redirect user I/O to a temporary buffer space

How can we redirect user I/O to a temporary buffer space so that all changes are written to the buffer space during the current session. When the computer is rebooted, the I/O reads should be
redirected back to the original file location and all changes made in the previous session
are discarded.(ie. after logg off/reboot)
[333 byte] By [rohitdivas] at [2007-11-18 1:39:59]