help: howto prealloc a large file in disk?

[reviver]

Hi, everyone
I want to process a large image(maybe 1G), and write the processed result into an output file, but the procession is not a sequential one, so i maybe start to write from the EOF. My question is how to pre-allocate the required space in disk so i can write anywhere in the file when needed. I have tried using WriteFile API but it is too slow, it cost 55 milliseconds to wirte a 1G file , does anyone has better solutions?
When i use bittorrent, I notice that its pre-allocation is very fast, perhaps less than 1s.
Any comments will be appreciated
Thanks for helping!

[Scrambler]

Try following sequence:

SetFilePointer(hFile, 1024*1024*1024, NULL, FILE_BEGIN);
SetEndOfFile(hFile);
SetFilePointer(hFile, 0, NULL, FILE_BEGIN);

I don't checked this, but I think that it should work.

[reviver]

Thanks! it works.
I thought SetFilePointer can only move pointer in the actual range before.
Thanks again!