Tutorial 13: Memory Mapped Files
Download the example here.
.386
.model flat,stdcall
WinMain proto :DWORD,:DWORD,:DWORD,:DWORD
include \masm32\include\windows.inc
include \masm32\include\user32.inc
include \masm32\include\kernel32.inc
include \masm32\include\comdlg32.inc
includelib \masm32\lib\user32.lib
includelib \masm32\lib\kernel32.lib
includelib \masm32\lib\comdlg32.lib
.const
IDM_OPEN equ 1
IDM_SAVE equ 2
IDM_EXIT equ 3
MAXSIZE equ 260
.data
ClassName db "Win32ASMFileMappingClass",0
AppName db "Win32 ASM File Mapping Example",0
MenuName db "FirstMenu",0
ofn OPENFILENAME <>
FilterString db "All Files",0,"*.*",0
db "Text Files",0,"*.txt",0,0
buffer db MAXSIZE dup(0)
hMapFile HANDLE 0
; Handle to the memory mapped file, must be
;initialized with 0 because we also use it as
;a flag in WM_DESTROY section too
.data?
hInstance HINSTANCE ?
CommandLine LPSTR ?
hFileRead HANDLE ?
; Handle to the source file
hFileWrite HANDLE ?
; Handle to the output file
hMenu HANDLE ?
pMemory DWORD ?
; pointer to the data in the source file
SizeWritten DWORD ?
; number of bytes actually written by WriteFile
.code
start:
invoke GetModuleHandle, NULL
mov hInstance,eax
invoke GetCommandLine
mov CommandLine,eax
invoke WinMain, hInstance,NULL,CommandLine,
SW_SHOWDEFAULT
invoke ExitProcess,eax
WinMain proc hInst:HINSTANCE,hPrevInst:HINSTANCE,CmdLine:LPSTR,CmdShow:DWORD
LOCAL wc:WNDCLASSEX
LOCAL msg:MSG
LOCAL hwnd:HWND
mov wc.cbSize,SIZEOF WNDCLASSEX
mov wc.style, CS_HREDRAW or CS_VREDRAW
mov wc.lpfnWndProc, OFFSET WndProc
mov wc.cbClsExtra,NULL
mov wc.cbWndExtra,NULL
push hInst
pop wc.hInstance
mov wc.hbrBackground,COLOR_WINDOW+1
mov wc.lpszMenuName,OFFSET MenuName
mov wc.lpszClassName,OFFSET ClassName
invoke LoadIcon,NULL,IDI_APPLICATION
mov wc.hIcon,eax
mov wc.hIconSm,eax
invoke LoadCursor,NULL,IDC_ARROW
mov wc.hCursor,eax
invoke RegisterClassEx, addr wc
invoke CreateWindowEx,WS_EX_CLIENTEDGE,ADDR ClassName,\
ADDR AppName, WS_OVERLAPPEDWINDOW,CW_USEDEFAULT,\
CW_USEDEFAULT,300,200,NULL,NULL,\
hInst,NULL
mov hwnd,eax
invoke ShowWindow, hwnd,SW_SHOWNORMAL
invoke UpdateWindow, hwnd
.WHILE TRUE
invoke GetMessage, ADDR msg,NULL,0,0
.BREAK .IF (!eax)
invoke TranslateMessage, ADDR
msg
invoke DispatchMessage, ADDR msg
.ENDW
mov eax,msg.wParam
ret
WinMain endp
WndProc proc hWnd:HWND, uMsg:UINT, wParam:WPARAM, lParam:LPARAM
.IF uMsg==WM_CREATE
invoke GetMenu,hWnd
;Obtain the menu handle
mov hMenu,eax
mov ofn.lStructSize,SIZEOF ofn
push hWnd
pop ofn.hWndOwner
push hInstance
pop ofn.hInstance
mov ofn.lpstrFilter, OFFSET
FilterString
mov ofn.lpstrFile, OFFSET
buffer
mov ofn.nMaxFile,MAXSIZE
.ELSEIF uMsg==WM_DESTROY
.if hMapFile!=0
call CloseMapFile
.endif
