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Chapter 16: Create and include header files

Introduction

In modern C, header files are crucial tools that must be designed and used correctly. They allow
the compiler to cross-check independently compiled parts of a program.

Headers declare types, functions, macros etc that are needed by the consumers of a set of
facilities. All the code that uses any of those facilities includes the header. All the code that defines
those facilities includes the header. This allows the compiler to check that the uses and definitions
match.

Examples

Introduction

There are a number of guidelines to follow when creating and using header files in a C project:

• Idemopotence

If a header file is included multiple times in a translation unit (TU), it should not break builds.

• Self-containment

If you need the facilities declared in a header file, you should not have to include any other
headers explicitly.

• Minimality

You should not be able to remove any information from a header without causing builds to
fail.

• Include What You Use (IWYU)

Of more concern to C++ than C, but nevertheless important in C too. If the code in a TU (call
it code.c) directly uses the features declared by a header (call it "headerA.h"), then code.c
should #include "headerA.h" directly, even if the TU includes another header (call it
"headerB.h") that happens, at the moment, to include "headerA.h".

Occasionally, there might be good enough reasons to break one or more of these guidelines, but
you should both be aware that you are breaking the rule and be aware of the consequences of
doing so before you break it.

Idempotence

If a particular header file is included more than once in a translation unit (TU), there should not be
any compilation problems. This is termed 'idempotence'; your headers should be idempotent.

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