This document discusses preprocessor directives in C. The preprocessor processes source code before it is passed to the compiler. There are three types of preprocessor directives: macro substitution directives like #define that replace identifiers with predefined strings; file inclusion directives like #include that incorporate external files; and compiler control directives like #pragma that provide instructions to the compiler. Macro substitution allows identifiers to be replaced by strings, files can be included to reuse code, and additional directives provide functionality like conditional compilation.

2. PRE-PROCESSOR DIRECTIVES IN C
BY
JOHN JOSE

3. INTRODUCTION
• One of the unique feature of C language is
preprocessor.
• The pre processor is a program that processes the
source code before it passes through compiler
• Preprocessor directives are divided into three
categories,
1.Macro substitution directives
2.file inclusion directives
3.compiler control directives

4. Macro Substitution:
• Macro substitution is a process where an identifier
in a program replaced by predefined string
composed of one or more tokens
• It is achieved by #define directives

5. Examples
• #define PI 3.142
• #define TRUE 1
• #define AND &&
• #define LESSTHAN <
• #define MESSAGE "welcome to C"

6. Example(program by programmer)
#include<stdio.h>
#define LESSTHAN <
int main()
{
int a = 30;
if(a LESSTHAN 40)
printf("a is Smaller");
return(0);
}

7. Program is processed by pre-processor
int main()
{
int a = 30;
if(a < 40)
printf("a is Smaller");
return(0); }
Program is processed by compiler
a is Smaller

8. FILE INCLUSSION
• An external file containing functions or macro
definition can be included as a part of program so
that we need not rewrite those function or macro
definition.
• This is achieved by #include directives

9. Example
• Step1 : Type this Code
• In this Code write only function definition as you
write in General C Program
int add(int a,int b)
{
return(a+b);
}

10. Step 2
• Save Above Code with [.h ] Extension .
• Let name of our header file be myhead [ myhead.h ]
• Compile Code if required.

11. Step 3 : Write Main Program
#include<stdio.h>
#include"myhead.h"
void main()
{
int number1=10,number2=10,number3;
number3 = add(number1,number2);
printf("Addition of Two numbers : %d",number3);
}

12. • Include Our New Header File .
• Instead of writing < myhead.h> use this
terminology “myhead.h”
• All the Functions defined in the myhead.h header file are
now ready for use .
• Directly call function add(); [ Provide proper parameter
and take care of return type ]
Note
While running your program precaution to be taken :
Both files [ myhead.h and sample.c ] should be in same
folder.

13. ADDITIONAL PREPROCESSOR DIRECTIVES
• #ELIF DIRECTIVE: #ELIF Enable us to establish an if
else sequence for testing multiple conditions.
• #PRAGMA DIRECTIVES: #PRAGMA Is an
implementation oriented directive that allow us to
specify various instruction to be given to compiler
Syntax: #pragma name
• #ERROR : #ERROR Directive is used to produce
diagonastic messages during debugging
• Syntax :#ERROR error message.

14. PREPROCESSOR OPERATORS
• There are two preprocessor operators namely,
• 1. Stringizing operator (#): it is used in
definition of macro functions. This operators
allows a formal argument within macro
definition to be converted to string
• Syntax: # define sum(xy)
printf(#xy”=%f/n”,xy);

15. 2.TOKEN PASTING OPERATORS
The token pasting operator enables us to
combine two within a macro definition to
form a single token
Syntax : #define
combine(string1,string2)s1##s2