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C Preprocessor Cheatsheet

A comprehensive cheat sheet for the C preprocessor, covering directives, macros, conditional compilation, and more. This guide provides a quick reference for developers working with C and C++.

Directives Overview

Include Directives

#include <file.h>

Searches standard system directories for file.h.

#include "file.h"

Searches the current directory first, then standard system directories.

#include MACRO

If MACRO expands to <...> or "...", the include directive is processed accordingly.

Example:

 
#define HEADER_FILE "my_header.h"
#include HEADER_FILE

Define and Undefine

#define SYMBOL

Defines a simple symbol. Commonly used for conditional compilation.

#define SYMBOL value

Defines a symbol with a value. The value can be any text.

#undef SYMBOL

Undefines a previously defined symbol.

Example:

 
#define PI 3.14159
#undef PI

Conditional Compilation 

#ifdef SYMBOL
  // Code to compile if SYMBOL is defined
#endif

#ifndef SYMBOL
  // Code to compile if SYMBOL is not defined
#endif

#if expression
  // Code to compile if expression is true
#elif expression2
  // Code to compile if expression2 is true
#else
  // Code to compile if none of the above are true
#endif

#if defined(SYMBOL)
  // Code to compile if SYMBOL is defined
#endif

Macros in Detail

Basic Macros

#define MACRO(arg) ((arg) * 2)

A simple macro that doubles its argument.

Example Usage:

 
int x = 5;
int y = MACRO(x); // y will be 10

Important:

Always enclose macro arguments in parentheses to avoid unexpected behavior due to operator precedence.

Stringification Operator (#)

#define STRINGIFY(x) #x

Converts a macro parameter into a string literal.

Example:

 
STRINGIFY(hello world) // expands to "hello world"

Token Pasting Operator (##)

#define CONCAT(x, y) x ## y

Concatenates two tokens into a single token.

Example:

 
CONCAT(var, 123) // creates a single token var123

Variadic Macros

#define LOG(format, ...) printf(format, __VA_ARGS__)

Creates macros that accept a variable number of arguments.

Example:

 
LOG("Error: %d", errno); // Prints an error message with the error number

Predefined Macros

Standard Predefined Macros

__FILE__

The name of the current input file, as a string literal.

__LINE__

The current line number in the input file, as a decimal integer.

__DATE__

The date of compilation, as a string literal in “Mmm dd yyyy” format.

__TIME__

The time of compilation, as a string literal in “hh:mm:ss” format.

__STDC__

Indicates that the implementation conforms to the C standard. Defined as 1.

Example Usage:

 
printf("File: %s, Line: %d\n", __FILE__, __LINE__);

Compiler-Specific Macros

Compilers often define their own macros (e.g., __GNUC__ for GCC, _MSC_VER for Microsoft Visual C++).

These can be used for compiler-specific conditional compilation.

Example:

 
#ifdef __GNUC__
  // GCC specific code
#endif

Advanced Directives and Techniques

Error and Warning Directives

#error message

Causes the preprocessor to issue a fatal error message and halt compilation.

#warning message

Causes the preprocessor to issue a warning message, but compilation continues.

Example:

 
#if !defined(DEBUG)
  #error "DEBUG must be defined for this build."
#endif

Pragma Directive

#pragma directive

Specifies various compiler-specific instructions. Behavior varies between compilers.

Common Usage:

#pragma once - Prevents a header file from being included more than once in a single compilation unit.

Line Control

#line number "filename"

Resets the line number and filename reported by the compiler for subsequent code.

Example:

 
#line 100 "newfile.c"
// The next line will be reported as line 100 of newfile.c
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