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winapimemory-mapped-files

Are MapViewOfFile memory mappings reused?

If I create 2 separate mappings of the same file in the same process will the pointers be shared?

in other words: 

LPCTSTR filename = //...

HANDLE file1 = CreateFile(filename, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, 0);
HANDLE fileMapping1 = CreateFileMapping(file1, NULL, PAGE_READONLY, 0, 0, 0);
void* pointer1 = MapViewOfFile(fileMapping1, FILE_MAP_READ, 0, 0, 0);

CloseHandle(fileMapping1);
CloseHandle(file1);

HANDLE file2 = CreateFile(filename, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, 0);
HANDLE fileMapping2 = CreateFileMapping(file2, NULL, PAGE_READONLY, 0, 0, 0);
void* pointer2 = MapViewOfFile(fileMapping2, FILE_MAP_READ, 0, 0, 0);

CloseHandle(fileMapping2);
CloseHandle(file2);

Will pointer1 ever be equal to pointer2?

The reason I am asking is that I have several threads that need to search in a large (300+MB) file and I want to use memory mapping for that. However the process needs to be able to run on an old 32bit xp machine, so if each thread allocated their own copy in virtual memory then I could run out of memory.

Solution

  • Will pointer1 ever be equal to pointer2?

    The pointers might be the equal in case MapViewOfFile chooses the same address for the mapping. You don't control this with MapViewOfFile, and you have some control over this with MapViewOfFileEx (last argument lpBaseAddress there).

    Each separate MapViewOfFile can create a new mapping over the same physical data, so OS does not need to map the file mapping into the same addresses even if you open two mappings simultaneously, preserving the coherence of data. It is easy to see this by modifying your code slightly:

    HANDLE file1 = CreateFile(filename, GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, 0, NULL);
HANDLE fileMapping1 = CreateFileMapping(file1, NULL, PAGE_READWRITE, 0, 0, 0);
void* pointer1 = MapViewOfFile(fileMapping1, FILE_MAP_READ | FILE_MAP_WRITE, 0, 0, 0);

//CloseHandle(fileMapping1);
//CloseHandle(file1);

HANDLE file2 = CreateFile(filename, GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, 0, NULL);
HANDLE fileMapping2 = CreateFileMapping(file2, NULL, PAGE_READWRITE, 0, 0, 0);
void* pointer2 = MapViewOfFile(fileMapping2, FILE_MAP_READ | FILE_MAP_WRITE, 0, 0, 0);

INT& n1 = *((INT*) pointer1);
INT& n2 = *((INT*) pointer2);

ATLASSERT(&n1 != &n2);  // The pointers are not equal even though they point 
            // the same data!
INT n3 = 0;
n1 = 2;
n3 += n2;
n1 = 3;
n3 += n2;
ATLASSERT(n3 == 5); // That's 2+3 we wrote through n1 and read through n2

//CloseHandle(fileMapping2);
//CloseHandle(file2);

    That is, pointer equivalence is not something you should expect or rely on. Especially if your mapping is large, and reopening does not take place immediately.