Preprocessor Directives

What is Preprocessor?

The C preprocessor runs before compilation. It handles lines starting with #, such as #include, #define, #ifdef and #ifndef.

#include Directive

#include adds the content of a header file into the program.

#include <stdio.h>     // standard library header
#include "myheader.h"  // user-defined header file

#define and Macros

#define is used to create constants and macros.

#include <stdio.h>
#define PI 3.14159
#define SQUARE(x) ((x) * (x))
#define MAX(a,b) ((a) > (b) ? (a) : (b))

int main() {
    printf("Area = %.2f\n", PI * SQUARE(5));
    printf("Max = %d", MAX(10, 20));
    return 0;
}

Conditional Compilation

#include <stdio.h>
#define DEBUG 1

int main() {
    int x = 10;

#ifdef DEBUG
    printf("Debug: x = %d\n", x);
#endif

    printf("Program running");
    return 0;
}

Include Guards

Include guards prevent the same header file from being included multiple times.

#ifndef MYHEADER_H
#define MYHEADER_H

void greet();
int add(int a, int b);

#endif

Common preprocessor directives

Here are the directives you will meet most often, all beginning with # and processed before compilation:

Common mistakes

• Putting a semicolon after a directive, e.g. #define PI 3.14; — the ; becomes part of the value.
• Forgetting parentheses in a macro, so SQUARE(a+b) expands wrongly.
• Using a macro where a const or real function would be safer and type-checked.
• Leaving out include guards, causing the same header to be included twice.

Summary

• Preprocessor runs before compiler
• #include adds header files
• #define creates constants and macros
• Conditional compilation helps in debug and platform-specific code
• Include guards prevent duplicate inclusion