Complying with standard C

Updating existing code

If you have an existing application and want the benefits of function prototypes, there are several possibilities for updating, depending on how much of the code you care to change:

Recompile without making any changes.
Even with no coding changes, the compiler will warn you about mismatches in parameter type and number when invoked with the -v option.
Add function prototypes just to the headers.
All calls to global functions are covered.
Add function prototypes to the headers and start each source file with function prototypes for its local (static) functions.
All calls to functions are covered, but this requires typing the interface for each local function twice in the source file.
Change all function declarations and definitions to use function prototypes.

For most programmers, choices 2 and 3 are probably the best cost/benefit compromise. Unfortunately, these options are precisely the ones that require detailed knowledge of the rules for mixing of old and new styles.