A Problem in Writing Shared Memory to windows from a C++ application

D_code_writer

Hey Guys, I'm writing an application in C++ that needs to write shared memory to windows and I've hit a stumbling block. Even though I'm not getting any error messages (I've stepped through with the debugger) it would appear nothing is writing to shared memory. Here is the code that writes the shared memory,

void Write_Physics(PhysicsStruct *s_physics)
{
// Create the shared memory
TCHAR szName[] = TEXT("Local\\physics");
hMapFile = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, sizeof(PhysicsStruct), szName);
if (!hMapFile)
{
return;
}

//	Open the buffer for file read and write
mapFileBuffer = (unsigned char\*)MapViewOfFile(m\_physics.hMapFile, FILE\_MAP\_ALL\_ACCESS, 0, 0, sizeof(PhysicsStruct));
if (!mapFileBuffer)
{
	return;
}

//	Copy the physics pointer over
CopyMemory(mapFileBuffer,s\_physics,sizeof(PhysicsStruct));
//\_getch();


//	Write the shared memory
UnmapViewOfFile(m\_physics.mapFileBuffer);
CloseHandle(m\_physics.hMapFile);

}

My difficulty is that when I try another process to read the shared memory nothing is happening. For completeness here is the code that reads the structure from the shared memory,

void Read_Physics(PhysicsStruct *s_physics)
{
// Create the shared memory
TCHAR szName[] = TEXT("Local\\physics");
hMapFile = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, sizeof(PhysicsStruct), szName);
if (!m_physics.hMapFile)
{
return;
}

//	Open the buffer for file read and write
mapFileBuffer = (unsigned char\*)MapViewOfFile(hMapFile, FILE\_MAP\_READ, 0, 0, sizeof(PhysicsStruct));
if (!mapFileBuffer)
{
	return;
}

s\_physics = (PhysicsStruct\*)mapFileBuffer;

//	Write the shared memory
UnmapViewOfFile(mapFileBuffer);
CloseHandle(hMapFile);

}
//-----------------------------------------------------------------------

If anyone has any suggestions it would be greatly appreciated. What the problem resembles is like trying to write something to file and forgetting the fprintf or file write commands. Thanks in advance guys.

Richard Andrew x64

Where is hMapFile declared?

void Read_Physics(PhysicsStruct *s_physics)
{
// Create the shared memory
TCHAR szName[] = TEXT("Local\\physics");
here >>>> hMapFile = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, sizeof(PhysicsStruct), szName);
if (!m_physics.hMapFile)
{
return;
}

//	Open the buffer for file read and write
mapFileBuffer = (unsigned char\*)MapViewOfFile(hMapFile, FILE\_MAP\_READ, 0, 0, sizeof(PhysicsStruct));
if (!mapFileBuffer)
{
	return;
}

s\_physics = (PhysicsStruct\*)mapFileBuffer;

//	Write the shared memory
UnmapViewOfFile(mapFileBuffer);
CloseHandle(hMapFile);

}

The difficult we do right away... ...the impossible takes slightly longer.

Richard Andrew x64

There is another problem with the code for the Read function: You're passing the pointer s_physics by value. This is not going to do what you want. You need to pass the pointer's address. You would do that like so:

void Read_Physics(PhysicsStruct **s_physics)

Then inside the function:

*s_physics = (PhysicsStruct*)mapFileBuffer;

And the read function would be called thusly:

Read_Physics(&s_physics);

The difficult we do right away... ...the impossible takes slightly longer.

D_code_writer

Richard, Many thanks for answering. My apologies I wrote this as pseudo code to try and save time. The declarations for those are, HANDLE hmapFile; unsigned char *mapFileBuffer; They are at the beginning of each function. The joys of doing this at 1:00am! Copy you on the declaration of the read function - leave that with me to sort out. Apart from this is there any other reason you can see for this and particular the write function not working? Many thanks in advance.

Richard Andrew x64

D_code_writer wrote:

I wrote this as pseudo code to try and save time.

It's not helpful if you post code that is not the actual code that is causing the problem. I can see one other BIG problem: You're setting s_physics to point to the memory of the file map, but then you're closing the file mapping! EDIT: The problem is the same with the write function. You're writing to the memory, but then you're closing the file mapping! The read function cannot read from a file mapping that doesn't exist. I think this is the main reason it's not working. You need to copy the memory pointed to by mapFileBuffer, and return that as your result. Then you'll need to free this memory somewhere else. So, in pseudo code: Read Function: Create File Map; Allocate new memory = sizeof(PhysicsStruct); Copy memory from File Map to new memory; Set s_physics = new memory; return s_physics;

The difficult we do right away... ...the impossible takes slightly longer.

D_code_writer

Richard, Copy that - I actually figured it out. I found an old VC++ 6 project called MMFAPP1. That actually worked. Here is the crux of the fix. Declare the following variables,

HANDLE m_hFileMMF; // memory mapped file
LPVOID m_pViewMMFFile; // view of file, contains text in edit box

Then create the file handle,

m_hFileMMF = CreateFileMapping(INVALID_HANDLE_VALUE,NULL,PAGE_READWRITE,0,4*1024,"MyMMF");
DWORD dwError = GetLastError();
if ( ! m_hFileMMF )
{
MessageBox(_T("Creation of file mapping failed"));
}
else
{
m_pViewMMFFile = MapViewOfFile(m_hFileMMF,FILE_MAP_ALL_ACCESS,0,0,0); // map all file

    if(! m\_pViewMMFFile )
    {
        MessageBox(\_T("MapViewOfFile function failed"));
    }
}

Once that is done to write the data to the shared memory we have PhysicsStruct s_physics;

if(m_pViewMMFFile )
CopyMemory(m_pViewMMFFile,&s_physics,sizeof(PhysicsStruct));

To read from the shared memory we have,

if(m_pViewMMFFile )
CopyMemory((PVOID)&s_physics,m_pViewMMFFile,sizeof(SPageFilePhysics));

I tested that and it worked like a charm. Also many thanks to the guy who wrote MMFAPP1 in the first place. It saved my neck. Also Richard thanks for your help as well.

Richard Andrew x64

Glad you got it sorted. :)

The difficult we do right away... ...the impossible takes slightly longer.

D_code_writer

So am I!! Thanks again for your help