C Preprocessor Directives

#include <stdio.h>

// Object-like macros (constants)
#define PI        3.14159265
#define MAX_SIZE  100
#define NEWLINE   '\n'

// Function-like macros (no type checking — use with care)
#define SQUARE(x)    ((x) * (x))
#define MAX(a, b)    ((a) > (b) ? (a) : (b))
#define MIN(a, b)    ((a) < (b) ? (a) : (b))
#define ABS(x)       ((x) < 0 ? -(x) : (x))

// Multi-line macro using backslash
#define PRINT_INFO(name, val) \
    printf("%-15s = %d\n", name, val)

int main() {
    printf("PI = %.5f\n", PI);
    printf("MAX_SIZE = %d\n", MAX_SIZE);

    int x = 5;
    printf("SQUARE(%d) = %d\n", x, SQUARE(x));   // 25
    printf("MAX(3, 7)  = %d\n", MAX(3, 7));       // 7
    printf("ABS(-10)   = %d\n", ABS(-10));        // 10

    // Caution: macro side effects
    int a = 3;
    printf("SQUARE(a++) = %d\n", SQUARE(a++));  // (3)*(4) = 12, not 9!
    printf("a after = %d\n", a);                // 5

    PRINT_INFO("MAX_SIZE", MAX_SIZE);

    // #undef — remove a macro definition
    #undef MAX_SIZE
    // printf("%d", MAX_SIZE);  // ERROR: MAX_SIZE not defined

    return 0;
}

#include <stdio.h>

#define DEBUG    // comment this out to disable debug output
#define VERSION  2

int main() {
    int x = 42;

    // #ifdef — compile only if DEBUG is defined
    #ifdef DEBUG
        printf("[DEBUG] x = %d\n", x);
    #endif

    // #ifndef — compile only if macro is NOT defined
    #ifndef RELEASE
        printf("Running in development mode\n");
    #endif

    // #if / #elif / #else — numeric conditions
    #if VERSION == 1
        printf("Version 1 features\n");
    #elif VERSION == 2
        printf("Version 2 features\n");
    #else
        printf("Unknown version\n");
    #endif

    // Header guard pattern (prevents double inclusion)
    // Typically in .h files:
    // #ifndef MY_HEADER_H
    // #define MY_HEADER_H
    // ... header content ...
    // #endif

    return 0;
}

/*
[DEBUG] x = 42
Running in development mode
Version 2 features
*/

// #pragma once — modern alternative to header guards
// (supported by most compilers: GCC, Clang, MSVC)
// #pragma once

#include <stdio.h>

// Useful debug macro using predefined macros
#define LOG(msg) printf("[%s:%d] %s\n", __FILE__, __LINE__, msg)

int main() {
    // Predefined macros
    printf("File:    %s\n", __FILE__);
    printf("Line:    %d\n", __LINE__);
    printf("Date:    %s\n", __DATE__);
    printf("Time:    %s\n", __TIME__);

    // Using LOG macro
    LOG("Program started");
    int x = 10;
    LOG("About to compute");
    printf("x = %d\n", x);
    LOG("Done");

    // #pragma message — print message at compile time
    // #pragma message("Compiling main.c...")

    // #pragma pack — control struct alignment
    // #pragma pack(1)  // pack structs with 1-byte alignment

    return 0;
}

/*
File:    predefined.c
Line:    12
Date:    Jan 15 2025
Time:    10:30:00
[predefined.c:16] Program started
x = 10
[predefined.c:19] Done
*/