Preprocessor Directives

Before the compiler parses C, the preprocessor handles directives beginning with #. It includes headers, expands macros, and selects conditional source. Because macros perform token substitution rather than typed function calls, use them deliberately.

Include headers

#include <stdio.h>       // implementation search path
#include "project/io.h" // usually search project path first

A header should declare a cohesive public interface and be safe to include more than once. Use a portable include guard:

#ifndef PROJECT_BUFFER_H
#define PROJECT_BUFFER_H

#include <stddef.h>
size_t buffer_capacity(void);

#endif

Headers should include what their own declarations require. Do not rely on another header being included first.

Object-like and function-like macros

#define APP_VERSION "2.1.0"
#define ARRAY_COUNT(array) (sizeof(array) / sizeof((array)[0]))

Parenthesize each parameter and the complete replacement expression:

#define SQUARE(value) ((value) * (value))

Even this macro evaluates its argument twice, so SQUARE(i++) is wrong. Prefer a real function or static inline function when type checking and single evaluation matter.

The # operator stringifies a macro parameter; ## pastes tokens. These are useful for carefully designed logging, testing, and code-generation utilities, but can make diagnostics harder.

Conditional compilation

#if defined(_WIN32)
    #include <windows.h>
#elif defined(__unix__)
    #include <unistd.h>
#else
    #error Unsupported platform
#endif

Use build-system feature checks rather than assuming platforms from names where possible. #if DEBUG tests a numeric macro; #ifdef DEBUG tests only whether it is defined.

Built-in context and diagnostics

__FILE__ and __LINE__ help diagnostics. #error stops translation with a message. #pragma is implementation-defined, though standardized #pragma STDC forms and widely supported pragmas exist.

Keep macros uppercase when they behave like constants or syntax, keep their scope small, and never use a macro merely to save a few typed lines.