invoke PostQuitMessage,NULL
.ELSEIF uMsg==WM_COMMAND
mov eax,wParam
.if lParam==0
.if ax==IDM_OPEN
mov ofn.Flags, OFN_FILEMUSTEXIST or \
OFN_PATHMUSTEXIST or OFN_LONGNAMES or\
OFN_EXPLORER or OFN_HIDEREADONLY
invoke GetOpenFileName, ADDR ofn
.if eax==TRUE
invoke CreateFile,ADDR buffer,\
GENERIC_READ ,\
0,\
NULL,OPEN_EXISTING,FILE_ATTRIBUTE_ARCHIVE,\
NULL
mov hFileRead,eax
invoke CreateFileMapping,hFileRead,NULL,PAGE_READONLY,0,0,NULL
mov hMapFile,eax
mov eax,OFFSET buffer
movzx edx,ofn.nFileOffset
add eax,edx
invoke SetWindowText,hWnd,eax
invoke EnableMenuItem,hMenu,IDM_OPEN,MF_GRAYED
invoke EnableMenuItem,hMenu,IDM_SAVE,MF_ENABLED
.endif
.elseif
ax==IDM_SAVE
mov ofn.Flags,OFN_LONGNAMES or\
OFN_EXPLORER or OFN_HIDEREADONLY
invoke GetSaveFileName, ADDR ofn
.if eax==TRUE
invoke CreateFile,ADDR buffer,\
GENERIC_READ or GENERIC_WRITE ,\
FILE_SHARE_READ or FILE_SHARE_WRITE,\
NULL,CREATE_NEW,FILE_ATTRIBUTE_ARCHIVE,\
NULL
mov hFileWrite,eax
invoke MapViewOfFile,hMapFile,FILE_MAP_READ,0,0,0
mov pMemory,eax
invoke GetFileSize,hFileRead,NULL
invoke WriteFile,hFileWrite,pMemory,eax,ADDR SizeWritten,NULL
invoke UnmapViewOfFile,pMemory
call CloseMapFile
invoke CloseHandle,hFileWrite
invoke SetWindowText,hWnd,ADDR AppName
invoke EnableMenuItem,hMenu,IDM_OPEN,MF_ENABLED
invoke EnableMenuItem,hMenu,IDM_SAVE,MF_GRAYED
.endif
.else
invoke DestroyWindow, hWnd
.endif
.endif
.ELSE
invoke DefWindowProc,hWnd,uMsg,wParam,lParam
ret
.ENDIF
xor eax,eax
ret
WndProc endp
CloseMapFile PROC
invoke CloseHandle,hMapFile
mov hMapFile,0
invoke CloseHandle,hFileRead
ret
CloseMapFile endp
end start
When the user selects a file in the open file dialog, we call CreateFile to open it. Note that we specify GENERIC_READ to open this file for read-only access and dwShareMode is zero because we don't want some other process to modify the file during our operation.
invoke CreateFileMapping,hFileRead,NULL,PAGE_READONLY,0,0,NULL
Then we call CreateFileMapping to create a memory mapped file from the opened file. CreateFileMapping has the following syntax:
CreateFileMapping proto hFile:DWORD,\
lpFileMappingAttributes:DWORD,\
flProtect:DWORD,\
dwMaximumSizeHigh:DWORD,\
dwMaximumSizeLow:DWORD,\
lpName:DWORD
You should know first that CreateFileMapping doesn't have to map the whole
file to memory. You can use this function to map only a part of the actual file
to memory. You specify the size of the memory mapped file in dwMaximumSizeHigh
and dwMaximumSizeLow params. If you specify the size that 's larger than the
actual file, the actual file will be expanded to the new size. If you want the
memory mapped file to be the same size as the actual file, put zeroes in both
params.
