how should a daemon drop privileges in a PAM-compatible way?

[Eric Cooper <ecc@cmu.edu>]

I wrote a daemon that is started from an init-script as root, and then
uses setuid and setgid to drop to a less-privileged user & group.

A user discovered that the program breaks when he uses the
libpam-tmpdir module, because TMPDIR doesn't get changed to the
/tmp/user/NNN directory, so the daemon tries to create files in /tmp
without permission.

So, what is the correct way to do this?  Is there a high level
function to "change userid, groupid and do the related PAM things"
that I can use, or an example program to copy?  Thanks for any pointers.

-- 
Eric Cooper             e c c @ c m u . e d u