How to use ReadFile() in c++ to read bytes of length that are not multiple of 512

Question

I am trying to read raw bytes to a drive. But the function ReadFile() only allowing me to read bytes of length 512 or its multiple. I cannot read bytes of length 10, 180,1000 etc.

DWORD NumberOfBytesRead=0;

ReadFile(hDevice, nullbuffer, (DWORD)512, &NumberOfBytesRead, (LPOVERLAPPED)NULL); //works

ReadFile(hDevice, nullbuffer, (DWORD)1024, &NumberOfBytesRead, (LPOVERLAPPED)NULL); //works

ReadFile(hDevice, nullbuffer, (DWORD)1000, &NumberOfBytesRead, (LPOVERLAPPED)NULL); //error 87

ReadFile(hDevice, nullbuffer, (DWORD)300, &NumberOfBytesRead, (LPOVERLAPPED)NULL);  //error 87

Is there some way to overcome this?

Answer 1

The rules for direct access of the device mandate that you must read aligned blocks of data. That's not something that you can change. It's a hard rule that you must follow.

Given this rule, you must read into a buffer that is a multiple of the required block size. If you wish only to access some part of that buffer, you can pick that part out after you have read from the device.