C 치트시트

#include <stdio.h>

int main(void) {
    printf("Hello, World!\n");
    return 0;
}

int age = 30;
float height = 5.7f;
double pi = 3.14159;
char grade = 'A';
char name[] = "Alice";
long big = 100000L;
unsigned int count = 42;
printf("%s is %d\n", name, age);

int n;
printf("Enter a number: ");
scanf("%d", &n);
printf("You entered %d\n", n);

char name[50];
printf("Enter name: ");
scanf("%49s", name);  // limit to prevent overflow
printf("Hi, %s!\n", name);

printf("%d\n", 42);        // integer
printf("%f\n", 3.14);       // float/double
printf("%.2f\n", 3.14159);  // 3.14 (2 decimals)
printf("%c\n", 'A');        // char
printf("%s\n", "hello");    // string
printf("%x\n", 255);        // ff (hex)
printf("%5d\n", 42);        // right-aligned, width 5
printf("%-5d|\n", 42);      // left-aligned

#include <stdio.h>   // system header
#include "myheader.h"  // local header

#define PI 3.14159
#define SQUARE(x) ((x) * (x))

#ifndef GUARD_H
#define GUARD_H
// header content
#endif

#include <string.h>
char s[20] = "Hello";
printf("Length: %zu\n", strlen(s));   // 5
printf("Size: %zu\n", sizeof(s));     // 20

char dest[20];
strcpy(dest, s);   // copy
printf("%s\n", dest);  // Hello

#include <string.h>
char s[30] = "Hello";
strcat(s, ", World!");   // s = "Hello, World!"
printf("%s\n", s);

int cmp = strcmp("apple", "banana");
// returns <0 if a<b, 0 if equal, >0 if a>b
if (strcmp(s, "Hello") == 0) {
    printf("Equal!\n");
}

char buf[100];
int age = 30;
char name[] = "Alice";
sprintf(buf, "%s is %d years old", name, age);
printf("%s\n", buf);

int a, b;
sscanf("10 20", "%d %d", &a, &b);
printf("a=%d, b=%d\n", a, b);  // a=10, b=20

#include <string.h>
char s[] = "Hello, World!";
char *p = strchr(s, 'W');   // find first 'W'
printf("%s\n", p);          // World!

char *sub = strstr(s, "World");
printf("%s\n", sub);        // World!

char tokens[] = "a,b,c";
char *tok = strtok(tokens, ",");
while (tok) {
    printf("%s\n", tok);
    tok = strtok(NULL, ",");
}

char line[100];
printf("Enter text: ");
fgets(line, sizeof(line), stdin);
// removes trailing newline
line[strcspn(line, "\n")] = 0;
printf("You said: %s\n", line);

#include <stdint.h>
#include <limits.h>
int32_t a = 100;
int64_t big = 9223372036854775807LL;
uint8_t byte = 255;

printf("INT_MAX = %d\n", INT_MAX);     // 2147483647
printf("INT_MIN = %d\n", INT_MIN);     // -2147483648
printf("UINT_MAX = %u\n", UINT_MAX);   // 4294967295

#include <float.h>
double d = 3.141592653589793;
float f = 3.14f;

printf("DBL_MAX = %e\n", DBL_MAX);
printf("DBL_EPSILON = %e\n", DBL_EPSILON);

if (d == 0.1 + 0.2) {
    // likely false! floating point imprecision
}

#include <math.h>
double x = 2.5;
pow(x, 3);      // 15.625
sqrt(x);        // 1.581
fabs(-5.0);     // 5.0
floor(3.7);     // 3.0
ceil(3.2);      // 4.0
fmod(10.5, 3);  // 1.5
exp(1);         // 2.718 (e^1)
log(2.718);     // 1.0 (natural log)

#include <stdlib.h>
#include <time.h>

srand(time(NULL));  // seed once at start
int r = rand() % 100;       // 0-99
int dice = rand() % 6 + 1;  // 1-6

float fr = (float)rand() / RAND_MAX;  // 0.0 - 1.0

int i = 65;
char c = (char)i;        // 'A'
double d = 3.99;
int truncated = (int)d;  // 3

// Implicit promotion
int a = 5;
double result = a / 2.0;  // 2.5 (promoted to double)
int bad = a / 2;          // 2 (integer division)

// String to number
int n = atoi("42");
double f = atof("3.14");

int score = 85;
if (score >= 90) {
    printf("A\n");
} else if (score >= 80) {
    printf("B\n");
} else if (score >= 70) {
    printf("C\n");
} else {
    printf("F\n");
}

int day = 3;
switch (day) {
    case 1: printf("Mon\n"); break;
    case 2: printf("Tue\n"); break;
    case 3: printf("Wed\n"); break;
    case 6:
    case 7: printf("Weekend\n"); break;
    default: printf("Invalid\n");
}

for (int i = 0; i < 5; i++) {
    printf("%d\n", i);
}

// Iterate an array
int nums[] = {10, 20, 30};
int n = sizeof(nums) / sizeof(nums[0]);
for (int i = 0; i < n; i++) {
    printf("%d\n", nums[i]);
}

int count = 0;
while (count < 3) {
    printf("%d\n", count++);
}

int x;
do {
    printf("Enter positive: ");
    scanf("%d", &x);
} while (x <= 0);  // runs at least once

for (int i = 0; i < 10; i++) {
    if (i == 3) continue;  // skip 3
    if (i == 7) break;     // stop at 7
    printf("%d ", i);      // 0 1 2 4 5 6
}

// goto for breaking nested loops
for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
        if (found) goto done;
    }
}
done: printf("exited\n");

int add(int a, int b) {
    return a + b;
}

void greet(const char *name) {
    printf("Hello, %s!\n", name);
}

int main(void) {
    int sum = add(3, 4);
    greet("Alice");
    return 0;
}

int factorial(int n) {
    if (n <= 1) return 1;       // base case
    return n * factorial(n - 1); // recursive case
}

int fib(int n) {
    if (n < 2) return n;
    return fib(n - 1) + fib(n - 2);
}
// factorial(5) == 120

int add(int a, int b) { return a + b; }
int sub(int a, int b) { return a - b; }

int (*op)(int, int) = add;
printf("%d\n", op(3, 4));  // 7

op = sub;
printf("%d\n", op(3, 4));  // -1

// As a parameter
int apply(int (*f)(int, int), int a, int b) {
    return f(a, b);
}

#include <stdarg.h>
int sum(int count, ...) {
    va_list args;
    va_start(args, count);
    int total = 0;
    for (int i = 0; i < count; i++) {
        total += va_arg(args, int);
    }
    va_end(args);
    return total;
}
// sum(3, 10, 20, 30) == 60

// static: internal linkage (file-local)
static int counter = 0;
static int next_id(void) { return ++counter; }

// inline: hint to expand inline
static inline int square(int x) { return x * x; }

// static local: persists across calls
int call_count(void) {
    static int n = 0;
    return ++n;
}

int nums[5] = {1, 2, 3, 4, 5};
printf("%d\n", nums[0]);     // 1
printf("%d\n", nums[4]);     // 5

int len = sizeof(nums) / sizeof(nums[0]);  // 5

// Array of strings
char *fruits[] = {"apple", "banana", "cherry"};
printf("%s\n", fruits[1]);   // banana

int x = 10;
int *ptr = &x;     // ptr holds address of x
printf("%p\n", (void*)ptr);  // address
printf("%d\n", *ptr);        // 10 (dereference)

*ptr = 20;         // modify x through pointer
printf("%d\n", x);  // 20

int *p = NULL;     // null pointer (points to nothing)
if (p) { /* safe to dereference */ }

int arr[] = {10, 20, 30, 40, 50};
int *p = arr;       // points to arr[0]

printf("%d\n", *p);       // 10
printf("%d\n", *(p + 1)); // 20
printf("%d\n", *(p + 2)); // 30

p += 3;              // now points to arr[3]
printf("%d\n", *p);  // 40

int diff = (p - arr); // 3 (number of elements)

int arr[5] = {1, 2, 3, 4, 5};
int *ptr = arr;  // arr decays to &arr[0]

// These are equivalent
printf("%d\n", arr[2]);
printf("%d\n", ptr[2]);
printf("%d\n", *(arr + 2));

// But sizeof differs
printf("%zu\n", sizeof(arr));   // 20 (5 * 4 bytes)
printf("%zu\n", sizeof(ptr));   // 8 (pointer size)

int grid[2][3] = {
    {1, 2, 3},
    {4, 5, 6}
};

printf("%d\n", grid[0][1]);  // 2
printf("%d\n", grid[1][2]);  // 6

for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 3; j++) {
        printf("%d ", grid[i][j]);
    }
    printf("\n");
}

struct Point {
    int x;
    int y;
};

struct Point p = {3, 4};
printf("(%d, %d)\n", p.x, p.y);  // (3, 4)

p.x = 10;
p.y = 20;
printf("(%d, %d)\n", p.x, p.y);  // (10, 20)

typedef struct {
    char name[50];
    int age;
    float gpa;
} Student;

Student s = {"Alice", 20, 3.8};
printf("%s: %d, GPA %.1f\n", s.name, s.age, s.gpa);

// typedef for other types
typedef unsigned long ulong;
typedef int (*CompareFn)(const void*, const void*);

typedef struct {
    int x, y;
} Point;

Point p = {3, 4};
Point *ptr = &p;

// Arrow operator (->) for pointer members
printf("%d\n", ptr->x);   // 3
ptr->y = 10;
printf("%d\n", p.y);      // 10

// Equivalent: (*ptr).x

union Value {
    int i;
    float f;
    char bytes[4];
};

union Value v;
v.i = 65;
printf("%d\n", v.i);          // 65
printf("%c\n", v.bytes[0]);   // 'A' (same memory)

v.f = 3.14f;
printf("%d\n", v.i);  // reinterpreted bits!

