how to flush out cache.?

[Julian Elischer]

Ok so I have an application where I need to 
reread a file I have just written to ensure that it went to disk
correctly..

Other than reading a few GB of data, is there a way to flush
out the cache copy of a file I've written?

possibilities include:

a file flag saying "don't keep a copy after it's written to disk"?
a syscall discard_cached_blocks(fd);


?
any other suggestions?

julian
(BTW this would be for 4.x initially)