Memory mapped file C++

Question

please help me with reading memory mapped file. I open file in code below. And then i want to read bytes from 8 to 16. How can i do that?

// 0. Handle or create and handle file
m_hFile = CreateFile(file_path.c_str(), GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
if (m_hFile == INVALID_HANDLE_VALUE)
{
    if (GetLastError() == ERROR_FILE_NOT_FOUND)
    {
        m_hFile = createNewFile(file_path.c_str());
    }
    else throw GetLastError();
}

// 1. Create a file mapping object for the file
m_hMapFile = CreateFileMapping(m_hFile, NULL, PAGE_READWRITE, 0, 0, NULL);
if (m_hMapFile == NULL) throw GetLastError();

// 2. Map the view.
m_lpMapAddress = MapViewOfFile(m_hMapFile, FILE_MAP_ALL_ACCESS, 0, 0, 0);
// to map
if (m_lpMapAddress == NULL) throw GetLastError();

Is a dublicate from this one: [link](http://stackoverflow.com/questions/9889557/mapping-large-files-using-mapviewoffile). You have the pointer m_lpMapAddress, which is hopefully a byte pointer. Add 8 bytes to the pointer and read the memory. That's all — mrAtari, Aug 03 '16 at 09:36
@mrAtari: That won't work. `MapViewOfFile` returns `void*`. If the assignment to `m_lpMapAddress` works without a cast, then its type must be `void*` as well. You cannot use `operator+=` on a `void*`. You'll have to cast to a specific type (`unsigned char` for bytes) first. — IInspectable, Aug 03 '16 at 10:56
Have you ever heard about pointer casting? char* cursor = (char*)m_lpMapAddress; cursor+=8; — mrAtari, Aug 03 '16 at 10:59
@mrAtari: Given the fact, that I suggested you cast the `void*` to a specific type, it seems plausible, that I *"have [...] heard about [...] casting"*. What point are you trying to make? — IInspectable, Aug 03 '16 at 11:05

score 6 · Accepted Answer · answered Aug 03 '16 at 09:37

6

You can access it like any other memory block. Here's an example that prints those bytes interpreted as unsigned chars:

unsigned char *mappedDataAsUChars = (unsigned char*)m_lpMapAddress;

for(int k = 8; k < 16; k++)
    std::cout << "Byte at " << k << " is " << mappedDataAsUChars[k] << std::endl;

answered Aug 03 '16 at 09:37

user253751

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is there more beautiful soulution? What if i want read bytes in structure? – ExiD Aug 03 '16 at 09:56
1

@ExiD: A *"more beautiful"* way to access memory? None that I know of. More convoluted? Probably. But what exactly do you dislike about the canonical way to do it? – IInspectable Aug 03 '16 at 09:57
1

@ExiD You can easily do `MyStruct *mappedDataAsMyStruct = (MyStruct*)m_lpMapAddress;` or similar. Watch out for alignment that might make your structure not be laid out the way you want. – user253751 Aug 03 '16 at 09:59
1

i am sorry for my english. I think it may use memcpy, but i am not sure. – ExiD Aug 03 '16 at 10:02
@ExiD You can also use memcpy. – user253751 Aug 03 '16 at 10:03
Define "more beautiful", @ExiD. Beauty is in the eye of the beholder. FWIW, this is a very clean and simple solution. – Lightness Races in Orbit Aug 03 '16 at 10:05

Memory mapped file C++

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