4.2.a. There shall always be precisely one header file for each source file and they shall always have the same root name.

4.2.b. Each header file shall contain a preprocessor guard against multiple inclusion, as shown in the example below.6

4.2.c. The header file shall identify only the procedures, constants, and data types (via prototypes or macros, #define, and typedefs, respectively) about which it is strictly necessary for other modules to be informed.

           i. It is a preferred practice that no variable ever be declared (via extern) in a header file.

          ii. No storage for any variable shall be allocated in a header file.

4.2.d. No public header file shall contain a #include of any private header file.


#ifndef ADC_H
#define ADC_H
#endif /* ADC_H */

Reasoning: The C language standard gives all variables and functions global scope by default. The downside of this is unnecessary (and dangerous) coupling between modules. To reduce inter-module coupling, keep as many procedures, constants, data types, and variables as possible privately hidden within a module’s source file.

   See also What Belongs in a C .h Header File?:

Enforcement: These rules shall be enforced during code reviews.


[6] The preprocessor directive “#pragma once” has the same purpose but is non-portable.