memory backed files

I want to write to a memory backed file.

I create a file -
I create a section with the file handle
I open a view on the section.

Question: It appears that once I've got a section, I just write to the mapped section. Then I close everything up and viola, stuff just shows up on the disk when the MM is done having its way with it.

Fine, how big is the file ? There is a MaximumSize arg to ZwCreateSection, and a flag SECTION_EXTEND_SIZE. But how do I guess at a size, then if turns out I did not need so much memory, how do I shrink it down? And conversely, If I need more, how do I grow it?

Thanks
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Gak