Using ifstream to move data to and from disk

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VC6

C++

I'm trying to write a program that moves doubles out of memory onto disk for later recovery and processing. A typical file size is around 30GBytes. I'm using ofstream for binary output and ifstream for binary retrieval.

The problem I've encountered is with the maximum size of the types "steamsize" and "ifstream::pos_type" both of which, being variants of int, are too small to manage file transfers of the size needed. Outputting to disk doesn't seem to be a problem but reading blocks of the binary file from disk means that the read byte position in the file is too large for the default "pos_type" and the size of the block read back is too large for "streamsize".

Can anyone tell me if it's possible to alter the type def of "pos_type" and "streamsize" within ifstream and ofstream to __int64 and if so how it can be done? Are there any other methods of moving blocks of say 30Mb from within 30GB of binary stuff on disk to memory fast?

Any help would be very much appreciated.
David G