[Boost-users] [iostreams] Flushing a zlib_compressor... how to obtain a “zlib sync flush” ?

Subject: [Boost-users] [iostreams] Flushing a zlib_compressor... how to obtain a “zlib sync flush” ?
From: Tim Day (timday_at_[hidden])
Date: 2010-03-18 08:31:10

Is there some magic required to obtain a "zlib sync flush" when using
boost::iostreams::zlib_compressor ? Just invoking flush on the filter,
or strict_sync on a filtering_ostream containing it doesn't see to do
the job (ie I want the compressor to flush enough that the decompressor
can recover all the bytes consumed by the compressor so far, without
closing the stream; I remember using such a feature when driving zlib
directly in the past, and there's a page
  http://www.bolet.org/~pornin/deflate-flush.html 
describing it nicely).

Looking at the header, there seem to be some "flush codes" defined
(notably a sync_flush) but it's unclear to me how they should be used
(bearing in mind my compressor is just added into a filtering_ostream's
chain, although I do have a reference to it if some specific method
needs invoking).

Thanks for any tips.
Tim