Question

I’m trying to use CreateFileMapping and OpenFileMapping to share memory between processes. This isn’t working as I want it to – OpenFileMapping returns null and GetLastError is 5 – access denied. Any ideas what I am doing wrong? Name is something like MemoryTest.

Edit:

using CreateFileMapping both times I can read the data written in the other process. The reason this is a problem is that I get Error 183 – memory area already exists. However, it still returns a handle to the existing memory.

var map_handle = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, sizeof(int), name.c_str());

....

var handle = MapViewOfFile(map_handle, FILE_MAP_ALL_ACCESS , 0, 0, 0)

*handle = 10;

UnMapViewOfFile(map_handle);

getchar();

Other process:

var map_handle = OpenFileMapping(PAGE_READWRITE, false, name.c_str())

....

var handle = MapViewOfFile(map_handle, FILE_MAP_ALL_ACCESS , 0, 0, 0) //returns null

var out = *handle;

getchar();

This works for the second process though:

var map_handle = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, sizeof(int), name.c_str());

....

var handle = MapViewOfFile(map_handle, FILE_MAP_ALL_ACCESS , 0, 0, 0) //returns null

var out = *handle;

getchar();

Answer 1

Simple things to be aware of from the very start:

• Error code 5: ERROR_ACCESS_DENIED “Access is denied.”
• Error code 183: ERROR_ALREADY_EXISTS “Cannot create a file when that file already exists.”

ERROR_ALREADY_EXISTS is a documented behavior and is an indication of scenario that you do receive handle, but it is a handle to already existing object, not created.

The problem with not working OpenFileMapping is around its first argument: the API function expects values/flags from another enumeration, it takes FILE_MAP_* values and not PAGE_*. Incorrect argument results in failure to open you the mapping you want.