manually truncating histfile?

"Anthony Heading" <[email protected]>
Newsgroups gmane.comp.shells.zsh.user
Message-ID <[email protected]> 
I have SAVEHIST=0x4000000 to prevent the history file ever being automatically truncated before e.g. I can archive it.

What's the best way then to explicitly truncate?  If I run a (non-interactive?) shell script with SAVEHIST set to a lower value, I'm not sure what triggers a rewrite.   Alternatively I could flock the histfile and do it manually, but I'm also not sure if flock is the appropriate lock on all platforms.
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