Memory Mapped File changes not reflected in underlying file - c

Question

I am learning MMF in win32 C.
I can't seem to see what I am missing...
Here is my code:
#include "stdafx.h"
#include <Windows.h>
int main()
{
wchar_t szfilename[] = L"c:\\temp\\mmf.txt";
HANDLE hFile = CreateFile(
szfilename,
GENERIC_READ | GENERIC_WRITE, //dwDesiredAccess
FILE_SHARE_READ | FILE_SHARE_WRITE, //dwShareMode
NULL, //lpSecurityAttributes
CREATE_NEW, //dwCreationDisposition
FILE_ATTRIBUTE_NORMAL, //dwFlagsAndAttributes
NULL); //hTemplateFile
if (hFile == INVALID_HANDLE_VALUE)
{
DWORD error = GetLastError();
if (error != ERROR_FILE_EXISTS)
exit(-1);
}
//Now create the file mapping
HANDLE hMap = CreateFileMapping(
hFile,
NULL, //LPSECURITY_ATTRIBUTES lpAttributes,
PAGE_READWRITE, //DWORD flProtect,
0, //DWORD dwMaximumSizeHigh,
1024, //DWORD dwMaximumSizeLow,
L"myMappingFile"); //LPCTSTR lpName
if (hMap == (HANDLE)ERROR_ALREADY_EXISTS || hMap == NULL)
{
DWORD error = GetLastError();
exit(-2);
}
LPCTSTR pBuf = (LPTSTR)MapViewOfFile(hMap, // handle to map object
FILE_MAP_ALL_ACCESS, // read/write permission
0,
0,
0);
if (pBuf == NULL)
{
printf("Could not map view of file (%d).\n",
GetLastError());
return -2;
}
wchar_t szMsg[] = L"Msg from first process";
CopyMemory((PVOID)pBuf, szMsg, wcslen(szMsg) * sizeof(wchar_t));
UnmapViewOfFile(pBuf);
CloseHandle(hMap);
CloseHandle(hFile);
return 0;
}
I open a file, associate it with a memory mapping and create a view. Then I write data (copy) into the buffer (view) and close things down.
I am trying to figure out why my underlying file (c:\temp\mmf.txt) does not contain the msg after the routine executes. Am I missing something? The file size remains 0.

Answer 1

Here is a snippet of code that demonstrates memory mapping a file. In the end I think you will just benefit from reading the MSDN docs for each API call you are using a little more carefully.
HANDLE hFile = INVALID_HANDLE_VALUE;
HANDLE hFileMapping = NULL;
LPVOID pBase = NULL;
__try
{
hFile = CreateFile(filename,
GENERIC_READ,
FILE_SHARE_READ,
NULL,
OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL,
0);
if(hFile == INVALID_HANDLE_VALUE) __leave;
hFileMapping = CreateFileMapping(hFile,
NULL,
PAGE_READONLY,
0, 0, NULL);
if(hFileMapping == NULL) __leave;
pFileBase = MapViewOfFile(hFileMapping,
FILE_MAP_READ,
0, 0, 0);
if(pBase == NULL) __leave;
}
__finally
{
if(hFile != INVALID_HANDLE_VALUE) CloseHandle(hFile);
if(hFileMapping != NULL) CloseHandle(hFileMapping);
}