Rules:
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.
Example:
#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?:
embeddedgurus.com/barr-code/2010/11/what-belongs-in-a-c-h-header-file/
Enforcement: These rules shall be enforced during code reviews.
Footnotes
[6] The preprocessor directive “#pragma once” has the same purpose but is non-portable.