Read some data from the handle at the specified offset.
template< typename MutableBufferSequence> std::size_t read_some_at( boost::uint64_t offset, const MutableBufferSequence & buffers); template< typename MutableBufferSequence> std::size_t read_some_at( boost::uint64_t offset, const MutableBufferSequence & buffers, asio::error_code & ec);