Or maybe a feature? Or maybe I'm just using it incorrectly. I'm trying to create a new file of a certain size, regardless of whether or not a file of the same name exists. If I run a test program along the lines of boost::iostreams::mapped_file file; boost::iostreams::mapped_file_params p("test.txt"); p.new_file_size = 1201; p.length = 1; p.mode = ios_base::in | ios_base::out; file.open(p); It fails with the error 'failed setting file size: Cannot create a file when that file already exists.' if file 'test.txt' already exists. It seems to me that this error is a bug caused by incorrect error checking in mapped_file.cpp in function mapped_file_impl::open_file. At the call to CreateFileA, dwCreationDisposition=CREATE_ALWAYS and a valid handle value is returned. The Win32 docs say that in this case the call to CreateFileA has succeeded and the last-error code is set to ERROR_ALREADY_EXISTS. http://msdn.microsoft.com/en-us/library/aa363858%28VS.85%29.aspx Immediately after the call to CreateFileA, the handle is checked for validity but the last-error code is not reset. Shortly thereafter is a call to SetFilePointer. The docs say that the proper way to check for an error from SetFilePointer is to compare its return value to INVALID_SET_VALUE_POINTER. http://msdn.microsoft.com/en-us/library/aa365541%28VS.85%29.aspx This isn't done in the code... Instead, GetLastError() is called, which apparently returns the ERROR_ALREADY_EXISTS code generated by CreateFileA, which wasn't an error at all. It seems to me that a couple of things need to be changed to make this correct- 1) after the check for valid handle creation by CreateFileA, call SetLastError(NO_ERROR) 2) when resizing the file the code if (::GetLastError() != NO_ERROR || !::SetEndOfFile(handle_)) should be something like if (::SetFilePointer(handle_, sizelow, &sizehigh, FILE_BEGIN) == INVALID_SET_VALUE_POINTER || !::SetEndOfFile(handle_)) Does that make sense? Or am I just setting the mapped_file_params incorrectly to force creation of a new file? Erik