struct Flags {
    unsigned int bold : 1;
    unsigned int italic : 1;
    unsigned int size : 6;  // 0-63
};

struct Flags f = {1, 0, 12};
printf("bold=%d, size=%d\n", f.bold, f.size);

enum Color { RED, GREEN, BLUE };
enum Color c = GREEN;
printf("%d\n", c);  // 1

#include <stdlib.h>
int *arr = malloc(5 * sizeof(int));
if (arr == NULL) {
    fprintf(stderr, "malloc failed\n");
    return 1;
}
for (int i = 0; i < 5; i++) arr[i] = i * 2;
free(arr);  // release memory
arr = NULL; // avoid dangling pointer

#include <stdlib.h>
// calloc: zero-initialized
int *arr = calloc(5, sizeof(int));  // all zeros

// realloc: resize
arr = realloc(arr, 10 * sizeof(int));
if (!arr) { /* handle failure, original still valid */ }

free(arr);

// Stack: automatic, fast, limited size
int local_var = 42;
int arr[100];  // on the stack

// Heap: manual, large, slower
int *heap_arr = malloc(1000000 * sizeof(int));

// Stack frame is freed when function returns
// Heap memory persists until explicitly freed

// Memory leak: lost the pointer, can't free
void leak(void) {
    int *p = malloc(100 * sizeof(int));
    // function returns without free -> leaked!
}

// Dangling pointer: using freed memory
int *p = malloc(sizeof(int));
free(p);
*p = 42;  // UNDEFINED BEHAVIOR!

// Double free: also undefined
free(p);  // crash likely

#include <stdlib.h>
#include <string.h>

// Dynamic string copy
char *dup_str(const char *s) {
    char *copy = malloc(strlen(s) + 1);  // +1 for null
    if (copy) strcpy(copy, s);
    return copy;  // caller must free
}

char *name = dup_str("Alice");
printf("%s\n", name);
free(name);

#include <stdio.h>
FILE *f = fopen("data.txt", "r");
if (!f) {
    perror("fopen failed");
    return 1;
}

char line[256];
while (fgets(line, sizeof(line), f)) {
    printf("%s", line);
}

fclose(f);

FILE *f = fopen("output.txt", "w");
fprintf(f, "Name: %s\n", "Alice");
fprintf(f, "Age: %d\n", 30);
fclose(f);

FILE *in = fopen("output.txt", "r");
char name[50];
int age;
fscanf(in, "Name: %49s\n", name);
fscanf(in, "Age: %d\n", &age);
printf("%s, %d\n", name, age);
fclose(in);

typedef struct { int id; float score; } Record;

Record r = {1, 95.5f};
FILE *f = fopen("data.bin", "wb");
fwrite(&r, sizeof(Record), 1, f);
fclose(f);

Record r2;
FILE *in = fopen("data.bin", "rb");
fread(&r2, sizeof(Record), 1, in);
printf("id=%d, score=%.1f\n", r2.id, r2.score);
fclose(in);

FILE *f = fopen("data.txt", "r");
fseek(f, 0, SEEK_END);   // jump to end
long size = ftell(f);     // get position = file size
printf("Size: %ld bytes\n", size);

rewind(f);                // back to start
// or: fseek(f, 0, SEEK_SET);

fseek(f, 10, SEEK_SET);   // 10 bytes from start
char c = fgetc(f);
printf("Char at 10: %c\n", c);
fclose(f);

#include <stdio.h>
// Three standard streams: stdin, stdout, stderr
fprintf(stdout, "Normal output\n");
fprintf(stderr, "Error: something went wrong\n");

int c;
while ((c = fgetc(stdin)) != EOF) {
    fputc(c, stdout);  // echo input
}

// stderr is unbuffered (appears immediately)
// stdout is line-buffered (flushes on newline)

#define MAX_SIZE 100
#define PI 3.14159
#define VERSION "2.0"

// Function-like macro
#define SQUARE(x) ((x) * (x))
#define MAX(a, b) ((a) > (b) ? (a) : (b))

int area = SQUARE(5);    // 25
int big = MAX(3, 7);     // 7

#define DEBUG 1

#if DEBUG
    printf("Debug: x=%d\n", x);
#endif

#ifdef _WIN32
    // Windows-specific code
#elif defined(__linux__)
    // Linux-specific code
#endif

#ifndef BUFFER_SIZE
#define BUFFER_SIZE 1024
#endif

// myheader.h
#ifndef MYHEADER_H
#define MYHEADER_H

struct Point { int x, y; };
void init_point(struct Point *p);

#endif // MYHEADER_H

// Alternative (non-standard but widely supported):
#pragma once

#pragma once              // include guard
#pragma pack(1)            // struct packing (no padding)
#pragma GCC diagnostic ignored "-Wunused-variable"

// Common pragmas
#pragma message("Compiling " __FILE__)

// C99 __attribute__ (GCC/Clang)
__attribute__((deprecated)) void old_func(void);
__attribute__((noreturn)) void fatal(void);

#define STR(x) #x
#define XSTR(x) STR(x)
#define CONCAT(a, b) a##b

printf("%s\n", STR(Hello World));  // "Hello World"
printf("%s\n", XSTR(VERSION));      // expands VERSION first

int CONCAT(foo, bar) = 42;  // creates variable foobar
printf("%d\n", foobar);     // 42

#include <stdio.h>

int main() {
    unsigned int a = 0b1100;  // 12
    unsigned int b = 0b1010;  // 10

    // AND: both bits must be 1
    printf("%u\n", a & b);   // 8  (0b1000)

    // OR: either bit is 1
    printf("%u\n", a | b);   // 14 (0b1110)

    // XOR: bits differ (exclusive or)
    printf("%u\n", a ^ b);   // 6  (0b0110)

    // NOT: flip all bits
    printf("%u\n", ~a);      // 4294967283 (on 32-bit)

    // Left shift: multiply by 2^n
    printf("%u\n", a << 2);  // 48 (12 * 4)

    // Right shift: divide by 2^n (unsigned)
    printf("%u\n", a >> 1);  // 6  (12 / 2)

    return 0;
}

#include <stdio.h>

// Flag definitions (powers of 2)
#define FLAG_READ    (1 << 0)  // 0b0001
#define FLAG_WRITE   (1 << 1)  // 0b0010
#define FLAG_EXECUTE (1 << 2)  // 0b0100
#define FLAG_ADMIN   (1 << 3)  // 0b1000

int main() {
    unsigned int permissions = 0;

    // SET a bit (OR with mask)
    permissions |= FLAG_READ | FLAG_WRITE;  // 0b0011

    // CHECK if a bit is set (AND, compare to 0)
    if (permissions & FLAG_READ) {
        printf("Read permission granted\n");
    }

    // CLEAR a bit (AND with inverted mask)
    permissions &= ~FLAG_WRITE;  // 0b0001

    // TOGGLE a bit (XOR with mask)
    permissions ^= FLAG_EXECUTE;  // 0b0101 (execute now on)
    permissions ^= FLAG_EXECUTE;  // 0b0001 (execute now off)

    // SET multiple bits at once
    permissions = FLAG_READ | FLAG_EXECUTE | FLAG_ADMIN;

    printf("Permissions: 0x%X\n", permissions);  // 0xD
    return 0;
}

#include <stdio.h>

int main() {
    int x = 42;

    // Check if odd/even (faster than x % 2)
    if (x & 1) printf("odd\n"); else printf("even\n");

    // Check if power of 2 (only one bit set)
    // x & (x-1) clears the lowest set bit
    if (x && !(x & (x - 1))) printf("power of 2\n");

    // Count set bits (popcount / Hamming weight)
    unsigned int n = 0b10110110;
    int count = 0;
    while (n) { count += n & 1; n >>= 1; }
    printf("Set bits: %d\n", count);  // 6
    // Or use __builtin_popcount(n) (GCC/Clang)

    // Swap two values without temp (XOR swap)
    int a = 5, b = 10;
    a ^= b; b ^= a; a ^= b;
    // a=10, b=5 (avoid in practice — less readable)

    // Get lowest set bit
    unsigned int lowest = x & (-x);  // isolates lowest 1-bit

    // Round up to next power of 2
    unsigned int v = 5;
    v--; v |= v >> 1; v |= v >> 2; v |= v >> 4; v |= v >> 8; v |= v >> 16; v++;

    return 0;
}

#include <stdio.h>

// Bit fields: pack multiple small fields into one int
struct Date {
    unsigned int day   : 5;   // 0-31  (5 bits)
    unsigned int month : 4;   // 0-15  (4 bits)
    unsigned int year  : 12;  // 0-4095 (12 bits)
    unsigned int is_leap : 1; // 0 or 1 (1 bit)
};  // Total: 22 bits (padded to 32)

struct Flags {
    unsigned int visible  : 1;
    unsigned int editable : 1;
    unsigned int locked   : 1;
    unsigned int          : 5;  // unnamed padding (5 bits)
    unsigned int priority : 4;  // 0-15
};

int main() {
    struct Date d = { 15, 6, 2024, 0 };
    printf("Size: %zu bytes\n", sizeof(d));  // 4 bytes
    printf("Date: %u/%u/%u\n", d.day, d.month, d.year);

    struct Flags f = { .visible = 1, .editable = 0, .locked = 1, .priority = 7 };
    printf("Size: %zu bytes\n", sizeof(f));  // 4 bytes

    return 0;
}

#include <stdio.h>
#include <stdint.h>

// Pack RGB into a single 32-bit integer (0xRRGGBB)
uint32_t make_color(uint8_t r, uint8_t g, uint8_t b) {
    return ((uint32_t)r << 16) | ((uint32_t)g << 8) | b;
}

