/* switch to _view_write. normally, this is a bad idea since your changes will not
show up in _view_private if there have been changes there; thus the leading underscore
as a tad of a "warning". but useful when done with some care, such as during
initialization.
*/
void* MongoMMF::_switchToWritableView(void *p) {
size_t ofs;
MongoMMF *f = privateViews.find(p, ofs);
assert( f );
return (((char *)f->_view_write)+ofs);
}