Hello, On Windows platform, I noticed that how to treat boost::file_descriptor and std::fstream's openmode was different. Here's the test code: (I tested on VC++8.0 with Boost 1.33.1) #include <boost/test/unit_test.hpp> #include <boost/iostreams/detail/ios.hpp> #include <windows.h> DWORD make_create_flags(BOOST_IOS::openmode m) { // copy from file_descriptor::open() DWORD dwDesiredAccess; DWORD dwCreationDisposition; if ( (m & (BOOST_IOS::in | BOOST_IOS::out)) == (BOOST_IOS::in | BOOST_IOS::out) ) { assert(!(m & BOOST_IOS::app)); dwDesiredAccess = GENERIC_READ | GENERIC_WRITE; dwCreationDisposition = (m & BOOST_IOS::trunc) ? #ifndef FIX OPEN_ALWAYS : #else CREATE_ALWAYS : #endif OPEN_EXISTING; } else if (m & BOOST_IOS::in) { assert(!(m & (BOOST_IOS::app |BOOST_IOS::trunc))); dwDesiredAccess = GENERIC_READ; dwCreationDisposition = OPEN_EXISTING; } else if (m & BOOST_IOS::out) { dwDesiredAccess = GENERIC_WRITE; #ifndef FIX dwCreationDisposition = OPEN_ALWAYS; #else dwCreationDisposition = (m & BOOST_IOS::app) ? OPEN_ALWAYS : CREATE_ALWAYS; #endif } return dwCreationDisposition; } struct test : BOOST_IOS { static void run() { BOOST_CHECK_EQUAL(make_create_flags(out), CREATE_ALWAYS); BOOST_CHECK_EQUAL(make_create_flags(out|app), OPEN_ALWAYS); BOOST_CHECK_EQUAL(make_create_flags(out|trunc), CREATE_ALWAYS); BOOST_CHECK_EQUAL(make_create_flags(in), OPEN_EXISTING); BOOST_CHECK_EQUAL(make_create_flags(in|out), OPEN_EXISTING); BOOST_CHECK_EQUAL(make_create_flags(in|out|trunc), CREATE_ALWAYS); // binary modes are omitted... } }; using boost::unit_test::test_suite; test_suite* init_unit_test_suite(int, char*[]) { test_suite* test = BOOST_TEST_SUITE("openmode test"); test->add(BOOST_TEST_CASE(&test::run)); return test; } These test cases are the one having derived it from the standard and some implementation of std::fstream. If a macro "FIX" is defined, all test cases are passed. Regards, Takeshi Mouri