// Extract components
uint8_t get_red(uint32_t color)   { return (color >> 16) & 0xFF; }
uint8_t get_green(uint32_t color) { return (color >> 8)  & 0xFF; }
uint8_t get_blue(uint32_t color)  { return color & 0xFF; }

// Blend two colors (50/50 mix)
uint32_t blend(uint32_t c1, uint32_t c2) {
    uint8_t r = (get_red(c1) + get_red(c2)) / 2;
    uint8_t g = (get_green(c1) + get_green(c2)) / 2;
    uint8_t b = (get_blue(c1) + get_blue(c2)) / 2;
    return make_color(r, g, b);
}

int main() {
    uint32_t red   = make_color(255, 0, 0);    // 0xFF0000
    uint32_t blue  = make_color(0, 0, 255);    // 0x0000FF
    uint32_t purple = blend(red, blue);          // 0x7F007F

    printf("Red:   0x%06X\n", red);
    printf("Blue:  0x%06X\n", blue);
    printf("Mix:   0x%06X (R=%d G=%d B=%d)\n",
           purple, get_red(purple), get_green(purple), get_blue(purple));

    return 0;
}

#include <stdio.h>
#include <signal.h>
#include <unistd.h>

// Signal handler function (must match signature)
void handler(int sig) {
    // WARNING: only async-signal-safe functions allowed here!
    // printf is NOT safe — use write() instead
    const char *msg = "Caught SIGINT\n";
    write(STDOUT_FILENO, msg, 14);
}

int main() {
    // Register handler for Ctrl+C (SIGINT)
    signal(SIGINT, handler);

    // Ignore SIGINT entirely
    // signal(SIGINT, SIG_IGN);

    // Reset to default behavior (terminate)
    // signal(SIGINT, SIG_DFL);

    printf("PID %d waiting. Press Ctrl+C...\n", getpid());
    while (1) {
        sleep(1);
    }
    return 0;
}

#include <stdio.h>
#include <signal.h>
#include <string.h>

void handler(int sig, siginfo_t *info, void *context) {
    // siginfo_t provides details about the signal
    printf("Signal %d from PID %d\n", sig, info->si_pid);
}

int main() {
    struct sigaction sa;
    memset(&sa, 0, sizeof(sa));

    sa.sa_sigaction = handler;  // use sa_sigaction (not sa_handler)
    sa.sa_flags = SA_SIGINFO;   // enable siginfo_t parameter

    // Block other signals during handler execution
    sigemptyset(&sa.sa_mask);
    sigaddset(&sa.sa_mask, SIGQUIT);  // block SIGQUIT during handler

    // Register (more portable than signal())
    sigaction(SIGINT, &sa, NULL);

    // Send a signal to self
    raise(SIGINT);  // like kill(getpid(), SIGINT)

    printf("Done\n");
    return 0;
}

#include <stdio.h>
#include <signal.h>
#include <unistd.h>

volatile sig_atomic_t got_alarm = 0;

void alarm_handler(int sig) {
    got_alarm = 1;  // safe: sig_atomic_t is atomic
}

int main() {
    signal(SIGALRM, alarm_handler);

    // Set a timer: deliver SIGALRM after 3 seconds
    alarm(3);
    printf("Waiting for alarm...\n");

    // Wait for the alarm
    while (!got_alarm) {
        pause();  // sleep until any signal arrives
    }
    printf("Alarm fired!\n");

    // Send signal to another process
    // kill(pid, SIGTERM);  // request termination
    // kill(pid, SIGKILL);  // force kill (can't be caught)

    // Send signal to self
    raise(SIGUSR1);

    return 0;
}

#include <signal.h>

// Common POSIX signals:
// SIGINT  (2)  - Ctrl+C interrupt (terminate)
// SIGQUIT (3)  - Ctrl+\ quit (core dump)
// SIGKILL (9)  - Force kill (CANNOT be caught/ignored)
// SIGSEGV (11) - Segmentation fault (invalid memory access)
// SIGPIPE (13) - Write to broken pipe
// SIGTERM (15) - Termination request (graceful shutdown)
// SIGSTOP (19) - Pause process (CANNOT be caught/ignored)
// SIGCONT (18) - Resume paused process
// SIGCHLD (17) - Child process exited
// SIGALRM (14) - Timer alarm
// SIGUSR1 (10) - User-defined signal 1
// SIGUSR2 (12) - User-defined signal 2
// SIGFPE  (8)  - Arithmetic error (divide by zero)
// SIGBUS  (7)  - Bus error (misaligned access)

// Graceful shutdown pattern:
void cleanup_handler(int sig) {
    // Save state, close files, release resources
    // Then exit cleanly
    _exit(0);  // use _exit in signal handlers (not exit)
}

// In main:
signal(SIGTERM, cleanup_handler);
signal(SIGINT, cleanup_handler);

// Prevent zombie children:
signal(SIGCHLD, SIG_IGN);  // auto-reap children

#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <fcntl.h>

int pipe_fd[2];  // [0]=read, [1]=write

void handler(int sig) {
    // Write one byte to the pipe — wakes up select()/poll()
    write(pipe_fd[1], &sig, sizeof(sig));
}

int main() {
    pipe(pipe_fd);
    // Make read end non-blocking
    fcntl(pipe_fd[0], F_SETFL, O_NONBLOCK);

    signal(SIGINT, handler);
    signal(SIGTERM, handler);

    printf("Waiting (select-based)...\n");

    while (1) {
        fd_set readfds;
        FD_ZERO(&readfds);
        FD_SET(pipe_fd[0], &readfds);

        // select() blocks until pipe is writable (signal received)
        int ready = select(pipe_fd[0] + 1, &readfds, NULL, NULL, NULL);
        if (ready > 0 && FD_ISSET(pipe_fd[0], &readfds)) {
            int sig;
            read(pipe_fd[0], &sig, sizeof(sig));
            printf("Handled signal %d in main loop\n", sig);
            if (sig == SIGTERM) break;
        }
    }
    return 0;
}

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>

int main() {
    pid_t pid = fork();  // create a child process

    if (pid < 0) {
        perror("fork failed");
        return 1;
    } else if (pid == 0) {
        // CHILD process (fork returned 0)
        printf("Child: PID=%d, Parent PID=%d\n",
               getpid(), getppid());
        sleep(2);
        printf("Child exiting\n");
        return 42;  // child exit code
    } else {
        // PARENT process (fork returned child's PID)
        printf("Parent: PID=%d, Child PID=%d\n",
               getpid(), pid);

        int status;
        waitpid(pid, &status, 0);  // wait for child

        if (WIFEXITED(status)) {
            printf("Child exited with code %d\n",
                   WEXITSTATUS(status));
        }
    }
    return 0;
}

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>

int main() {
    pid_t pid = fork();

    if (pid == 0) {
        // CHILD: replace self with a new program
        // exec never returns on success (only on failure)

        // execlp: search PATH, list arguments
        execlp("ls", "ls", "-la", "/tmp", NULL);

        // execvp: search PATH, array of arguments
        char *args[] = {"ls", "-la", "/tmp", NULL};
        execvp("ls", args);

        // execl: full path, list arguments
        execl("/bin/ls", "ls", "-la", NULL);

        // Only reached if exec failed
        perror("exec failed");
        _exit(1);
    } else {
        wait(NULL);  // parent waits for child
        printf("Child finished\n");
    }
    return 0;
}

// exec variants:
// execl  (path, arg1, arg2, ..., NULL)        — list args, full path
// execlp (file, arg1, arg2, ..., NULL)        — list args, search PATH
// execv  (path, argv[])                        — array args, full path
// execvp (file, argv[])                        — array args, search PATH
// execve (path, argv[], envp[])                — array args, custom env

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>

int main() {
    pid_t pid = fork();

    if (pid == 0) {
        printf("Child PID=%d\n", getpid());
        _exit(0);  // child exits immediately
    }

    // If parent doesn't wait(), child becomes a ZOMBIE
    // (process table entry remains until reaped)
    sleep(5);  // parent sleeps — child is now a zombie
    // Run 'ps' during this window to see the zombie (state 'Z')

    // Reap the zombie:
    int status;
    waitpid(pid, &status, 0);
    printf("Zombie reaped\n");

    // Orphan: if parent exits before child
    pid_t pid2 = fork();
    if (pid2 == 0) {
        sleep(3);  // parent will exit first
        printf("Orphan adopted by init (PID 1), new parent=%d\n",
               getppid());
        _exit(0);
    }
    // Parent exits immediately — child becomes orphan
    // init/systemd (PID 1) adopts and reaps it

    // Prevent zombies: ignore SIGCHLD
    // signal(SIGCHLD, SIG_IGN);  // kernel auto-reaps children
    // Or use SA_NOCLDWAIT with sigaction

    return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>

void daemonize() {
    // 1. Fork and exit parent (child continues in background)
    pid_t pid = fork();
    if (pid > 0) exit(0);  // parent exits
    if (pid < 0) exit(1);

    // 2. Create new session (detach from controlling terminal)
    setsid();

    // 3. Fork again (prevent reacquiring a terminal)
    pid = fork();
    if (pid > 0) exit(0);
    if (pid < 0) exit(1);

    // 4. Change working directory to / (don't hold filesystem)
    chdir("/");

    // 5. Set umask to 0 (full control over file permissions)
    umask(0);

    // 6. Close standard file descriptors (detach from terminal)
    close(STDIN_FILENO);
    close(STDOUT_FILENO);
    close(STDERR_FILENO);

    // 7. Redirect them to /dev/null (in case code writes to them)
    open("/dev/null", O_RDWR);  // fd 0 = stdin
    dup(0);                      // fd 1 = stdout
    dup(0);                      // fd 2 = stderr
}

int main() {
    daemonize();
    // Now running as a daemon (background, no terminal)
    while (1) {
        // Daemon work here (e.g., log to file, listen on socket)
        sleep(60);
    }
    return 0;
}

