1. Win32 Programming Lesson 17: Memory Mapped Files (Finally, cool stuff again, all this work is getting tedious!)

2. Where are we?  We’ve gotten pretty familiar with memory  But now it’s time to look at some of the clever tricks you can play with memory

3. Memory mapped files  Three primary purposes Loading and executing.exe and DLL files Accessing files on disk quickly without need to buffer Allow multiple processes to share the same data in memory

4. CreateProcess Revisited  Five steps: Find the.exe file on disk Create a new Process kernel object Create a private address space for the process Reserve space in memory at 0x00400000 (by default) and load the.exe image Physical storage is from the.exe image, not the paging file

5. Next…  System uses LoadLibrary to load the needed DLLs from disk  Once again, physical storage underlying the DLLs is on disk, not the system paging file (clever eh?)  If everything can’t be mapped CreateProcess fails and returns FALSE

6. Multiple Instances  Very clever – remember COPY_ON_WRITE?  DLLs share the same image in memory – but get new pages when they modify their memory image  However, what if we want to share variables that change?

7. Background  To work out how to do this, we need to understand a little more of the foundations  Every.exe or DLL has multiple sections  Each section has one or more of the following attributes: READ WRITE EXECUTE SHARED (effectively turns off COPY_ON_WRITE)  Can see this using DumpBin

8. Using a Shared Data segment  See example

9. Memory mapped data files  How to reverse the order of all bytes in a file? Four ways… One file, one buffer (load it all!) Two files, one buffer (read it in in chunks, reverse each chunk) One file, two buffers (read in 1024 bytes from the start and the end, and swap them) One file, no buffers!

10. Whassat?  Yep, one file, zero buffers  Map the file into memory  Work on the memory image directly, letting the OS handle the buffering

11. Six step process  You must: Create or open the kernel object that references the file Create a file-mapping kernel object Tell the system to map all or part of the file Tell the system when mapping is no longer needed Close the file-mapping object Close the file

12. Step 1: Opening the file  Easy: Createfile HANDLE CreateFile( PCSTR pszFileName, DWORD dwDesiredAccess, DWORD dwShareMode, PSECURITY_ATTRIBUTES psa, DWORD dwCreationDisposition, DWORD dwFlagsAndAttributes, HANDLE hTemplateFile);

13. Parms  dwDesiredAccess 0 – just get the attributes GENERIC_READ GENERIC_WRITE GENERIC_READ | GENERIC_WRITE  dwShareMode 0 – not shared FILE_SHARE_READ FILE_SHARE_WRITE FILE_SHARE_READ | FILE_SHARE_WRITE

14. Step 2: Creating the file-mapping  HANDLE CreateFileMapping( HANDLE hFile, PSECURITY_ATTRIBUTES psa, DWORD fdwProtect, DWORD dwMaximumSizeHigh, DWORD dwMaximumSizeLow, PCTSTR pszName);  See MSDNMSDN

15. Parms  dwMaximumSizeHigh/Low are important  64-bit representation of the file length so there is enough disk space if you want to make writes  See example for what happens as we step through…

16. Step 3: Map the file in memory  Using: PVOID MapViewOfFile( HANDLE hFileMappingObject, DWORD dwDesiredAccess, DWORD dwFileOffsetHigh, DWORD dwFileOffsetLow, SIZE_T dwNumberOfBytesToMap);

17. Parms  FILE_MAP_WRITE: You can read and write file data. CreateFileMapping had to be called by passing PAGE_READWRITE.  FILE_MAP_READ: You can read file data. CreateFileMapping could be called with any of the protection attributes: PAGE_READONLY, PAGE_READWRITE, or PAGE_WRITECOPY.  FILE_MAP_ALL_ACCESS: Same as FILE_MAP_WRITE.  FILE_MAP_COPY: You can read and write file data. Writing causes a private copy of the page to be created.

18. Step 4: Unmap the file  BOOL UnmapViewOfFile(PVOID pvBaseAd dress);  And flush, if you need to… BOOL FlushViewOfFile( PVOID pvAddress, SIZE_T dwNumberOfBytesToFlush); pvAddress is what you want to flush…

19. Step 5/6 Cleaning up  CloseHandle(hFileMapping);  CloseHandle(hFile);  Easy  But remember kernel usage counts – means that we can close the file much earlier for neater code if we want to

20. Example  Let’s look at the one in the book – file reverse…

21. Processing a really big file  __int64 qwFileOffset = 0, qwNumOf0s = 0; while (qwFileSize > 0) { // Determine the number of bytes to be mapped in this view DWORD dwBytesInBlock = sinf.dwAllocationGranularity; if (qwFileSize > 32), // Starting byte (DWORD) (qwFileOffset & 0xFFFFFFFF), // in file dwBytesInBlock); // # of bytes to map // Count the number of Js in this block. for (DWORD dwByte = 0; dwByte < dwBytesInBlock; dwByte++) { if (pbFile[dwByte] == 0) qwNumOf0s++; } // Unmap the view; we don't want multiple views // in our address space. UnmapViewOfFile(pbFile); // Skip to the next set of bytes in the file. qwFileOffset += dwBytesInBlock; qwFileSize -= dwBytesInBlock; }

22. Specifying the Base Address  Say you want to share a memory-mapped file which contains a linked-list  Need to have the same base address in both processes  Can call MapViewOfFileEx

23. Assignment  Recreate my little shared-memory form program  EASY!