C Cheat Sheet

The C programming language is a foundational, low-level language used for operating systems, embedded systems, performance‑critical software, and systems programming. This C cheatsheet provides a structured reference for C syntax, pointers, memory management, data structures, and common programming patterns.

Compilation

gcc main.c -o app
./app

Debug buildC standard

gcc -g -Wall -Wextra main.c -o app

gcc -std=c11 main.c -o app

Basic Program Structure

#include <stdio.h>

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

Data Types

int integer_value = 10;
float float_value = 3.14f;
double double_value = 3.14159;
char character = 'A';

Sizes

sizeof(int);
sizeof(double);
sizeof(char);

Exact-width integers

#include <stdint.h>

int8_t   a;
int32_t  b;
uint64_t c;

Variables & Constants

int count = 0;
const int max_value = 100;

#define PI 3.14159

Operators

a + b
a - b
a * b
a / b
a % b

a == b
a != b
a > b
a < b

a && b
a || b
!a

Control Flow

if (value > 10) {
    result = 1;
} else {
    result = 0;
}

switch (value) {
    case 1:
        break;
    case 2:
        break;
    default:
        break;
}

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

while (count > 0) {
    count--;
}

Functions

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

Function prototypes

int add(int a, int b);

Arrays

int numbers[3] = {1, 2, 3};

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

Pointers

int value = 10;
int *ptr = &value;

*ptr = 20;

Pointer basics

& → address of
* → dereference

printf("%p\n", (void*)ptr);

Pointer Arithmetic

int values[3] = {1, 2, 3};
int *p = values;

p++;
printf("%d\n", *p);

Out-of-bounds

Pointer arithmetic outside array bounds causes undefined behavior.

Strings

#include <string.h>

char text[16] = "hello";
strlen(text);
strcpy(text, "hi");
strcmp(text, "hi");

Null-terminated

C strings must end with '\0'.

Structs

struct Point {
    int x;
    int y;
};

struct Point p = {10, 20};
p.x = 5;

struct Point *ptr = &p;
ptr->y = 15;

Enums

enum Status {
    STATUS_OK,
    STATUS_ERROR
};

Memory Management

#include <stdlib.h>

int *data = malloc(sizeof(int) * 10);

if (data == NULL) {
    return 1;
}

free(data);

calloc vs malloc

int *arr = calloc(10, sizeof(int));

Memory leaks

Every malloc / calloc must have a matching free.

File I/O

#include <stdio.h>

FILE *file = fopen("data.txt", "r");

if (file != NULL) {
    fclose(file);
}

fprintf(file, "text\n");
fscanf(file, "%d", &value);

Header Files

// math_utils.h
int add(int a, int b);

// math_utils.c
#include "math_utils.h"

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

Preprocessor

#define DEBUG 1

#if DEBUG
printf("debug mode\n");
#endif

Common Low-Level Patterns

int tmp = a;
a = b;
b = tmp;

memset(buffer, 0, size);

for (;;) {
    break;
}