文件映射的三个功能：
1、File mapping allows the process to use both random input and output (I/O) and sequential I/O.
2、It also allows the process to work efficiently with a large data file, such as a database, without having to map the whole file into memory.
3、Multiple processes can also use memory-mapped files to share data.
https://msdn.microsoft.com/en-us/library/windows/desktop/aa366556(v=vs.85).aspx