sessions & concurrency

Hi If I set up a session while generating a page, and I store some data in it, when does that data become available for other connections from the same user (same session)? I.e. If I have a complex page with some session data that is used to dynamically generate some of the images in the page, it is conceivably possible that the browser could receive the HTML and IMG tags for the images before the page itself finishes building. I'm thinking that the session data probably isnt written until the PHP script exits, and that there probably isnt any locking. Is this the case? I imagine that if this is true I could write my own session save/load handlers to install with session_set_save_handler() and handle locking/concurrency issues - if so would these work if written in C as a php extension? Any info appreciated, Thanks -- Shane