[boost] [iostreams] input_sequence() questions

I am attempting to adapt a windowed mmap reader, one which manages its mapping window using iterators, to a boost::iostreams direct source. I wrote this for reading large files via mmap on 32-bit platforms. The documentation shows that input_sequence() returns a pair of char pointers, not iterators. Is that really true or am I misreading the docs? http://www.boost.org/doc/libs/1_41_0/libs/iostreams/doc/concepts/source.html And it also shows that the input_sequence pointers are non-const. Why is that required for sources? Thanks, Rob