currentpos initially 0

READ                                                WRITE
-----------------------------------------------------------------
                        get filesize
                                                set realSize

seekg:                                          seekp:
(reducelength()?)
reduce nTodo (pos) to                           pos > fileSize ->
# bytes to fileSize                             enlarge fileSize (setwrite)
-> EOF if nTdoDo
(setRead)

                        map:
                        fd = open
                        cpt offet, bufSize
                        bufPos = pos - offset
                        d_cp = mmap(bufSize, offset)

                                                
imapbuf:                                        omapbuf:
setg...                                         setp...
                                                maybe enlarge realSize

                                                at destruction:
                                                resize file to realSize