#include <stdio.h>
// C provides several IPC mechanisms:

// 1. PIPES: unidirectional byte stream between parent/child
//    pipe(fd) creates fd[0]=read, fd[1]=write
//    Only works between related processes (fork)

// 2. NAMED PIPES (FIFOs): like pipes but have a filesystem path
//    mkfifo("/tmp/myfifo", 0666);
//    Works between unrelated processes

// 3. SHARED MEMORY: fastest IPC (both processes access same RAM)
//    shmget/shmat (System V) or shm_open/mmap (POSIX)

// 4. MESSAGE QUEUES: structured messages (not byte stream)
//    msgget/msgsnd/msgrcv (System V) or mq_open (POSIX)

// 5. SEMAPHORES: synchronization (not data transfer)
//    semget/semop (System V) or sem_open (POSIX)

// 6. SOCKETS: bidirectional, works across machines (network)
//    socket/bind/listen/accept/connect

// 7. SIGNALS: minimal data (just signal number)
//    kill(pid, SIGUSR1)

// Choosing IPC:
// - Same machine, related processes → pipes
// - Same machine, unrelated processes → named pipes, shared memory
// - Different machines → sockets
// - Need synchronization → semaphores, mutexes
// - Need structured messages → message queues

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>

int main() {
    int fd[2];  // fd[0]=read end, fd[1]=write end
    pipe(fd);   // create pipe

    pid_t pid = fork();

    if (pid == 0) {
        // CHILD: read from pipe
        close(fd[1]);  // close unused write end

        char buf[256];
        int n = read(fd[0], buf, sizeof(buf));
        buf[n] = '\0';
        printf("Child received: %s", buf);

        close(fd[0]);
    } else {
        // PARENT: write to pipe
        close(fd[0]);  // close unused read end

        const char *msg = "Hello from parent!\n";
        write(fd[1], msg, strlen(msg));

        close(fd[1]);  // close write end → child's read returns 0 (EOF)
        wait(NULL);
    }
    return 0;
}

#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <string.h>

// Process A (writer):
int writer_main() {
    mkfifo("/tmp/myfifo", 0666);  // create named pipe

    int fd = open("/tmp/myfifo", O_WRONLY);
    write(fd, "Hello via FIFO!", 15);
    close(fd);
    return 0;
}

// Process B (reader) — can be a completely separate program:
int reader_main() {
    int fd = open("/tmp/myfifo", O_RDONLY);
    char buf[256];
    int n = read(fd, buf, sizeof(buf));
    buf[n] = '\0';
    printf("Received: %s\n", buf);
    close(fd);
    return 0;
}

// Named pipes persist in the filesystem (use unlink to remove):
// unlink("/tmp/myfifo");

// open() blocks until BOTH a reader and writer are connected
// (unless O_NONBLOCK is used)

#include <stdio.h>
#include <sys/shm.h>
#include <sys/ipc.h>
#include <string.h>
#include <unistd.h>

#define SHM_SIZE 1024

int main() {
    key_t key = ftok("/tmp/shmfile", 65);  // generate unique key

    // Create shared memory segment
    int shmid = shmget(key, SHM_SIZE, 0666 | IPC_CREAT);

    pid_t pid = fork();

    if (pid == 0) {
        // CHILD: attach and read
        char *shared = (char *)shmat(shmid, NULL, 0);
        sleep(1);  // wait for parent to write
        printf("Child reads: %s\n", shared);
        shmdt(shared);  // detach
    } else {
        // PARENT: attach and write
        char *shared = (char *)shmat(shmid, NULL, 0);
        strcpy(shared, "Hello from shared memory!");
        printf("Parent wrote to shared memory\n");
        shmdt(shared);
        wait(NULL);

        // Destroy shared memory after use
        shmctl(shmid, IPC_RMID, NULL);
    }
    return 0;
}

// WARNING: shared memory has NO synchronization!
// Use semaphores or mutexes to prevent race conditions.

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/wait.h>

// Implement shell-like pipeline: ls | sort
int main() {
    int pipefd[2];
    pipe(pipefd);

    pid_t pid1 = fork();
    if (pid1 == 0) {
        // First child: ls (writes to pipe instead of stdout)
        close(pipefd[0]);                    // close read end
        dup2(pipefd[1], STDOUT_FILENO);      // stdout → pipe write
        close(pipefd[1]);                    // close original (dup'd)
        execlp("ls", "ls", NULL);
    }

    pid_t pid2 = fork();
    if (pid2 == 0) {
        // Second child: sort (reads from pipe instead of stdin)
        close(pipefd[1]);                    // close write end
        dup2(pipefd[0], STDIN_FILENO);       // stdin → pipe read
        close(pipefd[0]);                    // close original (dup'd)
        execlp("sort", "sort", NULL);
    }

    // Parent: close both ends and wait
    close(pipefd[0]);
    close(pipefd[1]);
    waitpid(pid1, NULL, 0);
    waitpid(pid2, NULL, 0);

    return 0;
}

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

int main() {
    // popen opens a process with a pipe (like shell command | ...)
    FILE *fp = popen("ls -la /tmp", "r");
    if (fp == NULL) {
        perror("popen failed");
        return 1;
    }

    // Read command output line by line
    char buf[256];
    while (fgets(buf, sizeof(buf), fp) != NULL) {
        printf(">> %s", buf);
    }

    pclose(fp);  // closes pipe and waits for child

    // Writing to a process (like ... | command):
    FILE *wp = popen("grep hello", "w");
    fprintf(wp, "hello world\n");
    fprintf(wp, "goodbye\n");
    pclose(wp);  // grep outputs "hello world"

    return 0;
}

// popen is simpler than fork+pipe+exec but:
// - Runs via /bin/sh (shell injection risk with user input!)
// - Less control over the child process
// - Use fork+exec directly for untrusted input

# Makefile — build automation for C/C++ projects
# Rule syntax: target: prerequisites
#               recipe (must start with TAB, not spaces)

# Variables
CC = gcc
CFLAGS = -Wall -Wextra -g -O2
TARGET = myapp
SRCS = main.c utils.c parser.c
OBJS = $(SRCS:.c=.o)  # substitute .c with .o

# Default target (first rule)
all: $(TARGET)

# Link object files into executable
$(TARGET): $(OBJS)
	$(CC) $(CFLAGS) -o $@ $^

# Compile each .c to .o
%.o: %.c
	$(CC) $(CFLAGS) -c $< -o $@

# Clean build artifacts
clean:
	rm -f $(OBJS) $(TARGET)

# Run the program
run: $(TARGET)
	./$(TARGET)

# Phony targets (not files)
.PHONY: all clean run

# Usage:
# $ make        — builds 'all' (the default)
# $ make clean  — removes build files
# $ make run    — builds and runs
# $ make -j4    — parallel build (4 jobs)

# Automatic variables in recipes:
# $@   — the target filename
# $<   — the first prerequisite
# $^   — all prerequisites (no duplicates)
# $+   — all prerequisites (with duplicates)
# $?   — prerequisites newer than the target
# $*   — the stem (matching % part)

# Example showing all automatic variables:
program: main.o utils.o
	@echo "Target: $@"        # program
	@echo "First dep: $<"     # main.o
	@echo "All deps: $^"      # main.o utils.o
	@echo "Newer deps: $?"    # (whichever changed)
	gcc -o $@ $^

# Pattern rule: compile any .c to .o
%.o: %.c
	gcc -c $< -o $@
# $< = source (.c file), $@ = target (.o file)

# Static pattern rule (specific files):
$(OBJS): %.o: %.c
	gcc -c $< -o $@

# Built-in functions:
SRCS = $(wildcard src/*.c)           # find all .c files
OBJS = $(patsubst src/%.c,build/%.o,$(SRCS))  # path substitution
DIRS = $(sort $(dir $(SRCS)))         # unique directories

# When a header file changes, dependent .c files must recompile
# Make doesn't track this automatically — use gcc -MMD

CC = gcc
CFLAGS = -Wall -MMD -MP  # generate .d dependency files

SRCS = $(wildcard src/*.c)
OBJS = $(SRCS:.c=.o)
DEPS = $(OBJS:.o=.d)    # dependency files

all: myapp

myapp: $(OBJS)
	$(CC) -o $@ $^

%.o: %.c
	$(CC) $(CFLAGS) -c $< -o $@

# Include auto-generated dependency files
-include $(DEPS)  # '-' means don't error if missing

clean:
	rm -f $(OBJS) $(DEPS) myapp

.PHONY: all clean

# How it works:
# 1. gcc -MMD creates main.d next to main.o
# 2. main.d contains: main.o: main.c utils.h parser.h
# 3. -include pulls these in, so Make knows header dependencies
# 4. If utils.h changes, main.o rebuilds automatically

# Project structure:
# project/
#   src/       — source files
#   include/   — header files
#   build/     — object files (created by make)
#   bin/       — final executable

CC = gcc
CFLAGS = -Wall -Iinclude -g
SRCDIR = src
INCDIR = include
BUILDDIR = build
BINDIR = bin

TARGET = $(BINDIR)/myapp
SRCS = $(wildcard $(SRCDIR)/*.c)
OBJS = $(patsubst $(SRCDIR)/%.c,$(BUILDDIR)/%.o,$(SRCS))
DEPS = $(OBJS:.o=.d)

all: $(TARGET)

$(TARGET): $(OBJS) | $(BINDIR)
	$(CC) -o $@ $^

# Order-only prerequisite: create build dir before compiling
$(BUILDDIR)/%.o: $(SRCDIR)/%.c | $(BUILDDIR)
	$(CC) $(CFLAGS) -MMD -c $< -o $@

