1. C Preprocessor
Seema Chandak

2. What is PREPROCESSING ?
A separate compilation step before the compiler tries to “understand” your program
What is PREPROCESSOR Statement
Simple text substitution “On the fly”

3. Features of C preprocessor
Preprocessing
Occurs before program compiled
Inclusion of external files
Definition of symbolic constants
Macros
Conditional compilation
Conditional execution
All directives begin with #
Can only have whitespace before directives
Directives not C statements
Do not end with ;

4. The #include Preprocessor Directive
#include directive
Puts copy of file in place of directive
Seen many times in example code
Two forms
#include <filename>
For standard library header files
Searches predestinated directories
#include "filename"
Searches in current directory
Normally used for programmer-defined files

5. The #include Preprocessor Directive
Usage
Loading header files
#include <iostream>
Programs with multiple source files
Header file
Has common declarations and definitions
Classes, structures, enumerations, function prototypes
Extract commonality of multiple program files

6. #define The #define Preprocessor Directive: Symbolic Constants
Constants represented as symbols
When program compiled, all occurrences replaced
Format
#define identifier replacement-text
#define PI
Everything to right of identifier replaces text
#define PI=
Replaces PI with "= "
Probably an error
Cannot redefine symbolic constants

7. The #define Preprocessor Directive: Symbolic Constants
Advantages
Takes no memory
Disadvantages
Name not be seen by debugger (only replacement text)
Do not have specific data type
const variables preferred

8. Macro The #define Preprocessor Directive: Macros
Operation specified in #define
Intended for legacy C programs
Macro without arguments
Treated like a symbolic constant
Macro with arguments
Arguments substituted for replacement text
Macro expanded
Performs a text substitution
No data type checking

9. Example The #define Preprocessor Directive: Macros
Declaration of macro
#define CIRCLE_AREA( x ) ( PI * ( x ) * ( x ) )
Calling macro
area = CIRCLE_AREA( 4 );
Becomes
area = ( * ( 4 ) * ( 4 ) );
Use parentheses very important
Without them
#define CIRCLE_AREA( x ) PI * x * x
Call will
area = CIRCLE_AREA( c + 2 );
area = * c + 2 * c + 2;
Which evaluates incorrectly

10. Example
# define M(x) x*x main() { printf(“%d”,M(2+3)); } What is the output ? Why ?

11. ((x+1) > (y-2) ? (x+1) : (y-2))
Macro Substitution with Arguments
#define NAME(A, B) repl_text with A & B
Defines a macro in which
text of argument A is inserted into repl_text wherever parameter A occurs
text of argument B is inserted into repl_text wherever parameter B occurs
Example:–
#define max(A, B) ((A) > (B) ? (A) : (B))
Replace max(x+1, y-2) with
((x+1) > (y-2) ? (x+1) : (y-2))

12. Multiple arguments : #undef The #define Preprocessor Directive: Macros
#define RECTANGLE_AREA( x, y ) ( ( x ) * ( y ) )
rectArea = RECTANGLE_AREA( a + 4, b + 7 );
Becomes
rectArea = ( ( a + 4 ) * ( b + 7 ) );
#undef
Undefined symbolic constant or macro
Can later be redefined

13. Control preprocessor directives and compilation
Conditional Compilation
Control preprocessor directives and compilation
Cannot evaluate cast expressions, sizeof, enumeration constants
Structure similar to if
#if !defined( NULL )
#define NULL 0
#endif
Determines if symbolic constant NULL defined
If NULL defined,
defined( NULL ) evaluates to 1
#define statement skipped
Otherwise
#define statement used
Every #if ends with #endif

14. Can use else Abbreviations Conditional Compilation #else
#elif is "else if"
Abbreviations
#ifdef short for
#if defined(name)
#ifndef short for
#if !defined(name)

15. Examples :: #include<stdio.h> #define a 10 main() { }
int a = 50;
printf(“\n\t %d”,a); }
#incldue <stdio.h>
#define square(X) X*X
main() {
int y ;
y= 30/square(4);
printf(“\n\t %d “, y); }