C Preprocessor and Macros

The preprocessor runs before the compiler. It handles the lines that start with #. It copies header files, replaces macros and removes code which is not needed.

Common directives

Directive	Work
#include	Adds the content of a file
#define	Defines a macro (a name for a value or code)
#undef	Removes a macro
#ifdef / #ifndef	Compiles code only if a macro is defined / not defined
#if, #else, #endif	Conditional compilation

#include

#include <stdio.h>      // standard header (system folder)
#include "myfile.h"     // your own header (current folder)

Macro with arguments

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

int main(void) {
    printf("%d\n", SQUARE(5));
    printf("%d\n", MAX(8, 3));
    return 0;
}

Output:

Always put brackets around each argument and around the whole macro. Without them the result can be wrong. For example, #define SQUARE(x) x * x gives 11 for SQUARE(2 + 3) instead of 25.

Conditional compilation

#include <stdio.h>
#define DEBUG

int main(void) {
#ifdef DEBUG
    printf("Debug mode is on\n");
#endif
    printf("Program running\n");
    return 0;
}

Output:

If you remove the line #define DEBUG, the debug message is not even compiled. This is used to keep one code for different systems.

Header guard

In your own header files, a guard stops the file from being included two times.

#ifndef MYHEADER_H
#define MYHEADER_H

// declarations here

#endif

Next: C File Handling.

• Previous Page
• Next Page

---

Exercises & Assignments
No Content Found.

Interview Questions & Answers
No Content Found.