OT: Programming portability

[Chris Zakelj]

I'm curious as to how programs actually get ported from one OS to 
another, and if certain directions are easier than others.  That is, how 
does one figure out what needs to be changed in order to make OpenNTPD 
work on Linux?  Is it generally easier to move a program from $some_bsd 
to $some_other_os, or from $some_other_os to $some_bsd?  How would you 
even begin to port something like OpenSSH to a non-Unix system like 
Windows?  Does the chosen language (C, C++, Java, etc) make a difference 
in difficulty?  When I've built from ports, I can see make files doing 
OS detection, but from there (not being a very good coder), I can't 
really make out how it changes the code based on that.  Any 
recommendations for "casual programmer" books would be cool...  I'm not 
interested in becoming a kernel hacker or driver programmer, but at 
least being able to understand what's going on would be nice.