$(BUILDDIR):
	mkdir -p $(BUILDDIR)

$(BINDIR):
	mkdir -p $(BINDIR)

-include $(DEPS)

clean:
	rm -rf $(BUILDDIR) $(BINDIR)

.PHONY: all clean

# Order-only prerequisites (| syntax) create directories
# without triggering rebuilds when the dir timestamp changes

# CMakeLists.txt — CMake is a cross-platform build generator
# It generates Makefiles (or Ninja, VS, Xcode projects)

cmake_minimum_required(VERSION 3.10)
project(MyApp C)

set(CMAKE_C_STANDARD 11)
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra")

# Add executable from source files
add_executable(myapp src/main.c src/utils.c src/parser.c)

# Include directory
target_include_directories(myapp PRIVATE include)

# Link a library
target_link_libraries(myapp m)  # math library (-lm)

# Build type flags
set(CMAKE_C_FLAGS_DEBUG "-g -O0")
set(CMAKE_C_FLAGS_RELEASE "-O3 -DNDEBUG")

# Usage:
# $ mkdir build && cd build
# $ cmake ..            # generate Makefiles
# $ make                # build
# $ make install        # install (optional)
# $ cmake .. -DCMAKE_BUILD_TYPE=Debug  # debug build

# Out-of-source builds keep source tree clean
# CMake is the standard for C/C++ cross-platform projects

#include <stdio.h>

// Function pointer syntax: return_type (*name)(param_types)
int add(int a, int b) { return a + b; }
int subtract(int a, int b) { return a - b; }

int main() {
    // Declare a function pointer
    int (*operation)(int, int);

    // Assign (function name decays to pointer)
    operation = add;          // or &add
    printf("5 + 3 = %d\n", operation(5, 3));  // 8

    operation = subtract;
    printf("5 - 3 = %d\n", operation(5, 3));  // 2

    // typedef for readability
    typedef int (*math_func)(int, int);
    math_func fn = add;
    printf("Result: %d\n", fn(10, 20));

    // Array of function pointers (dispatch table)
    math_func ops[] = {add, subtract};
    printf("op[0](4,2)=%d  op[1](4,2)=%d\n",
           ops[0](4, 2), ops[1](4, 2));

    return 0;
}

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

// Comparator function for qsort
// Returns: negative if a<b, 0 if equal, positive if a>b
int compare_asc(const void *a, const void *b) {
    return (*(int *)a - *(int *)b);
}

int compare_desc(const void *a, const void *b) {
    return (*(int *)b - *(int *)a);
}

int main() {
    int arr[] = {5, 2, 8, 1, 9, 3, 7, 4, 6};
    int n = sizeof(arr) / sizeof(arr[0]);

    // qsort takes a function pointer as the comparator
    qsort(arr, n, sizeof(int), compare_asc);
    printf("Ascending: ");
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
    printf("\n");

    // Same array, different comparator (descending)
    qsort(arr, n, sizeof(int), compare_desc);
    printf("Descending: ");
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
    printf("\n");

    return 0;
}

#include <stdio.h>

// Simulating OOP with structs + function pointers
typedef struct Shape Shape;

// Virtual function table (vtable)
typedef struct {
    double (*area)(Shape *);
    double (*perimeter)(Shape *);
    void (*describe)(Shape *);
} ShapeVTable;

struct Shape {
    const ShapeVTable *vtable;  // pointer to virtual functions
    char name[32];
};

// Circle implementation
typedef struct {
    Shape base;      // inherit from Shape
    double radius;
} Circle;

double circle_area(Shape *s) {
    return 3.14159 * ((Circle *)s)->radius * ((Circle *)s)->radius;
}

double circle_perimeter(Shape *s) {
    return 2 * 3.14159 * ((Circle *)s)->radius;
}

static const ShapeVTable circle_vtable = {
    circle_area, circle_perimeter, NULL
};

Circle *circle_create(double r) {
    Circle *c = malloc(sizeof(Circle));
    c->base.vtable = &circle_vtable;
    strcpy(c->base.name, "Circle");
    c->radius = r;
    return c;
}

// Polymorphic function (works with any Shape)
void print_shape(Shape *s) {
    printf("%s: area=%.2f, perimeter=%.2f\n",
           s->name, s->vtable->area(s), s->vtable->perimeter(s));
}

#include <stdio.h>

// Event system using function pointers
typedef void (*EventHandler)(const char *event, void *data);

// Simple event emitter
#define MAX_HANDLERS 10
static EventHandler handlers[MAX_HANDLERS];
static int handler_count = 0;

void on_event(EventHandler handler) {
    if (handler_count < MAX_HANDLERS) {
        handlers[handler_count++] = handler;
    }
}

void emit_event(const char *event, void *data) {
    for (int i = 0; i < handler_count; i++) {
        handlers[i](event, data);  // call each registered handler
    }
}

// Concrete handlers
void log_handler(const char *event, void *data) {
    printf("[LOG] Event: %s\n", event);
}

void alert_handler(const char *event, void *data) {
    if (strcmp(event, "error") == 0) {
        printf("[ALERT] Error occurred!\n");
    }
}

int main() {
    // Register handlers (subscribe)
    on_event(log_handler);
    on_event(alert_handler);

    // Emit events (publish)
    emit_event("click", NULL);
    emit_event("error", NULL);
    emit_event("scroll", NULL);

    return 0;
}

#include <stdio.h>

// PITFALL 1: Calling a NULL function pointer (crash!)
void bad_call() {
    void (*fp)(void) = NULL;
    fp();  // SEGFAULT — always check for NULL
    if (fp) fp();  // safe
}

// PITFALL 2: Wrong signature (undefined behavior)
void takes_int(int x) { printf("%d\n", x); }
void wrong_sig() {
    void (*fp)(void) = (void (*)(void))takes_int;  // WRONG cast
    fp();  // UB: missing argument, garbage value
}

// PITFALL 3: Function pointer to a local function (dangling)
typedef int (*callback_t)(int);
callback_t get_callback() {
    // Returning pointer to local function is OK (functions aren't local)
    // But returning a pointer to a local VARIABLE is not
    return NULL;  // functions have static storage, safe to return
}

// PITFALL 4: Comparing function pointers
int f1(int x) { return x; }
int f2(int x) { return x; }
void compare_fps() {
    int (*p1)(int) = f1;
    int (*p2)(int) = f1;
    if (p1 == p2) printf("Same function\n");  // OK
    // Comparing p1 == f2 is valid but they're different functions
}

// GOOD: Always use typedef for complex function pointers
// typedef int (*comparator_t)(const void *, const void *);
// This makes declarations readable and consistent

#include <stdio.h>
#include <stdarg.h>

// Variadic function: takes variable number of arguments
// The '...' must be the LAST parameter
int sum(int count, ...) {
    va_list args;           // argument list type
    va_start(args, count);  // initialize (needs last named param)

    int total = 0;
    for (int i = 0; i < count; i++) {
        int val = va_arg(args, int);  // get next argument (as int)
        total += val;
    }

    va_end(args);  // cleanup
    return total;
}

int main() {
    printf("%d\n", sum(3, 10, 20, 30));      // 60
    printf("%d\n", sum(5, 1, 2, 3, 4, 5));   // 15
    printf("%d\n", sum(0));                    // 0
    return 0;
}

// The 'count' parameter tells the function how many args follow.
// Without it, the function can't know when to stop.

#include <stdio.h>
#include <stdarg.h>
#include <string.h>

// Custom logging function with format string
void log_msg(const char *level, const char *format, ...) {
    printf("[%s] ", level);

    va_list args;
    va_start(args, format);

    // vprintf: like printf but takes va_list instead of ...
    vprintf(format, args);

    va_end(args);
    printf("\n");
}

// Variadic function with mixed types
void print_values(int count, ...) {
    va_list args;
    va_start(args, count);

    for (int i = 0; i < count; i++) {
        int type = va_arg(args, int);  // type tag
        switch (type) {
            case 0:  // int
                printf("int: %d\n", va_arg(args, int));
                break;
            case 1:  // double
                printf("double: %f\n", va_arg(args, double));
                break;
            case 2:  // string
                printf("string: %s\n", va_arg(args, char *));
                break;
        }
    }
    va_end(args);
}

int main() {
    log_msg("INFO", "User %s logged in from %s", "Alice", "192.168.1.1");
    log_msg("ERROR", "Failed to open %s (code %d)", "config.txt", 13);

    print_values(2, 0, 42, 2, "hello", 1, 3.14);
    return 0;
}

#include <stdio.h>
#include <stdarg.h>
#include <string.h>

// Sentinel-terminated: last argument is NULL
void concat_strings(char *dest, size_t size, ...) {
    va_list args;
    va_start(args, size);

    dest[0] = '\0';  // start with empty string
    size_t used = 0;

    while (1) {
        char *s = va_arg(args, char *);
        if (s == NULL) break;  // sentinel — stop reading

        size_t len = strlen(s);
        if (used + len < size) {
            strcat(dest, s);
            used += len;
        }
    }

    va_end(args);
}

int main() {
    char result[256];
    concat_strings(result, sizeof(result),
                   "Hello, ", "world", "! ", "How are you?", NULL);
    printf("%s\n", result);
    // Hello, world! How are you?

