If a and b contain the very same path you would expect both a == b and boost::filesystem::equivalent() to return true. However on Cygwin boost::filesystem::equivalent() returns false. If I use BOOST_CHECK(bfs::equivalent(a, b)) on Cygwin I get this error: unknown location(0): fatal error in "test_find_param": std::runtime_error: boost::filesystem::equivalent: No such file or directory: "\home\Boris\boost_1_36_0\libs\process\test\..\..\..\bin.v2\libs\process\test\gcc-3.4.4\debug\link-static\runtime-link-static\helpers.exe", "\home\Boris\boost_1_36_0\libs\process\test\..\..\..\bin.v2\libs\process\test\gcc-3.4.4\debug\link-static\runtime-link-static\helpers.exe" Both strings are exactly the same and contain valid Cygwin paths. Is Boost.Filesystem broken on Cygwin? Boris PS: I've a similar problem with boost::filesystem::create_directory() which can't create a directory for no obvious reason.