That's the idiom I've always seen used (seek then truncate), which is why I recommended it back in my post.

Consulting APUE (well, technically the 1st edition of the same tome; pp 91-92), the discussion there indicates that while SVR4's truncate(2) could be used to extend a file (creating a hole) the 4.3+BSD version wouldn't extend the size of the file. So while seeking then calling truncate is possibly more effort than strictly necessary, it's probably more likely to work regardless of the underlying OS' truncate(2) semantics.