unix - Truncating a file while it's being used (Linux)

Question

I have a process that's writing a lot of data to stdout, which I'm redirecting to a log file. I'd like to limit the size of the file by occasionally copying the current file to a new name and truncating it.

My usual techniques of truncating a file, like

cp /dev/null file

don't work, presumably because the process is using it.

Is there some way I can truncate the file? Or delete it and somehow associate the process' stdout with a new file?

FWIW, it's a third party product that I can't modify to change its logging model.

EDIT redirecting over the file seems to have the same issue as the copy above - the file returns to its previous size next time it's written to:

ls -l sample.log ; echo > sample.log ; ls -l sample.log ; sleep 10 ; ls -l sample.log
-rw-rw-r-- 1 user group 1291999 Jun 11  2009 sample.log
-rw-rw-r-- 1 user group 1 Jun 11  2009 sample.log
-rw-rw-r-- 1 user group 1292311 Jun 11  2009 sample.log

Answer 1

As of coreutils 7.0, there is a truncate command.