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Preprocessor DirectivesIn C

Muhammed Thanveer MelayiMuhammedthanveermelayi@yahoo.com Muhammed Thanveer MelayiTwitter ProfileLinkedin Profile

What is Preprocessors

The C Preprocessor is not part of the compiler, but is a separate step in the compilation process. In simplistic terms, a C Preprocessor is just a text substitution tool and they instruct compiler to do required pre-processing before actual compilation

All preprocessor commands begin with a pound symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in first column

important preprocessor directives

Directive and Description

The unconditional directives are:#include - Inserts a particular header from another file#define - Defines a preprocessor macro#undef - Undefines a preprocessor macro

The conditional directives are:#ifdef - If this macro is defined#ifndef - If this macro is not defined#if - Test if a compile time condition is true#else - The alternative for #if#elif - #else an #if in one statement#endif - End preprocessor conditional

Other directives include:# - Stringization, replaces a macro parameter with a string constant## - Token merge, creates a single token from two adjacent ones

Use of the preprocessor programs easier to develop easier to read easier to modify

example#include <stdio.h> #define PI 3.1415 int main(){ int radius; float area; printf("Enter the radius: "); scanf("%d",&radius); area=PI*radius*radius; printf("Area=%.2f",area); return 0; }

#ifdef and #endif in c

It only checks identifier has been defied or not.#ifdef <Identifer> ------------- -------------#else ------------- -------------#endif

Example #include<stdio.h>#define ABC 25#define PQR "Exact Help"int main(){ int num = 3; #ifdef ABC printf("%d",ABC * ABC); #else printf("%s",PQR); #endifreturn 0;}

Output: 625

# error directive in c

Syntax : #errorit only issue an error message and this error message includes .Example #include<stdio.h>#ifndef __MATH_H#error First include then compile#elseint main(){ float a,b=25; a=sqrt(b); printf("%f",a); return 0;}#endifOutput: compiler error --> Error directive :First include then compile

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