Error using CreateFileMapping - C - c

Question

I am using the tutorial on this MSDN link to implement a way of transferring data from one process to another. Although I was advised in an earlier question to use the Pipe methods, due to certain constraints I have no choice but to use the CreateFileMapping method.
Now, i've succesfully managed to make two seperate window form projects within the same solution and by editing some properties both of the forms load at the same time.
Furthermore I have managed to implement the code given in the MSDN sample into the first (Producer) and second (Consumer) program without any compilation errors.
The problem I am having now is when I run the first program and try to create the handle to the mapped file, I am given an error saying it was unsuccesful and I do not understand why this is happening.
I have added both the Producer and Consumer code files to demonstrate what I am trying to do.
Producer:
#include <windows.h>
#include <stdio.h>
#include <conio.h>
//File header definitions
#define IDM_FILE_ROLLDICE 1
#define IDM_FILE_QUIT 2
#define BUF_SIZE 256
TCHAR szName[]=TEXT("Global\\MyFileMappingObject");
TCHAR szMsg[]=TEXT("Message from first process!");
void AddMenus(HWND);
LRESULT CALLBACK WindowFunc(HWND, UINT, WPARAM, LPARAM);
////Standard windows stuff - omitted to save space.
//////////////////////
// WINDOWS FUNCTION //
//////////////////////
LRESULT CALLBACK WindowFunc(HWND hMainWindow, UINT message,
WPARAM wParam, LPARAM lParam)
{
WCHAR buffer[256];
LPCTSTR pBuf;
struct DiceData storage;
HANDLE hMapFile;
switch(message)
{
case WM_CREATE:
{
// Create Menus
AddMenus(hMainWindow);
}
break;
case WM_COMMAND:
// Intercept menu choices
switch(LOWORD(wParam))
{
case IDM_FILE_ROLLDICE:
{
//Roll dice and store results in variable
//storage = RollDice();
////Copy results to buffer
//swprintf(buffer,255,L"Dice 1: %d, Dice 2: %d",storage.dice1,storage.dice2);
////Show via message box
//MessageBox(hMainWindow,buffer,L"Dice Result",MB_OK);
hMapFile = CreateFileMapping(
(HANDLE)0xFFFFFFFF, // use paging file
NULL, // default security
PAGE_READWRITE, // read/write access
0, // maximum object size (high-order DWORD)
BUF_SIZE, // maximum object size (low-order DWORD)
szName); // name of mapping object
if (hMapFile == NULL)
{
MessageBox(hMainWindow,L"Could not create file mapping object",L"Error",NULL);
return 1;
}
pBuf = (LPTSTR) MapViewOfFile(hMapFile, // handle to map object
FILE_MAP_ALL_ACCESS, // read/write permission
0,
0,
BUF_SIZE);
if (pBuf == NULL)
{
MessageBox(hMainWindow,L"Could not map view of file",L"Error",NULL);
CloseHandle(hMapFile);
return 1;
}
CopyMemory((PVOID)pBuf, szMsg, (_tcslen(szMsg) * sizeof(TCHAR)));
_getch();
UnmapViewOfFile(pBuf);
CloseHandle(hMapFile);
}
break;
case IDM_FILE_QUIT:
SendMessage(hMainWindow, WM_CLOSE, 0, 0);
break;
}
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
}
return DefWindowProc(hMainWindow, message, wParam, lParam);
}
//
//Setup menus
//
Consumer:
#include <windows.h>
#include <stdio.h>
#include <conio.h>
//File header definitions
#define IDM_FILE_QUIT 1
#define IDM_FILE_POLL 2
#define BUF_SIZE 256
TCHAR szName[]=TEXT("Global\\MyFileMappingObject");
//Prototypes
void AddMenus(HWND);
LRESULT CALLBACK WindowFunc(HWND, UINT, WPARAM, LPARAM);
//More standard windows creation, again omitted.
//////////////////////
// WINDOWS FUNCTION //
//////////////////////
LRESULT CALLBACK WindowFunc(HWND hMainWindow, UINT message,
WPARAM wParam, LPARAM lParam)
{
HANDLE hMapFile;
LPCTSTR pBuf;
switch(message)
{
case WM_CREATE:
{
// Create Menus
AddMenus(hMainWindow);
break;
}
case WM_COMMAND:
{
// Intercept menu choices
switch(LOWORD(wParam))
{
case IDM_FILE_POLL:
{
hMapFile = OpenFileMapping(
FILE_MAP_ALL_ACCESS, // read/write access
FALSE, // do not inherit the name
szName); // name of mapping object
if (hMapFile == NULL)
{
MessageBox(hMainWindow,L"Could not open file mapping object",L"Error",NULL);
return 1;
}
pBuf = (LPTSTR) MapViewOfFile(hMapFile, // handle to map object
FILE_MAP_ALL_ACCESS, // read/write permission
0,
0,
BUF_SIZE);
if (pBuf == NULL)
{
MessageBox(hMainWindow,L"Could not map view of file",L"Error",NULL);
CloseHandle(hMapFile);
return 1;
}
MessageBox(NULL, pBuf, TEXT("Process2"), MB_OK);
UnmapViewOfFile(pBuf);
CloseHandle(hMapFile);
break;
}
case IDM_FILE_QUIT:
SendMessage(hMainWindow, WM_CLOSE, 0, 0);
break;
}
break;
}
case WM_DESTROY:
{
PostQuitMessage(0);
break;
}
}
return DefWindowProc(hMainWindow, message, wParam, lParam);
}
//
//Setup menus
//
It's by no means tidy and final but it's just a start, thanks for any help.
Edit: Error
Edit2: Output