    // Common C APIs using sentinels:
    // execl("/bin/ls", "ls", "-l", NULL);  // exec family
    // sqlite3_exec(db, sql, callback, NULL, NULL);
    return 0;
}

#include <stdio.h>
#include <stdarg.h>

// Wrapper that forwards varargs to another function
// Use va_copy for the copy (needed for multiple passes)
void custom_printf(const char *format, ...) {
    va_list args1, args2;
    va_start(args1, format);
    va_copy(args2, args1);  // copy for second use

    // First pass: count characters that would be printed
    int len = vsnprintf(NULL, 0, format, args1);
    printf("[len=%d] ", len);

    // Second pass: actually print
    vprintf(format, args2);

    va_end(args1);
    va_end(args2);
}

// Macro forwarding (common pattern for logging)
#define LOG(fmt, ...) \
    fprintf(stderr, "[LOG] %s:%d: " fmt "\n", \
            __FILE__, __LINE__, ##__VA_ARGS__)

// The ## operator removes the comma if __VA_ARGS__ is empty
// (GCC extension, widely supported)

int main() {
    custom_printf("Value: %d, Name: %s\n", 42, "test");
    LOG("Simple message");           // no extra args
    LOG("With value: %d", 100);      // with args
    return 0;
}

#include <stdio.h>

// C99 variadic macros: __VA_ARGS__ captures all extra args
#define DEBUG_PRINT(fmt, ...) \
    printf("DEBUG: " fmt "\n", ##__VA_ARGS__)

#define MAX(...) (max_of(__VA_ARGS__))

// Assert macro with message
#define ASSERT(cond, fmt, ...) \
    do { \
        if (!(cond)) { \
            fprintf(stderr, "Assertion failed: %s\n" fmt "\n", \
                    #cond, ##__VA_ARGS__); \
            exit(1); \
        } \
    } while (0)

// Count arguments (GCC __VA_OPT__ or recursive macros)
#define COUNT(...) COUNT_N(__VA_ARGS__, 5, 4, 3, 2, 1)
#define COUNT_N(_1, _2, _3, _4, _5, N, ...) N

// Stringification of all args
#define STR(...) #__VA_ARGS__

int main() {
    DEBUG_PRINT("x = %d", 42);
    DEBUG_PRINT("no args");  // ## removes comma

    ASSERT(x > 0, "x was %d", x);

    printf("Count: %d\n", COUNT(a, b, c));  // 3
    printf("Stringified: %s\n", STR(hello, world));  // "hello, world"

    return 0;
}

// Check if odd
int is_odd(int n) { return n & 1; }

// Multiply/divide by powers of 2
int doubled = x << 1;    // x * 2
int halved = x >> 1;     // x / 2
int times8 = x << 3;     // x * 8

// Swap without temp variable
void swap(int *a, int *b) {
    *a ^= *b; *b ^= *a; *a ^= *b;
}

// Check if power of 2
int is_pow2(int n) { return n > 0 && (n & (n-1)) == 0; }

// Count set bits (population count)
int popcount(unsigned int n) {
    int count = 0;
    while (n) { n &= (n-1); count++; }
    return count;
}

#define FLAG_READ    (1 << 0)  // 0x01
#define FLAG_WRITE   (1 << 1)  // 0x02
#define FLAG_EXECUTE (1 << 2)  // 0x04

// Set flags
unsigned int perms = FLAG_READ | FLAG_WRITE;

// Check if flag is set
if (perms & FLAG_WRITE) { /* write allowed */ }

// Toggle a flag
perms ^= FLAG_EXECUTE;

// Clear a flag
perms &= ~FLAG_APPEND;

// Check if ALL flags in mask are set
int has_all = (perms & (FLAG_READ|FLAG_WRITE))
            == (FLAG_READ|FLAG_WRITE);

// Pack multiple small fields into one int
struct Date {
    unsigned int day   : 5;   // 0-31 (5 bits)
    unsigned int month : 4;   // 0-15 (4 bits)
    unsigned int year  : 23;  // 0-8M (23 bits)
};  // Total: 32 bits = 4 bytes

struct Date today = { 21, 6, 2025 };
printf("Size: %zu bytes\n", sizeof(today));  // 4

today.day = 15;
if (today.month == 12) {
    today.year++;
    today.month = 1;
}

#include <arpa/inet.h>  // htonl, ntohl

// Host to network byte order (big-endian) and back
uint32_t host_val = 0x12345678;
uint32_t net_val = htonl(host_val);
uint32_t back = ntohl(net_val);

// Manual byte swap (portable)
uint32_t swap32(uint32_t v) {
    return ((v & 0xFF000000) >> 24) |
           ((v & 0x00FF0000) >> 8)  |
           ((v & 0x0000FF00) << 8)  |
           ((v & 0x000000FF) << 24);
}

// Detect endianness at compile time
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
    printf("Little-endian system\n");
#endif

// Absolute value without branching
int abs_val(int n) {
    int mask = n >> (sizeof(int)*8 - 1);
    return (n ^ mask) - mask;
}

// Round up to next power of 2
unsigned int next_pow2(unsigned int n) {
    n--;
    n |= n >> 1;  n |= n >> 2;
    n |= n >> 4;  n |= n >> 8;
    n |= n >> 16;
    return n + 1;
}

// Reverse bits in a byte
uint8_t reverse_byte(uint8_t b) {
    b = (b >> 4) | (b << 4);
    b = ((b & 0xCC) >> 2) | ((b & 0x33) << 2);
    b = ((b & 0xAA) >> 1) | ((b & 0x55) << 1);
    return b;
}

#include <signal.h>

sigset_t mask, oldmask;
sigemptyset(&mask);
sigaddset(&mask, SIGINT);
sigaddset(&mask, SIGTERM);

// Block signals (they will be queued)
sigprocmask(SIG_BLOCK, &mask, &oldmask);

// Critical section - signals are deferred
do_critical_work();

// Unblock - pending signals are now delivered
sigprocmask(SIG_SETMASK, &oldmask, NULL);

// Check for pending signals
sigpending(&mask);
if (sigismember(&mask, SIGINT)) {
    printf("SIGINT is pending\n");
}

#include <signal.h>
#include <unistd.h>

// Only safe type in handlers
volatile sig_atomic_t got_signal = 0;

void handler(int sig) {
    // ONLY use async-signal-safe functions!
    // write() is safe; printf() is NOT
    const char msg[] = "Signal received\n";
    write(STDERR_FILENO, msg, sizeof(msg)-1);
    got_signal = 1;
}

int main() {
    struct sigaction sa;
    sa.sa_handler = handler;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;  // Restart interrupted syscalls
    sigaction(SIGINT, &sa, NULL);

    while (!got_signal) pause();
    return 0;
}

#include <sys/signalfd.h>

// signalfd: handle signals as file descriptors (Linux)
int setup_signalfd() {
    sigset_t mask;
    sigemptyset(&mask);
    sigaddset(&mask, SIGINT);
    sigaddset(&mask, SIGTERM);
    sigprocmask(SIG_BLOCK, &mask, NULL);  // Must block first

    int fd = signalfd(-1, &mask, SFD_CLOEXEC);
    return fd;
}

// In event loop:
struct signalfd_siginfo si;
read(signalfd_fd, &si, sizeof(si));
printf("Got signal %d\n", si.ssi_signo);

// Self-pipe trick (portable):
// Write to pipe in handler, read in main loop
int pipefd[2];
void pipe_handler(int sig) {
    write(pipefd[1], &sig, sizeof(sig));
}

#include <sys/time.h>

volatile sig_atomic_t timer_fired = 0;

void timer_handler(int sig) { timer_fired = 1; }

int main() {
    struct sigaction sa = {0};
    sa.sa_handler = timer_handler;
    sigaction(SIGALRM, &sa, NULL);

    // Set interval timer: 2 sec initial, 1 sec repeat
    struct itimerval timer;
    timer.it_value.tv_sec = 2;
    timer.it_interval.tv_sec = 1;
    setitimer(ITIMER_REAL, &timer, NULL);

    int count = 0;
    while (count < 5) {
        pause();
        if (timer_fired) {
            printf("Timer %d\n", ++count);
            timer_fired = 0;
        }
    }
    return 0;
}

#include <signal.h>
#include <sys/wait.h>

pid_t child = fork();
if (child == 0) {
    while (1) { sleep(1); }
} else {
    sleep(3);
    kill(child, SIGTERM);  // Send SIGTERM to child

    int status;
    waitpid(child, &status, 0);
    if (WIFSIGNALED(status))
        printf("Killed by signal %d\n", WTERMSIG(status));
}

// Send signal to self
raise(SIGSTOP);  // Stop (resume with SIGCONT)

// Send to process group
kill(0, SIGUSR1);  // 0 = own process group

// Send with data (sigqueue)
union sigval value = { .sival_int = 42 };
sigqueue(child, SIGUSR1, value);

#include <unistd.h>
#include <sys/wait.h>

int run_program(const char *path, char *const args[]) {
    pid_t pid = fork();
    if (pid < 0) { perror("fork"); return -1; }

    if (pid == 0) {
        // Child: exec replaces process image
        execvp(path, args);
        perror("execvp");  // Only on failure
        _exit(127);
    }

    // Parent: wait for child
    int status;
    waitpid(pid, &status, 0);
    return WIFEXITED(status) ? WEXITSTATUS(status) : -1;
}

// Usage
char *args[] = {"ls", "-la", "/tmp", NULL};
int result = run_program("ls", args);

#include <unistd.h>

int main() {
    pid_t child = fork();
    if (child > 0) _exit(0);  // Parent exits
    if (child < 0) return 1;

    // First child: create new session
    setsid();

    // Second fork (prevent reacquiring terminal)
    pid_t grandchild = fork();
    if (grandchild > 0) _exit(0);

