Preprocessor & Build System

Master the C preprocessor. Understand macros, includes, header guards, and how to structure professional C projects.

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#define: Text Substitution

The preprocessor replaces text before compilation. No type checking.

macros.c

#define PI 3.14159
#define MAX_SIZE 1000

int main() {
    float area = PI * 5 * 5;
    int arr[MAX_SIZE];
}

Function-like Macros

Faster than functions (no overhead), but dangerous.

#define SQUARE(x) ((x) * (x))

int y = SQUARE(5); // 25

The Danger

Side effects are duplicated.

SQUARE(i++); 
// Expands to: ((i++) * (i++))
// i increments TWICE! Undefined.

#include: File Insertion

Literally copies the contents of one file into another.

System Headers

Uses angle brackets <>. Searches system paths.

#include <stdio.h>
#include <stdlib.h>

User Headers

Uses quotes "". Searches current directory first.

#include "myheader.h"
#include "config.h"

Header Guards

Preventing duplicate definitions when files are included multiple times.

myheader.h

#ifndef MYHEADER_H
#define MYHEADER_H

struct Point {
    int x, y;
};

void move(struct Point *p);

#endif

                Modern Alternative: #pragma once is supported by most modern compilers and does the same thing more cleanly, though it is non-standard.
            

Conditional Compilation

Compiling different code blocks based on conditions.

#ifdef _WIN32
    #include <windows.h>
#else
    #include <unistd.h>
#endif

#if DEBUG_LEVEL > 0
    printf("Debug Mode On\n");
#endif