Run through some list messages, and I'm curious about something:

is it possible to alter a file in such a way as to introduce sparse
file holes in a normal file? I know, to create a sparse file you just
seek past the end and then write something. But what about a file
(say, a database file) that's become mostly nulls and is taking up an
inordinate amount of space? The brutish way to fix that would be to
write out the file again. Is there something with a bit more elegance?
Some libc/system call I'm missing?

thanks,
Marc