Q1 What is C Preprocessor? Types of C Preprocessors? Marco expansion, File Inclusion, Conditional Compilation, Miscellaneous Directives

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What is C Preprocessor? Types of C Preprocessors? Marco expansion, File Inclusion, Conditional Compilation, Miscellaneous Directives

Definition:

It is a program that processes our source program before it is passed to the compiler. (C Preprocessor commands also known as directives)

C PREPROCESSOR

There are four types of directives exists in C preprocessor

1. Macro expansion
2. File Inclusion
3. Conditional Compilation
4. Miscellaneous directives

MACRO EXPANSION

#define PI 3.14
#define AND &&
#define OR ||

Macros with Arguments

#define AREA(x) (3.14 * x * x)

FILE INCLUSION

#include<stdio.h>
#include<math.h>

CONDITIONAL COMPILATION

ifdef

#include<stdio.h>
int main()
 {
     #ifdef INTEL
         code suitable for an Intel PC
     #else
         code suitable for a Motorola PC
     #endif
         code common to both the computer
 }

ifndef

#include<stdio.h>
#define INTEL
int main()
 {
     #ifndef INTEL
         code suitable for an Intel PC
     #else
         code suitable for a Motorola PC
     #endif
         code common to both the computer
 }

if and elif

include
 int main()
 {
     #if ADAPTER==VGA
         code for video graphics array
     #elif ADAPTER==SVGA
         code for super video graphics array
     #endif // ADAPTER
 }

MISCELLANEOUS DIRECTIVES

There are two more preprocessor directives available, through they are not very commonly used.

a. #undef
b. #pragma

#undef

It is used to undefine a macro that has been earlier #defined, the directive,

#undef PI

#pragma startup and #pragma exit

These directives allow us to specify functions that are called upon program startup (before main()) or program exit (just before the program terminates)

include
 void fun1();
 void fun2();
 pragma startup fun1
 pragma exit fun2
 int main()
 {
     printf("Inside main\n");
     return 0;
 }
 void fun1()
 {
     printf("Inside fun1\n");
 }
 void fun2()
 {
     printf("Inside fun2\n");
 }

Output

Inside fun1
Inside main
Inside fun2

#pragma warn

#pragma warn -rvl  /* return value */
#pragma warn -par /* parameter not used */
#pragma warn -rch /* unreachable code */