684: Mainline

The following general rules will help you to keep global variable declarations and function declarations properly separated from their definitions:

• You must define global variables and functions only once. Accordingly, you should keep definitions out of header files, which are likely to be included into many source-code files.
• You can declare global variables (using extern) and functions (using function prototypes) over and over, as long as each declaration is the same. Accordingly, you can place declarations in header files that you use to inform many source-code files about global variables and functions.