The different levels of portability:

1. Language
2. Standard Library
3. POSIX
4. 3rd party libraries
5. OS

The more features a program needs, the further out it must reach through the layers.#

Avoid – or wrap – compiler language extensions

C though portable is not suitable as much work as to be done to ensures it works on multiple systems.

In order to use OS extras, use an interface to reimplement it in systems that does not support it.