C Preprocessors

Preprocessor in C language, is a text substitution tool. They instruct the C compiler to do the required pre-processing before actual program compilation.

Here the following table lists all the important preprocessor directives available in C language:

C Preprocessors Examples

Read the following code fragments carefully to understand the various types of preprocessor directives one by one.

#define

The #define directive defines an identifier and a character sequence that will be substituted for the identifier each time it is encountered in the source file. Here is an example:

#define MAX 100

#error

The #error directive focuses the compiler to stop compilation. It primarily used for debugging purposes. Here is the general form of the #error directive:

#error error-message

Here error-message is not between double quotes.

#include

The #include directive tells the compiler to read another source file in addition to the one that contains the #include directive.

#include<stdio.h>
#include "myheader.h"

#undef and #define

#undef  FILE_SIZE
#define FILE_SIZE 42

This tells the preprocessor to un-define an existing FILE_SIZE and define it as 42

#ifndef, #define and #endif

#ifndef PRINTMESSAGE
   #define PRINTMESSAGE "You wish!"
#endif

This tells the preprocessor to define PRINTMESSAGE only if PRINTMESSAGE isn't already defined

#ifdef DEBUG
   /* Your debugging statements here */
#endif

This tells the preprocessor to do the process the statements enclosed if DEBUG is defined. This is useful if you pass the -DDEBUG flag to gcc compiler at the time of compilation. This will define DEBUG, so you can turn debugging on and off on the fly during compilation.

C Predefined Macros

Here is an example program, uses predefined macros available in C language:

#include<stdio.h>
#include<conio.h>

void main()
{
   clrscr();

   printf("File : %s\n", __FILE__ );
   printf("Date : %s\n", __DATE__ );
   printf("Time : %s\n", __TIME__ );
   printf("Line : %d\n", __LINE__ );
   
   getch();
}

Above C program will produce the following output:

C Preprocessor Operators

The C preprocessor offers following operators to help you in creating macros:

C Macro Continuation (\)

You can use macro continuation (\) to continue your macro definition in more than one line. Here is an example:

#define  message_for(a, b)  \
    printf(#a " and " #b ": We love you!\n")

C Stringize (#)

The stringize or number-sign operator ('#'), when used within a macro definition, converts a macro parameter into a string constant. Here is an example:

#include<stdio.h>
#include<conio.h>

#define  message_for(a, b)  \
    printf("Hello, " #a ".\nWelcome to " #b )

void main(void)
{
   clrscr();
   message_for(Sir, codescracker.com);
   getch();

}

Here is the output of this C program:

The defined() Operator

The preprocessor defined operator in C, used in constant expressions to determine if an identifier is defined using the #define. Here is an example:

#include<stdio.h>
#include<conio.h>

#if !defined (PRINTMESSAGE)
   #define PRINTMESSAGE "Welcome to codescracker.com"
#endif

void main(void)
{
   clrscr();
   printf("Here is the message:\n%s\n", PRINTMESSAGE);
   getch();
}

Here is the sample output of the above C program:

C Parameterized Macros

Preprocessor is the ability to simulate functions using parameterized macros. Here is an example:

int square(int num)
{
   return num * num;
}

We can rewrite above code using a macro like this:

#define square(num) ((num) * (num))

C Parameterized Macro Example

Here is an example, uses parameterized macro in C:

/* C Preprocessor - C Parameterized Macro - Example Program */
      
#include<stdio.h>
#include<conio.h>

#define FINDMAX(num1, num2) ((num1) > (num2) ? (num1) : (num2))

void main(void)
{
   clrscr();
   printf("Max between 200 and 100 is %d\n", FINDMAX(100, 200));
   getch();
}

Here is the sample output produced by the above C program:

C Online Test