Question

I have a newbie question, I’ve never read files bigger than 4Mb with function like fopen, ReadFile(WINAPI). My question is What is the best way to read a big file like 400Mb load blocks of 512 bits into memory?

Thanks

Answer 1

First of all – at first instance – you may not want to read that small – a few kBytes is better when it comes from disk. In your case – a simple implementation could read 16k – and then cycle through those 32 times for your 512 bytes operations.

In reality that is not so important -as the OS is clever and generally will second guess you – and read more anyway. So a naive implementation where you simply do a

do 
{ 
  bResult = ReadFile(hFile, &inBuffer, nBytesToRead, &nBytesRead, NULL); 
  do-something-with-nBytesRead; 
} while(!bResult &&  nBytesRead != 0); 
if (nBytesRead) error..; 

sort of thing is fine.

If you know you are always going to have such (rather large) files, performance is important and there is other use of those files too – then consider ‘memory mapping’ the file – i.e. open it in such a way that the entire file appears in virtual memory. Have a look at http://msdn.microsoft.com/en-us/library/windows/desktop/aa366556(v=vs.85).aspx for that.