Page 129 - C-Language

So, the double quoted form may look in more places than the angle-bracketed form. The standard
specifies by example that the standard headers should be included in angle-brackets, even though
the compilation works if you use double quotes instead. Similarly, standards such as POSIX use
the angle-bracketed format — and you should too. Reserve double-quoted headers for headers
defined by the project. For externally-defined headers (including headers from other projects your
project relies on), the angle-bracket notation is most appropriate.

Note that there should be a space between #include and the header, even though the compilers
will accept no space there. Spaces are cheap.

A number of projects use a notation such as:

#include <openssl/ssl.h>
#include <sys/stat.h>
#include <linux/kernel.h>

You should consider whether to use that namespace control in your project (it is quite probably a
good idea). You should steer clear of the names used by existing projects (in particular, both sys
and linux would be bad choices).

If you use this, your code should be careful and consistent in the use of the notation.

Do not use #include "../include/header.h" notation.

Header files should seldom if ever define variables. Although you will keep global variables to a
minimum, if you need a global variable, you will declare it in a header, and define it in one suitable
source file, and that source file will include the header to cross-check the declaration and
definition, and all source files that use the variable will use the header to declare it.

Corollary: you will not declare global variables in a source file — a source file will only contain
definitions.

Header files should seldom declare static functions, with the notable exception of static inline
functions which will be defined in headers if the function is needed in more than one source file.

• Source files define global variables, and global functions.
• Source files do not declare the existence of global variables or functions; they include the
header that declares the variable or function.
• Header files declare global variable and functions (and types and other supporting material).
• Header files do not define variables or any functions except (static) inline functions.

Cross-references

• Where to document functions in C?
• List of standard header files in C and C++
• Is inline without static or extern ever useful in C99?
• How do I use extern to share variables between source files?

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