You can use NULL in lpFileMappingAttributes parameter to have Windows creates
a memory mapped file with default security attributes.
flProtect defines the protection desired for the memory mapped file. In our
example, we use PAGE_READONLY to allow only read operation on the memory mapped
file. Note that this attribute must not contradict the attribute used in CreateFile
else CreateFileMapping will fail.
lpName points to the name of the memory mapped file. If you want to share this
file with other process, you must provide it a name. But in our example, our
process is the only one that uses this file so we ignore this parameter.
mov eax,OFFSET buffer
movzx edx,ofn.nFileOffset
add eax,edx
invoke SetWindowText,hWnd,eax
If CreateFileMapping is successful, we change the window caption to the name of the opened file. The filename with full path is stored in buffer, we want to display only the filename in the caption so we must add the value of nFileOffset member of the OPENFILENAME structure to the address of buffer.
invoke EnableMenuItem,hMenu,IDM_OPEN,MF_GRAYED
invoke EnableMenuItem,hMenu,IDM_SAVE,MF_ENABLED
As a precaution, we don't want the user to open multiple files at once, so
we gray out the Open menu item and enable the Save menu item. EnableMenuItem
is used to change the attribute of menu item.
After this, we wait for the user to select File/Save as menu item or close our
program. If the user chooses to close our program, we must close the memory
mapped file and the actual file like the code below:
.ELSEIF uMsg==WM_DESTROY
.if hMapFile!=0
call CloseMapFile
.endif
invoke PostQuitMessage,NULL
In the above code snippet, when the window procedure receives the WM_DESTROY message, it checks the value of hMapFile first whether it is zero or not. If it's not zero, it calls CloseMapFile function which contains the following code:
CloseMapFile PROC
invoke CloseHandle,hMapFile
mov hMapFile,0
invoke CloseHandle,hFileRead
ret
CloseMapFile endp
CloseMapFile closes the memory mapped file and the actual file so that there
'll be no resource leakage when our program exits to Windows.
If the user chooses to save that data to another file, the program presents
him with a save as dialog box. After he types in the name of the new file, the
file is created by CreateFile function.
invoke MapViewOfFile,hMapFile,FILE_MAP_READ,0,0,0
mov pMemory,eax
Immediately after the output file is created, we call MapViewOfFile to map the desired portion of the memory mapped file into memory. This function has the following syntax:
MapViewOfFile proto hFileMappingObject:DWORD,\
dwDesiredAccess:DWORD,\
dwFileOffsetHigh:DWORD,\
dwFileOffsetLow:DWORD,\
dwNumberOfBytesToMap:DWORD
dwDesiredAccess specifies what operation we want to do to the file.
In our example, we want to read the data only so we use FILE_MAP_READ.
dwFileOffsetHigh and dwFileOffsetLowspecify the starting file
offset of the file portion that you want to map into memory. In our case, we
want to read in the whole file so we start mapping from offset 0 onwards.
dwNumberOfBytesToMap specifies the number of bytes to map into memory.
If you want to map the whole file (specified by CreateFileMapping), pass 0 to
MapViewOfFile.
After calling MapViewOfFile, the desired portion is loaded into memory. You'll
be given the pointer to the memory block that contains the data from the file.
invoke GetFileSize,hFileRead,NULL
Find out how large the file is. The file size is returned in eax. If the file is larger than 4 GB, the high DWORD of the file size is stored in FileSizeHighWord. Since we don't expect to handle such large file, we can ignore it.
invoke WriteFile,hFileWrite,pMemory,eax,ADDR SizeWritten,NULL
Write the data that is mapped into memory into the output file.
invoke UnmapViewOfFile,pMemory
When we're through with the input file, unmap it from memory.
call CloseMapFile
invoke CloseHandle,hFileWrite
And close all the files.
invoke SetWindowText,hWnd,ADDR AppName
Restore the original caption text.
invoke EnableMenuItem,hMenu,IDM_OPEN,MF_ENABLED
invoke EnableMenuItem,hMenu,IDM_SAVE,MF_GRAYED
Enable the Open menu item and gray out the Save As menu item.