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<p align="center">Tutorial 13: Memory Mapped Files</p>
<hr size="1">
I'll show you what memory mapped files are and how to use them to your advantages.
Using a memory mapped file is quite easy as you'll see in this tutorial.
<p>Download the example <a href="files/tut13.zip">here</a>.
<h3> Theory:</h3>
If you examine the example in the previous tutorial closely, you'll find that
it has a serious shortcoming: what if the file you want to read is larger than
the allocated memory block? or what if the string you want to search for is cut
off in half at the end of the memory block? The traditional answer for the first
question is that you should repeatedly read in the data from the file until the
end of file is encountered. The answer to the second question is that you should
prepare for the special case at the end of the memory block. This is called a
boundary value problem. It presents headaches to programmers and causes innumerable
bugs. <br>
It would be nice if we can allocate a very large block of memory, enough to store
the whole file but our program would be a resource hog. File mapping to the rescue.
By using file mapping, you can think of the whole file as being already loaded
into memory and you can use a memory pointer to read or write data from the file.
As easy as that. No need to use memory API functions and separate File I/O API
functions anymore, they are one and the same under file mapping. File mapping
is also used as a means to share data between processes. Using file mapping in
this way, there's no actual file involved. It's more like a reserved memory block
that every process can *see*. But sharing data between processes is a delicate
subject, not to be treated lightly. You have to implement process and thread synchronization
else your applications will crash in very short order. <br>
We will not touch the subject of file mapping as a means to create a shared memory
region in this tutorial. We'll concentrate on how to use file mapping as a means
to "map" a file into memory. In fact, the PE loader uses file mapping to load
executable files into memory. It is very convenient since only the necessary portions
can be selectively read from the file on the disk. Under Win32, you should use
file mapping as much as possible. <br>
There are some limitation to file mapping though. Once you create a memory mapped
file, its size cannot be changed during that session. So file mapping is great
for read-only files or file operations that don't affect the file size. That doesn't
mean that you cannot use file mapping if you want to increase the file size. You
can estimate the new size and create the memory mapped file based on the new size
and the file will grow to that size. It's just inconvenient, that's all. <br>
Enough for the explanation. Let's dive into implementation of file mapping. In
order to use file mapping, these steps must be performed:
<ol>
<li> call <b>CreateFile</b> to open the file you want to map.</li>
<li> call <b>CreateFileMapping</b> with the file handle returned by CreateFile
as one of its parameter. This function creates a file mapping object from
the file opened by CreateFile.</li>
<li> call <b>MapViewOfFile</b> to map a selected file region or the whole file
to memory. This function returns a pointer to the first byte of the mapped
file region.</li>
<li> Use the pointer to read or write the file</li>
<li> call <b>UnmapViewOfFile</b> to unmap the file.</li>
<li> call <b>CloseHandle</b> with the handle to the mapped file as the parameter
to close the mapped file.</li>
<li> call <b>CloseHandle</b> again this time with the file handle returned by
CreateFile to close the actual file.</li>
</ol>
<h3> Example:</h3>
The program listed below lets you open a file via an open file dialog box. It
opens the file using file mapping, if it's successful, the window caption is changed
to the name of the opened file. You can save the file in another name by select
File/Save as menuitem. The program will copy the whole content of the opened file
to the new file. Note that you don't have to call GlobalAlloc to allocate a memory
block in this program.
<p><b>.386</b> <br>
<b>.model flat,stdcall</b> <br>
<b>WinMain proto :DWORD,:DWORD,:DWORD,:DWORD</b> <br>
<b>include \masm32\include\windows.inc</b> <br>
<b>include \masm32\include\user32.inc</b> <br>
<b>include \masm32\include\kernel32.inc</b> <br>
<b>include \masm32\include\comdlg32.inc</b> <br>
<b>includelib \masm32\lib\user32.lib</b> <br>
<b>includelib \masm32\lib\kernel32.lib</b> <br>
<b>includelib \masm32\lib\comdlg32.lib</b>
<p><b>.const</b> <br>
<b>IDM_OPEN equ 1</b> <br>
<b>IDM_SAVE equ 2</b> <br>
<b>IDM_EXIT equ 3</b> <br>
<b>MAXSIZE equ 260</b>
<p><b>.data</b> <br>
<b>ClassName db "Win32ASMFileMappingClass",0</b> <br>
<b>AppName db "Win32 ASM File Mapping Example",0</b> <br>
<b>MenuName db "FirstMenu",0</b> <br>
<b>ofn OPENFILENAME <></b> <br>
<b>FilterString db "All Files",0,"*.*",0</b> <br>
<b>
db "Text Files",0,"*.txt",0,0</b> <br>
<b>buffer db MAXSIZE dup(0)</b> <br>
<b>hMapFile HANDLE 0
; Handle to the memory mapped file, must be</b> <br>
<b>
;initialized with 0 because we also use it as</b> <br>
<b>
;a flag in WM_DESTROY section too</b>
<p><b>.data?</b> <br>
<b>hInstance HINSTANCE ?</b> <br>
<b>CommandLine LPSTR ?</b> <br>
<b>hFileRead HANDLE ?
; Handle to the source file</b> <br>
<b>hFileWrite HANDLE ?
; Handle to the output file</b> <br>
<b>hMenu HANDLE ?</b> <br>
<b>pMemory DWORD ?
; pointer to the data in the source file</b> <br>
<b>SizeWritten DWORD ?
; number of bytes actually written by WriteFile</b>
<p><b>.code</b> <br>
<b>start:</b> <br>
<b> invoke GetModuleHandle, NULL</b>
<br>
<b> mov hInstance,eax</b>
<br>
<b> invoke GetCommandLine</b> <br>
<b> mov CommandLine,eax</b> <br>
<b> invoke WinMain, hInstance,NULL,CommandLine,
SW_SHOWDEFAULT</b> <br>
<b> invoke ExitProcess,eax</b>
<p><b>WinMain proc hInst:HINSTANCE,hPrevInst:HINSTANCE,CmdLine:LPSTR,CmdShow:DWORD</b>
<br>
<b> LOCAL wc:WNDCLASSEX</b> <br>
<b> LOCAL msg:MSG</b> <br>
<b> LOCAL hwnd:HWND</b> <br>
<b> mov wc.cbSize,SIZEOF WNDCLASSEX</b> <br>
<b> mov wc.style, CS_HREDRAW or CS_VREDRAW</b>
<br>
<b> mov wc.lpfnWndProc, OFFSET WndProc</b> <br>
<b> mov wc.cbClsExtra,NULL</b> <br>
<b> mov wc.cbWndExtra,NULL</b> <br>
<b> push hInst</b> <br>
<b> pop wc.hInstance</b> <br>
<b> mov wc.hbrBackground,COLOR_WINDOW+1</b> <br>
<b> mov wc.lpszMenuName,OFFSET MenuName</b> <br>
<b> mov wc.lpszClassName,OFFSET ClassName</b>
<br>
<b> invoke LoadIcon,NULL,IDI_APPLICATION</b> <br>
<b> mov wc.hIcon,eax</b> <br>
<b> mov wc.hIconSm,eax</b> <br>
<b> invoke LoadCursor,NULL,IDC_ARROW</b> <br>
<b> mov wc.hCursor,eax</b> <br>
<b> invoke RegisterClassEx, addr wc</b> <br>
<b> invoke CreateWindowEx,WS_EX_CLIENTEDGE,ADDR ClassName,\</b>
<br>
<b>
ADDR AppName, WS_OVERLAPPEDWINDOW,CW_USEDEFAULT,\</b> <br>
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