[NCLUG] Need to write to non-owned file

[bsimpson at att.net]

I am writing a C++ application that must read a file, alter it,
then write it back out.  This file may be owned by another user,
and be writable only by that user.  In this case I display a
popup to enter that user's password.  Once I have that password,
how do I write to that user's file?  In a shell environment, it's
easy to switch users.  I need to do that within an application.

Brian S