http://www.freebsd.org/doc/en_US.ISO8859-1/books/developers-handbook/x86-portable-code.html

16.5.5 Using an Include File
If you are releasing your software as (or with) source code, you can use macros and place them in a separate file, which you include in your code.

Porters of your software will simply write a new include file. No library or external object file is necessary, yet your code is portable without any need to edit the code.

Note: This is the approach we will use throughout this chapter. We will name our include file system.inc, and add to it whenever we deal with a new system call.

We can start our system.inc by declaring the standard file descriptors:

%define stdin 0
%define stdout 1
%define stderr 2