Memory mapped files optional write possible?

When using memory-mapped files it seems it is either read-only, or write-only. By this I mean you can't:

• have one open for writing, and later decide not to save it
• have open open for reading, and later decide to save it

Our application uses a writable memory-mapped file to save data files, but since the user might want to exit without saving changes, we have to use a temporary file which the user actually edits. When the user opts to save the changes, the original file is overwritten with the temporary file so it has the latest changes. This is cumbersome because the files can be very large (>1GB) and it takes a long time to copy them.

I've tried many combinations of the flags used to create the file mapping but none seem to allow the flexibility of saving on demand. Can anyone confirm this is the case? Our application is written in Delphi, but it uses the standard Windows API to create the mapping, in our case

FMapHandle := CreateFileMapping(FFileHandle, nil, PAGE_READWRITE, 0, 2 * 65536, nil);
FBasePointer := MapViewOfFile(FileMapHandle, FILE_MAP_WRITE, FileOffsetHigh,
FileOffsetLow, NumBytes);

I don't think you can. By that I mean you may be able to, but it doesn't make any sense to me :-)

The whole point of a memory-mapped file is that it's a window onto the actual file. If you don't wany changes reflected in the file, you'll probably have to do something like batch up the changes in a data structure (e.g., an array of base address, size and data) and apply them when saving.

In which case, you wouldn't actually need the memory mapped file, just read in and maintain the chunks you want to change (lock the file first if there's a chance of multi-user access).

Update:

Have you thought of the possibility of, when doing a save, deleting the original file and just renaming the temporary file to the original file name? That's likely to be much faster than copying 1G of data from temporary to original. That way, if you don't want it saved, just delete the temporary file and keep the original.

You'll still have to copy the original data to the temporary file when loading but you won't have to copy the temporary data back (whether you save it or not) - that would halve the time taken.