Preprocessor & Code Quality

#include <stdio.h>

/* Constants — no type, no semicolon, ALL_CAPS by convention */
#define PI        3.14159265
#define MAX_ITEMS 10
#define GREETING  "Hello, COMP2017!"

/* Parameterized macros — ALL parameters wrapped in parens */
#define MAX(a, b)    ((a) > (b) ? (a) : (b))
#define MIN(a, b)    ((a) < (b) ? (a) : (b))
#define SQUARE(x)    ((x) * (x))
#define ABS(x)       ((x) >= 0 ? (x) : -(x))

int main(void) {
    printf("%s\n", GREETING);
    printf("PI = %.5f\n", PI);
    printf("MAX_ITEMS = %d\n", MAX_ITEMS);

    int a = 3, b = 7;
    printf("MAX(%d,%d) = %d\n", a, b, MAX(a, b));   /* 7 */
    printf("MIN(%d,%d) = %d\n", a, b, MIN(a, b));   /* 3 */
    printf("SQUARE(5) = %d\n", SQUARE(5));           /* 25 */
    printf("ABS(-4)   = %d\n", ABS(-4));             /* 4  */

    /* Arrays can use #define for size */
    int arr[MAX_ITEMS];
    for (int i = 0; i < MAX_ITEMS; i++) arr[i] = i;
    printf("arr[9] = %d\n", arr[9]);   /* 9 */

    return 0;
}

// Predefined macros — filled in by preprocessor automatically
printf("Error at %s line %d in %s()\n", __FILE__, __LINE__, __func__);

// __FILE__  → "myprogram.c" (source filename string literal)
// __LINE__  → 42 (current line number as integer)
// __func__  → "main" (current function name, C99)
// __DATE__  → "Jun 11 2026" (compilation date)
// __TIME__  → "14:32:01" (compilation time)

// Practical DEBUG macro:
#define DEBUG(msg) fprintf(stderr, "[%s:%d %s] %s\n", \
    __FILE__, __LINE__, __func__, msg)

/* ===== mymath.h ===== */
#ifndef MYMATH_H       /* Guard: skip if already included */
#define MYMATH_H

#define MY_PI 3.14159265

/* Function declarations (prototypes) */
double circle_area(double r);
int    factorial(int n);

#endif /* MYMATH_H */

/* ===== mymath.c ===== */
#include "mymath.h"    /* quotes = look in current dir first */
#include <stdio.h>

double circle_area(double r) {
    return MY_PI * r * r;
}

int factorial(int n) {
    if (n <= 1) return 1;
    return n * factorial(n - 1);
}

/* ===== main.c ===== */
#include "mymath.h"   /* safe to include twice — guard prevents re-processing */
#include "mymath.h"   /* would cause errors WITHOUT the guard */

int main(void) {
    printf("Area r=3: %.2f\n", circle_area(3.0));    /* 28.27 */
    printf("5! = %d\n", factorial(5));                /* 120   */
    return 0;
}

#include <stdio.h>
#include <assert.h>

/* Define DEBUG to enable debug output */
/* gcc -DDEBUG main.c  — pass on command line */
/* gcc main.c          — no debug output */

#ifdef DEBUG
  #define LOG(fmt, ...) printf("[DEBUG] " fmt "\n", ##__VA_ARGS__)
#else
  #define LOG(fmt, ...) /* nothing — expands to empty */
#endif

int divide(int a, int b) {
    assert(b != 0);  /* crashes if b==0 in debug mode */
    LOG("divide called: a=%d b=%d", a, b);
    return a / b;
}

int main(void) {
    LOG("program starting");
    int result = divide(10, 2);
    printf("10 / 2 = %d\n", result);
    LOG("done");
    return 0;
}

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

/* Predefined macros — automatically set by the preprocessor:
   __FILE__  expands to the current source filename (string literal)
   __LINE__  expands to the current line number (integer)
   __func__  expands to the current function name (string, C99)
   __DATE__  expands to compilation date, e.g. "Jun 11 2026"
   __TIME__  expands to compilation time, e.g. "14:23:01"        */

/* Practical DEBUG macro — never goes stale because the
   preprocessor fills in file/line/function automatically     */
#ifdef DEBUG
  #define DBG(msg) \
    fprintf(stderr, "[%s:%d %s] %s\n", __FILE__, __LINE__, __func__, msg)
#else
  #define DBG(msg) do {} while(0)   /* expands to nothing in release */
#endif

void check_pointer(int *ptr) {
    if (ptr == NULL) {
        /* Without predefined macros you'd hardcode "check_pointer"
           and the line number — and both go stale when code moves */
        fprintf(stderr, "Error at %s:%d in %s()\n",
                __FILE__, __LINE__, __func__);
        exit(1);
    }
    DBG("pointer is valid");
}

int main(void) {
    printf("Compiled from: %s\n",  __FILE__);
    printf("This is line:  %d\n",  __LINE__);
    printf("Compiled on:   %s at %s\n", __DATE__, __TIME__);

    int x = 42;
    check_pointer(&x);
    DBG("all good");
    return 0;
}

#include <stdio.h>

/* BAD macro — missing parentheses */
#define BAD_SQUARE(x) x * x
/* GOOD macro — all parentheses present */
#define GOOD_SQUARE(x) ((x) * (x))

/* BAD macro — side effect with ++ */
#define BAD_MAX(a,b) ((a)>(b)?(a):(b))

int main(void) {
    /* Precedence bug */
    printf("BAD_SQUARE(1+2) = %d\n",  BAD_SQUARE(1+2));
    /* Expands to: 1+2*1+2 = 1+2+2 = 5 (WRONG — expected 9) */

    printf("GOOD_SQUARE(1+2) = %d\n", GOOD_SQUARE(1+2));
    /* Expands to: ((1+2)*(1+2)) = 9 (correct) */

    /* Side-effect bug */
    int i = 3;
    int r = BAD_MAX(i++, 2);
    /* Expands to: ((i++)>(2)?(i++):(2)) — i is incremented TWICE */
    printf("i after BAD_MAX(i++,2): %d (expected 4, got %d)\n", 4, i);
    /* i is now 5, not 4! */

    return 0;
}

// BROKEN — multi-statement macro without do{}while(0)
#define SWAP(a, b)  int tmp = a; a = b; b = tmp;

if (x > 0)
    SWAP(x, y);   // expands to 3 statements — only first is in the if!
else
    doSomething();  // parse error: "else without if"

// CORRECT — wrap in do { } while(0)
#define SWAP(a, b)  do { int tmp = (a); (a) = (b); (b) = tmp; } while(0)

if (x > 0)
    SWAP(x, y);   // now a single statement — works perfectly
else
    doSomething(); // else matches the if correctly