Preprocessors

Introduction

The preprocessor runs before compilation — handles #include, #define, #ifdef, and conditional compilation.

Understanding the topic

#include Paste header contents into source.

#define Text substitution macros and constants.

#ifdef Include code only when macro is defined.

• #include — Paste header contents into source.
• #define — Text substitution macros and constants.
• #ifdef — Include code only when macro is defined.

Step-by-step explanation

1. #include — Paste header contents into source.
2. #define — Text substitution macros and constants.
3. #ifdef — Include code only when macro is defined.

Informative example

Example program:

#include <stdio.h>
#define PI 3.14159

int main(void) {
    printf("%.5f\n", PI);
    return 0;
}

3.14159

Execution workflow

Best practices

• Enable warnings: gcc -Wall -Wextra -std=c11 source.c -o app
• Give every variable a defined value before it is read.
• Stay inside array bounds — C will not stop you from over-running a buffer.

Common mistakes

• Reading uninitialized storage — behavior is undefined.
• Dismissing compiler warnings instead of fixing root causes.
• Ignoring NULL returns from malloc, fopen, and similar APIs.

Hands-on exercise

Practice problems:

• Define MAX macro
• Header guards with #ifndef

Summary

Preprocessors in C — Directives processed before the compiler proper runs.