C Reference - Preprocessor directives

Directive Syntax	Directive Description
File inclusion
#include <filename.h>	replace directive with file from C library path
#include "filename.h"	replace directive with file from source path
Macro definition
#define name text	replace every occurrence of name with text
#define name(args) text	replace every occurrence of name with argument with text
#undef name	suspends macro substitution
Conditional compilation
#if expression	if expression is true compile section
#ifdef name #if defined(name)	if name is defined compile section
#ifndef name #if !defined(name)	if name is not defined compile section
#elif	else if expression is true compile section
#else	else compile section
#endif	terminates #if block
\	continue definition in next line
##	parameters concatenation
Other directives
#pragma token	do compiler dependent action
#error token	write diagnostic message
/* text */	comment
// text	line comment (C++ feature - not ANSI C)

 

---

#include

Syntax:

#include <filename.h> #include "filename.h"

Description:

Include header file instead of directive.

Example:

#include <stdio.h> #include "my_defs.h"   int main() {    printf("Example"); }

 

---

#define

Syntax:

#define name text #define name(args) text

Description:

Replace every occurrence of name with text

Example:

#define DEBUG #define C       3.0e8 #define abs(X)  ((X)>0 ? (X) : -(X)

 

---

#undef

Syntax:

#undef name

Description:

Suspends macro substitution of name

Example:

#undef C #undef abs

 

---

#if

Syntax:

#if name

Description:

If expression is true compile section

Example:

#define DEBUG 1 #if DEBUG    :    code will be compiled when    DEBUG is 1    : #endif

 

---

#ifdef

Syntax:

#ifdef name

Description:

If name is defined compile section

Example:

#define DEBUG #ifdef DEBUG    :    code will be compiled when    DEBUG is defined    : #endif

 

---

#ifndef

Syntax:

#ifndef name

Description:

If name is not defined compile section

Example:

#define VER2 #ifndef VER2    :    code will be compiled when    VER2 is not defined    : #endif

 

---

#elif

Syntax:

#if name    : #elif name    : #endif

Description:

Else if expression is true compile section

Example:

#define VER1 0 #define VER2 1 #if VER1    :      //code will not be compiled #elif VER2    :      //code will be compiled #endif

 

---

#else

Syntax:

#if name    : #else    : #endif

Description:

Else compile section

Example:

#define VER 0 #if VER    :      //code will not be compiled #else    :      //code will be compiled #endif

 

---

#endif

Syntax:

#if name    : #elseif name    : #else    : #endif

Description:

Terminates #if block

Example:

#if VER    : #endif

 

---

\

Syntax:

#define name text \              text \              text

Description:

Continue definition in next line

Example:

#define X { {1,2}, \             {2,4}, \             {3,6}  }

 

---

##

Syntax:

#define name text(arg) arg##text

Description:

Continue definition in next line

Example:

#define con(X,Y) X##Y

 

---

/* */

Syntax:

/* text */

Description:

Text comment

Example:

/*   This is   a comment */   /* and also this is a comment */

 

---

//

Syntax:

// text

Description:

Line comment (non ANSI C - C++ feature)

Example:

// This is a comment // and also this is a comment