Answer 1

Your code for the producer works for me. What version of Windows are you using? In newer versions (like Vista and 7) there are additional security restrictions placed on accessing shared memory. There is a note about this in the MSDN article you referenced above, saying that you must be an Administrator to create global shared memory objects in Windows Vista/7.
You should also make a call to GetLastError() to see which error code is actually returned from CreateFileMapping(), that may be helpful in determining the root cause of the problem.

Answer 2

Ensure the Global name is unique; this can be done with a tool named Process Explorer.
If not unique, this would typically fail with Error code 6 (The handle is invalid) upon calling CreateFileMappinng
Download Process Explorer from SysInternals
Run Process Explorer
Run your application up to a break point just prior to the CreateFileMapping() failure
Search for MyFileMappingObject using Find (Ctrl-F)
If anything comes up such as another FileMap, Mutex, etc.. consider a more unique name, and ensure your application is not the one creating it.
Note: Consider naming your FileMapping using a GUID (File -> Tools > Create GUID) within Visual Studio

Answer 3

Maybe we learned from the same material/examples in the past. I had the same problem after migrating from XP to Windows 7:
NULL Handle return value and GetLastError() = 5.
ERROR_ACCESS_DENIED
5 (0x5)
Access is denied.
System Error Codes (0-499): http://msdn.microsoft.com/en-us/library/ms681382.aspx
I used a lpName with backslashes like in the Microsoft example from http://msdn.microsoft.com/en-us/library/windows/desktop/aa366537.aspx that you posted above.
After changing the name of the file mapping object (lpName) from "Global\MyFileMappingObject" to "GlobalMyFileMappingObject", the CreateFileMapping function works again under Windows 7 with no other changes.
"The name can have a "Global\" or "Local\" prefix to explicitly create the object in the global or session namespace. The remainder of the name can contain any character except the backslash character ('\'). Creating a file mapping object in the global namespace from a session other than session zero requires the SeCreateGlobalPrivilege privilege. For more information, see Kernel Object Namespaces".
This is not just a name-change! If you need access to the global namespace, then you have to go the SeCreateGlobalPrivilege way.

Answer 4

Under Windows 7
I found:
OpenFileMapping(FILE_MAP_ALL_ACCESS, ...);
Causes issues with:
CreateFileMapping(
(HANDLE)0xFFFFFFFF, // use paging file
Try:
OpenFileMappingA(SECTION_MAP_WRITE | SECTION_MAP_READ,...);