    // Daemon process
    chdir("/");
    umask(0);
    close(STDIN_FILENO);
    close(STDOUT_FILENO);
    close(STDERR_FILENO);

    while (1) {
        sleep(60);
        // Do daemon work
    }
    return 0;
}

#include <spawn.h>
#include <sys/wait.h>

extern char **environ;

int spawn_child(const char *cmd, char *const argv[]) {
    pid_t pid;
    posix_spawn_file_actions_t actions;
    posix_spawn_file_actions_init(&actions);
    posix_spawn_file_actions_addclose(&actions, STDIN_FILENO);

    posix_spawnattr_t attr;
    posix_spawnattr_init(&attr);

    int ret = posix_spawnp(&pid, cmd, &actions, &attr, argv, environ);

    posix_spawn_file_actions_destroy(&actions);
    posix_spawnattr_destroy(&attr);

    if (ret != 0) return -1;

    int status;
    waitpid(pid, &status, 0);
    return WIFEXITED(status) ? WEXITSTATUS(status) : -1;
}

#include <sys/wait.h>
#include <signal.h>

// Reap zombies with SIGCHLD handler
void sigchld_handler(int sig) {
    int saved_errno = errno;
    while (waitpid(-1, NULL, WNOHANG) > 0) {
        // Reap all available zombies
    }
    errno = saved_errno;
}

// Setup: handle SIGCHLD
struct sigaction sa;
sa.sa_handler = sigchld_handler;
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_RESTART | SA_NOCLDSTOP;
sigaction(SIGCHLD, &sa, NULL);

// Alternative: explicitly ignore (auto-reap)
// signal(SIGCHLD, SIG_IGN);

// Double-fork to prevent zombies
pid_t inter = fork();
if (inter == 0) {
    if (fork() == 0) {
        // Grandchild does the work
        execlp("sleep", "sleep", "10", NULL);
        _exit(1);
    }
    _exit(0);  // First child exits immediately
}
waitpid(inter, NULL, 0);

#include <sys/resource.h>

void set_limits() {
    struct rlimit lim;

    // Limit CPU time to 10 seconds
    lim.rlim_cur = 10; lim.rlim_max = 10;
    setrlimit(RLIMIT_CPU, &lim);

    // Limit memory to 256 MB
    lim.rlim_cur = 256*1024*1024;
    lim.rlim_max = 256*1024*1024;
    setrlimit(RLIMIT_AS, &lim);

    // Limit open files
    lim.rlim_cur = 32; lim.rlim_max = 64;
    setrlimit(RLIMIT_NOFILE, &lim);
}

void show_limits() {
    struct rlimit lim;
    getrlimit(RLIMIT_NOFILE, &lim);
    printf("Max files: soft=%lu, hard=%lu\n",
           lim.rlim_cur, lim.rlim_max);
}

#include <unistd.h>

int main() {
    int pipefd[2];
    pipe(pipefd);

    pid_t pid = fork();
    if (pid == 0) {
        // Child: read from pipe
        close(pipefd[1]);  // Close unused write end
        char buf[256];
        ssize_t n = read(pipefd[0], buf, sizeof(buf)-1);
        buf[n] = '\0';
        printf("Child received: %s", buf);
        close(pipefd[0]);
    } else {
        // Parent: write to pipe
        close(pipefd[0]);  // Close unused read end
        const char *msg = "Hello from parent!\n";
        write(pipefd[1], msg, strlen(msg));
        close(pipefd[1]);  // EOF signal to reader
        wait(NULL);
    }
    return 0;
}

#include <sys/stat.h>
#include <fcntl.h>

// Create a named pipe (persists in filesystem)
mkfifo("/tmp/myfifo", 0666);

// Writer process
int fd = open("/tmp/myfifo", O_WRONLY);
write(fd, "Hello FIFO", 10);
close(fd);

// Reader process (can be unrelated to writer)
int fd = open("/tmp/myfifo", O_RDONLY);
char buf[256];
ssize_t n = read(fd, buf, sizeof(buf));
close(fd);

// Non-blocking open
int fd = open("/tmp/myfifo", O_RDONLY | O_NONBLOCK);

// Clean up
unlink("/tmp/myfifo");

#include <sys/mman.h>
#include <fcntl.h>

// Create shared memory object
int fd = shm_open("/my_shm", O_CREAT | O_RDWR, 0666);
ftruncate(fd, 4096);  // Set size

// Map into process address space
char *shared = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
                    MAP_SHARED, fd, 0);
close(fd);  // Can close after mmap

// Write data (visible to other processes)
sprintf(shared, "Shared data at %p", (void*)shared);

// Synchronize (flush to backing store)
msync(shared, 4096, MS_SYNC);

// Unmap and clean up
munmap(shared, 4096);
shm_unlink("/my_shm");

#include <sys/socket.h>
#include <sys/un.h>

// Server
int sfd = socket(AF_UNIX, SOCK_STREAM, 0);
struct sockaddr_un addr;
addr.sun_family = AF_UNIX;
strcpy(addr.sun_path, "/tmp/mysocket");
unlink("/tmp/mysocket");
bind(sfd, (struct sockaddr*)&addr, sizeof(addr));
listen(sfd, 5);

int cfd = accept(sfd, NULL, NULL);
char buf[256];
read(cfd, buf, sizeof(buf));
write(cfd, "Response", 8);
close(cfd);

// Client
int sock = socket(AF_UNIX, SOCK_STREAM, 0);
struct sockaddr_un addr;
addr.sun_family = AF_UNIX;
strcpy(addr.sun_path, "/tmp/mysocket");
connect(sock, (struct sockaddr*)&addr, sizeof(addr));
write(sock, "Request", 7);
close(sock);

unlink("/tmp/mysocket");

#include <mqueue.h>

// Create/open a message queue
struct mq_attr attr = {
    .mq_maxmsg = 10,
    .mq_msgsize = 256
};
mqd_t mq = mq_open("/my_queue", O_CREAT | O_RDWR, 0666, &attr);

// Send a message (priority-based)
mq_send(mq, "Hello MQ", 8, 1);  // priority 1

// Receive a message (highest priority first)
char buf[256];
unsigned int prio;
ssize_t n = mq_receive(mq, buf, sizeof(buf), &prio);
printf("Received (priority %u): %.*s\n", prio, (int)n, buf);

// Non-blocking receive
mqd_t mq_nb = mq_open("/my_queue", O_RDONLY | O_NONBLOCK);

// Timed receive
struct timespec ts = { .tv_sec = time(NULL) + 5 };
mq_timedreceive(mq, buf, sizeof(buf), &prio, &ts);

mq_close(mq);
mq_unlink("/my_queue");

# Automatic variables:
# $@ = target name
# $< = first prerequisite
# $^ = all prerequisites
# $? = prerequisites newer than target

# Pattern rule: compiles any .c to .o
%.o: %.c %.h
	$(CC) $(CFLAGS) -c $< -o $@

OBJS = main.o utils.o parser.o

program: $(OBJS)
	$(CC) $(LDFLAGS) $^ -o $@ $(LDLIBS)

# Automatic dependency generation
%.d: %.c
	@$(CC) -MM $< > $@

-include $(OBJS:.o=.d)

clean:
	rm -f $(OBJS) program

# = recursive (evaluated when used)
# := simple (evaluated when defined)
# ?= conditional (only if not already set)

CC := gcc
CFLAGS ?= -Wall -Wextra -O2
DEBUG := $(filter debug,$(MAKECMDGOALS))

ifeq ($(DEBUG),debug)
    CFLAGS += -g -DDEBUG -O0
else
    CFLAGS += -DNDEBUG
endif

ifdef VERBOSE
    Q =
else
    Q = @
endif

build:
	$(Q)$(CC) $(CFLAGS) -c main.c

SRCS = main.c utils.c parser.c

# Substitution
OBJS = $(patsubst %.c,%.o,$(SRCS))  # main.o utils.o parser.o
# Or shorthand:
OBJS = $(SRCS:.c=.o)

# Filter files by extension
C_SRCS = $(filter %.c,$(wildcard *.c))

# Add prefix
FLAGS = $(addprefix -I,$(INCLUDE_DIRS))

# Foreach
DIRS = src lib include
CREATE = $(foreach dir,$(DIRS),mkdir -p $(dir);)

# Shell function
GIT_HASH = $(shell git rev-parse --short HEAD 2>/dev/null || echo "unknown")
CFLAGS += -DGIT_HASH=\"$(GIT_HASH)\"

SUBDIRS = lib src tests

.PHONY: all $(SUBDIRS) clean test

all: $(SUBDIRS)

# Pass variables to sub-makes
$(SUBDIRS):
	$(MAKE) -C $@ $(MAKECMDGOALS)

# Parallel build: make -j4

clean: $(SUBDIRS)
	rm -f *.o program

# Non-recursive alternative (single Makefile)
vpath %.c src:lib
vpath %.h include

CFLAGS += -Iinclude -Ilib

program: main.o lib/utils.o
	$(CC) $^ -o $@

# CMakeLists.txt (modern CMake 3.15+)
cmake_minimum_required(VERSION 3.15)
project(MyApp C)

set(CMAKE_C_STANDARD 11)
set(CMAKE_C_STANDARD_REQUIRED ON)

add_executable(myapp src/main.c src/utils.c)

target_include_directories(myapp PRIVATE include)
target_compile_options(myapp PRIVATE -Wall -Wextra)

# Per-configuration flags
target_compile_options(myapp PRIVATE
    $<$<CONFIG:Debug>:-g -O0 -DDEBUG>
    $<$<CONFIG:Release>:-O2 -DNDEBUG>
)

# Find and link library
find_package(MATH REQUIRED)
target_link_libraries(myapp PRIVATE m)

install(TARGETS myapp DESTINATION bin)

# Compile with debug symbols
gcc -g -O0 -o program program.c

# Start GDB
gdb ./program
gdb --args ./program arg1 arg2

# Breakpoints
(gdb) break main           # Function
(gdb) break 42             # Line 42
(gdb) break file.c:50      # Specific file
(gdb) break func if x > 10 # Conditional
(gdb) tbreak main          # Temporary (removed after hit)
(gdb) watch x              # Break when x changes

