too complicated!

How about a simple buffer-based stream that stores

pointers into the buffer itself:

bufstart
bufend
bufcur

position in file that bufstart corresponds to

file_offset

API

void seekg(pos, way)
if (pos >= file_offset && <= bufend - bufstart)
bufcur = bufstart + (pos - file_offset)
else {
     file_offset = pos;
     fseek(file_offset, SEEK_BEG);
     fread(file, bufstart, bufsize);
     bufcur = bufstart;
     }
    
read(target, size){
     remain = bufend - bufcur;
     
     if (size < remain){
     memcpy(target, bufcur, size);
}
    else {
    memcpy(target, bufcur, remain);
    seekg(remain, cur);
    read(target+remain, size - remain);
}


tellg(){
    return file_offset + bufcur - bufstart;
}

                        












calling seekg causes buffer reload from disk even if the seek destination
is within the buffer range



  template<typename _CharT, typename _Traits>
    basic_istream<_CharT, _Traits>&
    basic_istream<_CharT, _Traits>::
    seekg(off_type __off, ios_base::seekdir __dir)
    {
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // DR60.  Do not change _M_gcount.
      ios_base::iostate __err = ios_base::iostate(ios_base::goodbit);
      try
	{
	  if (!this->fail())
	    {
	      // 136.  seekp, seekg setting wrong streams?
	      const pos_type __p = this->rdbuf()->pubseekoff(__off, __dir,
							     ios_base::in);
	      
	      // 129.  Need error indication from seekp() and seekg()
	      if (__p == pos_type(off_type(-1)))
		__err |= ios_base::failbit;
	    }
	}
      catch(__cxxabiv1::__forced_unwind&)
	{
	  this->_M_setstate(ios_base::badbit);
	  __throw_exception_again;
	}
      catch(...)
	{ this->_M_setstate(ios_base::badbit); }
      if (__err)
	this->setstate(__err);
      return *this;
    }




from streambuf

      pos_type 
      pubseekoff(off_type __off, ios_base::seekdir __way, 
		 ios_base::openmode __mode = ios_base::in | ios_base::out)
      { return this->seekoff(__off, __way, __mode); }


from streambuf:  seekoff is virtual and overridden by filebuf

      virtual pos_type 
      seekoff(off_type, ios_base::seekdir,
	      ios_base::openmode /*__mode*/ = ios_base::in | ios_base::out)
      { return pos_type(off_type(-1)); } 


from filebuf: seekoff is also virtual
      virtual pos_type
      seekoff(off_type __off, ios_base::seekdir __way,
	      ios_base::openmode __mode = ios_base::in | ios_base::out);


from filebuf: seekoff has a substantial body even though it's virtual...

  // According to 27.8.1.4 p11 - 13, seekoff should ignore the last
  // argument (of type openmode).
  template<typename _CharT, typename _Traits>
    typename basic_filebuf<_CharT, _Traits>::pos_type
    basic_filebuf<_CharT, _Traits>::
    seekoff(off_type __off, ios_base::seekdir __way, ios_base::openmode)
    {
      int __width = 0;
      if (_M_codecvt)
	__width = _M_codecvt->encoding();
      if (__width < 0)
	__width = 0;

      pos_type __ret =  pos_type(off_type(-1));
      const bool __testfail = __off != 0 && __width <= 0;
      if (this->is_open() && !__testfail)
	{
	  // Ditch any pback buffers to avoid confusion.
	  _M_destroy_pback();

	  // Correct state at destination. Note that this is the correct
	  // state for the current position during output, because
	  // codecvt::unshift() returns the state to the initial state.
	  // This is also the correct state at the end of the file because
	  // an unshift sequence should have been written at the end.
	  __state_type __state = _M_state_beg;
	  off_type __computed_off = __off * __width;
	  if (_M_reading && __way == ios_base::cur)
	    {
	      if (_M_codecvt->always_noconv())
		__computed_off += this->gptr() - this->egptr();
	      else
		{
		  // Calculate offset from _M_ext_buf that corresponds
		  // to gptr(). Note: uses _M_state_last, which
		  // corresponds to eback().
		  const int __gptr_off =
		    _M_codecvt->length(_M_state_last, _M_ext_buf, _M_ext_next,
				       this->gptr() - this->eback());
		  __computed_off += _M_ext_buf + __gptr_off - _M_ext_end;

		  // _M_state_last is modified by codecvt::length() so
		  // it now corresponds to gptr().
		  __state = _M_state_last;
		}
	    }
	  __ret = _M_seek(__computed_off, __way, __state);
	}
      return __ret;
    }



from fstream.tcc: _M_seek seems to be the culprit.  The call to
_M_set_buffer(-1) resets _M_in_beg, _M_in_cur, and _M_in_end to
the same value, which means any read() call later will trigger a
re-read from disk to reload the entire buffer


  template<typename _CharT, typename _Traits>
    typename basic_filebuf<_CharT, _Traits>::pos_type
    basic_filebuf<_CharT, _Traits>::
    _M_seek(off_type __off, ios_base::seekdir __way, __state_type __state)
    {
      pos_type __ret = pos_type(off_type(-1));
      if (_M_terminate_output())
	{
	  // Returns pos_type(off_type(-1)) in case of failure.
	  __ret = pos_type(_M_file.seekoff(__off, __way));
	  if (__ret != pos_type(off_type(-1)))
	    {
	      _M_reading = false;
	      _M_writing = false;
	      _M_ext_next = _M_ext_end = _M_ext_buf;
	      _M_set_buffer(-1);
	      _M_state_cur = __state;
	      __ret.state(_M_state_cur);
	    }
	}
      return __ret;
    }