Is transfer_at_least supposed to return success only if requested number of bytes transferred at once or at all?

http://www.boost.org/doc/libs/1_37_0/doc/html/boost_asio/reference/transfer_at_least.html
"Return a completion condition function object that indicates that a read or write operation *should continue until* a minimum number of bytes has been transferred, or until an error occurs."

For example, I want to receive 800 bytes and put them into streambuf using transfer_at_least(800) as completion condition, but I receive 2 chunks of 400 bytes, therefore I never get a success, because transfer_at_least_t always wants more data.

When 800 bytes are received (at once or by chunks) or error occurs, the completion handler is invoked - that's the effect of this completion condition. If you encounter different behaviour, could you please provide minimal code demonstrating this?