Truncating an open file under BSD 4.3

[Badri Lokanathan]

What is the best way to empty an already open write-only file under BSD 4.3?
I have an application where I need large amounts of alphanumeric data dumped
to a file which then must be periodically emptied. I also need to be
able to scan this data occasionally with a pager.

I do not want to close and reopen the file since open is an
expensive call (prof claims it to take about 15ms/call on a SUN
3/110.) I tried truncate but it leaves holes in the file, which make
it unreadable. I also could not find a way with fcntl, which surprises me.
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