# Manage breakpoints
(gdb) info breakpoints
(gdb) delete 2             # Delete breakpoint #2
(gdb) disable 1            # Temporarily disable
(gdb) enable 1

(gdb) run                  # Start program
(gdb) continue             # Continue to next breakpoint
(gdb) next                 # Step over (no function entry)
(gdb) step                 # Step into (enters functions)
(gdb) finish               # Run until function returns
(gdb) until 50             # Run until line 50

# Inspect variables
(gdb) print x              # Print variable
(gdb) print *ptr           # Dereference pointer
(gdb) print arr[0]@5       # First 5 elements
(gdb) print/x x            # Print in hex
(gdb) display x            # Auto-print at each stop
(gdb) info locals          # All local variables

# Backtrace
(gdb) backtrace            # Call stack
(gdb) frame 2              # Switch to frame #2
(gdb) up / down            # Navigate stack

# x/[count][format][size] address
(gdb) x/10x &arr           # 10 words in hex
(gdb) x/20cb &str          # 20 bytes as chars
(gdb) x/4xw ptr            # 4 words in hex
(gdb) x/s str              # String

# Format: x(hex) d(decimal) u(unsigned) o(octal)
#         t(binary) f(float) a(addr) i(instr) c(char) s(string)
# Size: b(byte) h(half/2) w(word/4) g(giant/8)

# Disassemble
(gdb) disassemble main
(gdb) disassemble /r main  # With raw bytes

# Memory map
(gdb) info proc mappings   # Process memory layout

# Enable core dumps
ulimit -c unlimited
echo "core.%p" > /proc/sys/kernel/core_pattern

# Run program until it crashes
./program                  # Produces core.12345

# Analyze core dump
gdb ./program core.12345

(gdb) bt                   # Backtrace at crash
(gdb) bt full              # With local variables
(gdb) frame 0              # Top frame (crash location)
(gdb) info locals          # Variables at crash
(gdb) print *ptr           # What was the pointer?

# Multithreaded
(gdb) info threads         # List all threads
(gdb) thread 3             # Switch to thread 3
(gdb) thread apply all bt  # Backtraces for ALL threads

# .gdbinit file (loaded on startup)
set pagination off
set print pretty on
set print element 0        # No limit on string display

# Define custom commands
define print_array
    set $i = 0
    while $i < $arg0
        printf "[%d] = %d\n", $i, $arg1[$i]
        set $i = $i + 1
    end
end
# Usage: print_array 10 my_array

# Commands attached to breakpoints
break main
commands 1
  silent
  printf "x = %d\n", x
  continue
end

# Python scripting (GDB 7+)
python
import gdb
gdb.execute("break main")
gdb.execute("run")
val = gdb.parse_and_eval("x")
print(f"x = {int(val)}")
end

#include <stddef.h>

// Without alignment consideration (padded)
struct Bad {
    char a;     // 1 byte + 7 padding
    double b;   // 8 bytes
    char c;     // 1 byte + 7 padding
};  // sizeof = 24

// Reordered for efficiency
struct Good {
    double b;   // 8 bytes
    char a;     // 1 byte
    char c;     // 1 byte + 6 padding
};  // sizeof = 16

printf("Bad: a=%zu b=%zu c=%zu total=%zu\n",
    offsetof(struct Bad, a),
    offsetof(struct Bad, b),
    offsetof(struct Bad, c),
    sizeof(struct Bad));

#include <stdalign.h>

// C11 explicit alignment
struct alignas(16) Aligned16 {
    int data[4];  // 16 bytes, 16-byte aligned
};

// Check alignment
printf("Alignment of int: %zu\n", alignof(int));     // 4
printf("Alignment of struct: %zu\n",
       alignof(struct Aligned16));  // 16

// Packed struct (no padding) - GCC/Clang
struct __attribute__((packed)) Packed {
    char a;
    int b;    // No padding after a
    char c;
};  // sizeof = 6

// MSVC packed
#pragma pack(push, 1)
struct PackedMSVC { char a; int b; char c; };
#pragma pack(pop)

#include <stdlib.h>

// C99 flexible array member (last member with no size)
struct Buffer {
    size_t size;
    char data[];  // Flexible array
};

// Allocate with space for data
size_t data_len = 1024;
struct Buffer *buf = malloc(sizeof(struct Buffer) + data_len);
buf->size = data_len;
memset(buf->data, 0, data_len);

// Use data
strcpy(buf->data, "Hello");

// Single free (no separate allocation for data)
free(buf);

#include <stdio.h>

// Union: members share the same memory
union Data {
    int i;
    float f;
    char bytes[4];
};

int main() {
    union Data d;
    d.i = 0x41424344;

    printf("As int: %d\n", d.i);
    printf("As float: %f\n", d.f);
    printf("As bytes: %02x %02x %02x %02x\n",
           d.bytes[0], d.bytes[1], d.bytes[2], d.bytes[3]);

    // Safe type punning (legal in C)
    union { float f; int i; } u;
    u.f = 3.14f;
    printf("Float bits: 0x%08x\n", u.i);

    // Anonymous unions (C11)
    struct Variant {
        int type;
        union { int i; float f; char *s; };
    };
}

#include <stdint.h>

void dump_hex(void *ptr, size_t len) {
    unsigned char *bytes = ptr;
    for (size_t i = 0; i < len; i++) {
        printf("%02x ", bytes[i]);
        if ((i + 1) % 16 == 0) printf("\n");
    }
}

int main() {
    uint32_t val = 0x12345678;
    printf("Value: 0x%08x\n", val);
    printf("Bytes: ");
    dump_hex(&val, sizeof(val));

    // Little-endian: 78 56 34 12 (LSB first)
    // Big-endian:    12 34 56 78 (MSB first)

    // Serialize to big-endian (network order)
    unsigned char be[4];
    be[0] = (val >> 24) & 0xFF;
    be[1] = (val >> 16) & 0xFF;
    be[2] = (val >> 8)  & 0xFF;
    be[3] = val & 0xFF;

    // Deserialize from big-endian
    uint32_t recovered = (be[0]<<24)|(be[1]<<16)|(be[2]<<8)|be[3];
    printf("Recovered: 0x%08x\n", recovered);
    return 0;
}

#include <stdarg.h>

// Variadic function: variable number of arguments
int sum(int count, ...) {
    va_list args;
    va_start(args, count);  // Initialize with last named param

    int total = 0;
    for (int i = 0; i < count; i++) {
        total += va_arg(args, int);  // Get next argument
    }

    va_end(args);  // Cleanup
    return total;
}

// Usage
int result = sum(3, 10, 20, 30);  // 60
int result2 = sum(5, 1, 2, 3, 4, 5);  // 15

#include <stdarg.h>

// Sentinel value marks the end
void log_messages(const char *first, ...) {
    va_list args;
    va_start(args, first);

    const char *msg = first;
    while (msg != NULL) {  // NULL is the sentinel
        printf("%s\n", msg);
        msg = va_arg(args, const char *);
    }

    va_end(args);
}

// Usage: NULL terminates the list
log_messages("Error 1", "Error 2", "Error 3", NULL);

// GCC attribute to enforce sentinel
void log_messages(const char *first, ...)
    __attribute__((sentinel));

#include <stdarg.h>
#include <stdio.h>

// Custom printf-like function
void logf(const char *format, ...) {
    va_list args;
    va_start(args, format);

    // Add timestamp prefix
    printf("[LOG] ");
    vprintf(format, args);  // Pass va_list to vprintf
    printf("\n");

    va_end(args);
}

// Write to string with vsnprintf
void format_msg(char *buf, size_t size, const char *fmt, ...) {
    va_list args;
    va_start(args, fmt);
    vsnprintf(buf, size, fmt, args);  // Safe: bounded
    va_end(args);
}

// Usage
logf("User %s logged in (id=%d)", username, id);
char msg[256];
format_msg(msg, sizeof(msg), "Error %d: %s", code, text);

#include <stdlib.h>

// Function pointer type
typedef int (*compare_fn)(const void *, const void *);

// Comparison function for qsort
int cmp_int(const void *a, const void *b) {
    return *(const int *)a - *(const int *)b;
}

int main() {
    int arr[] = {5, 2, 8, 1, 9, 3};
    size_t n = sizeof(arr) / sizeof(arr[0]);

    // qsort takes a function pointer
    qsort(arr, n, sizeof(int), cmp_int);

    // Array of function pointers
    double (*ops[])(double, double) = {
        add, subtract, multiply, divide
    };
    double result = ops[2](10.0, 3.0);  // multiply

    // Function pointer as parameter
    void apply(int *arr, size_t n, int (*fn)(int)) {
        for (size_t i = 0; i < n; i++) arr[i] = fn(arr[i]);
    }
    apply(arr, n, square);
    return 0;
}

// C99 variadic macros
#define LOG(fmt, ...) printf("[LOG] " fmt "\n", __VA_ARGS__)

// Usage
LOG("Value: %d", x);
LOG("User %s, age %d", name, age);

// GCC __VA_OPT__ (C99/C2x): handle zero arguments
#define LOG2(fmt, ...)     printf("[LOG] " fmt "\n" __VA_OPT__(,) __VA_ARGS__)

// Count arguments (GCC extension)
#define COUNT(...) NARG_(__VA_ARGS__, 5, 4, 3, 2, 1)
#define NARG_(_1, _2, _3, _4, _5, N, ...) N

// Debug macro that compiles out in release
#ifdef NDEBUG
    #define DEBUG(fmt, ...) ((void)0)
#else
    #define DEBUG(fmt, ...) fprintf(stderr, fmt, ##__VA_ARGS__)
#endif

// ## removes comma if no variadic args (GCC extension)
DEBUG("just a message");
DEBUG("value = %d", x);