MapViewOfFile returns different addresses with same handle - c++

Question

I'm trying to implement IPC for a school assignment by sharing memory.
I made a class called SharedMemoryBuffer to deal with creating file mappings and views.
My Init() function looks like this:
BYTE * SharedMemoryBuffer::Init(const wchar_t * name, size_t bufferSize)
{
FileMapHandle = OpenFileMapping(
FILE_MAP_ALL_ACCESS, // read/write access
FALSE, // do not inherit the name
name); // name of mapping object
if (FileMapHandle == NULL)
{
FileMapHandle =
CreateFileMapping(INVALID_HANDLE_VALUE,
NULL,
PAGE_READWRITE,
0,
bufferSize,
name);
pBuf = (BYTE*)MapViewOfFile(FileMapHandle, // handle to map object
FILE_MAP_ALL_ACCESS, // read/write permission
0,
0,
bufferSize);
}
else
{
pBuf = (BYTE*)MapViewOfFile(
FileMapHandle,
FILE_MAP_ALL_ACCESS,
0,
0,
bufferSize
);
}
return this->GetBuffer();
}
Essentially, I pass it a name and size and it tries to open a mapping with this name. If it fails, it creates it instead.
I call it like so
this->ringBuffer.Init(widestr.c_str(), buffSize);
After this is done (I call Init 4 times for 2 buffers, from the same process) I print out the addresses of the buffers (pBuf from Init()) but theyre all different addresses.
I cant for the love of my life figure out why the addresses would be different!
I have made sure that the second time i call Init() with the same name that it does indeed open the file mapping successfully.
source: https://github.com/RavioliFinoli/SharedMemory

Answer 1

You are mapping the same region twice in your process. You will get two distinct addresses, but they are backed by the same physical memory. Writing into the buffer pointed by the first address modifies the buffer pointed to by the second address, since they are really the same memory.