C++: Reading / getting data from a memory mapped file

Question

I have used CreateFileMapping and MapViewOfFile to map a file under Window using C++ VS2010.

The only way to get / read data from this mapped file that I know is to use MemCpy. However, I was hoping that there might be a faster / more direct way.

Is there one? If yes, could somebody please post a sample?

Thank you!

Memcpy() requires a pointer to the view. Just use that pointer directly. This quacks like an XY question. You could only ask this if you have trouble with arbitrating access to the view. A named semaphore is required to prevent your app from accessing the view while the other process is updating it. Life is a lot easier with a pipe. — Hans Passant
– Hans Passant, Commented Sep 18, 2013 at 14:47

aah134 · Accepted Answer · 2013-09-19 13:33:42Z

you may cast the memory block to a data struct, as a pointer,

struct someStruct* data = (struct someStruct*)memAddress;

then you can access the data as a pointer

somefuction (data->var1, data->var2);

or

sum = data->var1 + data->var2;

you will have to be carful, and sure that the mapping struct matches the memory block, or you gonna get some junk

something like this

struct msgStructureCommon { unsigned int messageID; unsigned int messageSize; char bufferLimit[1024-(2*sizeof(unsigned int))]; };

struct msgStructureID911 { unsigned int messageID; unsigned int messageSize; //someData.... }; struct msgStructureID2013 { unsigned int messageID; unsigned int messageSize; //someData.... };

// in main

char* buffer = receiveData(socket); struct msgStructureCommon* msgRCV = (struct msgStructureCommon*)buffer; std::cout<< msgRCV->messageID << std::endl; if(msgRCV->messageID == 911) { struct msgStructureID911* msg911 = (struct msgStructureID911*)buffer; if(msg911->messageSize >= MSG_LIMIT_SIZE_911) { std::cout<< msg911->someData<< std::endl; } else { std::cout<< "ops, message is not correct!" << std::endl; } } else if (msgRCV->messageID == 2013) { //... }

Thank you. Could perhaps also post a sample or edit your sample so that I can define the byte position and the size of the struct or so?

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