Basics

Hello World

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

Compile & Run

gcc -o app main.c          # compile
gcc -Wall -Wextra -std=c17 main.c  # strict
./app                       # run

Comments

// single-line comment (C99+)
/* multi-line
   comment */

Data Types

Primitive Types

char	1 byte, character or small integer
short	At least 16 bits
int	At least 16 bits (typically 32)
long	At least 32 bits
long long	At least 64 bits (C99+)
float	32-bit IEEE-754
double	64-bit IEEE-754
_Bool / bool	`0` or `1` (use for `bool`)

Fixed-Width Types (stdint.h)

int8_t, uint8_t	Exact 8-bit signed / unsigned
int16_t, uint16_t	Exact 16-bit
int32_t, uint32_t	Exact 32-bit
int64_t, uint64_t	Exact 64-bit
size_t	Unsigned, result of `sizeof`

Type Casting

int i = (int)3.14;          // explicit cast
double d = (double)5 / 2;   // 2.5, not 2
char c = (char)65;          // 'A'

Control Flow

If / Else

if (x > 0) { printf("positive\n"); }
else if (x == 0) { printf("zero\n"); }
else { printf("negative\n"); }

Switch

switch (choice) {
    case 1:  printf("one\n"); break;
    case 2:  printf("two\n"); break;
    default: printf("other\n");
}

Loops

for (int i = 0; i < 10; i++) { }
while (condition) { }
do { } while (condition);

Jump Statements

break	Exit innermost loop or switch
continue	Skip to next iteration
return	Exit function with optional value
goto label	Jump to label (use sparingly)

Functions

Declaration & Definition

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

Function Pointers

int (*op)(int, int) = add;
int result = op(3, 4);       // calls add(3, 4)
typedef int (*MathFn)(int, int);
MathFn fn = add;

Static Functions

// visible only within this translation unit
static int helper(int x) {
    return x * 2;
}

Pointers

Pointer Basics

int x = 42;
int *p = &x;        // p points to x
printf("%d\n", *p); // dereference: 42
*p = 100;           // x is now 100

Pointer Arithmetic

int arr[] = {10, 20, 30};
int *p = arr;
printf("%d\n", *(p + 1));  // 20
printf("%d\n", p[2]);      // 30 (same as *(p+2))

Common Pointer Patterns

int *p = NULL	Null pointer (always initialize)
void *	Generic pointer (must cast to use)
const int *p	Pointer to constant (cannot modify value)
int *const p	Constant pointer (cannot reassign pointer)
int **pp	Pointer to pointer (double indirection)

Arrays & Strings

Arrays

int nums[5] = {1, 2, 3, 4, 5};
int matrix[2][3] = {{1,2,3}, {4,5,6}};
int len = sizeof(nums) / sizeof(nums[0]);

String Functions (string.h)

strlen(s)	Length (excluding null terminator)
strcpy(dst, src)	Copy string (unsafe, prefer `strncpy`)
strncpy(dst, src, n)	Copy at most n characters
strcat(dst, src)	Concatenate strings
strcmp(a, b)	Compare: 0 if equal, <0 or >0 otherwise
strchr(s, c)	Find first occurrence of character
strstr(haystack, needle)	Find substring

String Literals

char greeting[] = "hello";     // mutable array
const char *msg = "world";     // pointer to literal
char buf[64];
snprintf(buf, sizeof(buf), "%s %s", greeting, msg);

Structs

Definition & Usage

struct Point { double x; double y; };
struct Point p = {1.0, 2.0};
printf("(%g, %g)\n", p.x, p.y);

Typedef

typedef struct {
    char name[50];
    int age;
} Person;
Person p = {"Alice", 30};

Struct Pointers

void set_age(Person *p, int age) {
    p->age = age;    // arrow operator
}

Enums & Unions

enum Color { RED, GREEN, BLUE };
union Data { int i; float f; char c; };
// union members share the same memory

Memory Management

Dynamic Allocation (stdlib.h)

int *arr = malloc(10 * sizeof(int));
if (arr == NULL) { /* handle error */ }
arr = realloc(arr, 20 * sizeof(int));
free(arr);
arr = NULL;   // avoid dangling pointer

Allocation Functions

malloc(size)	Allocate uninitialized memory
calloc(count, size)	Allocate and zero-initialize
realloc(ptr, size)	Resize previously allocated block
free(ptr)	Release allocated memory

Common Pitfalls

Memory leak	Forgetting to `free()` allocated memory
Double free	Calling `free()` on same pointer twice
Dangling pointer	Using pointer after `free()` — set to NULL
Buffer overflow	Writing past allocated bounds

File I/O

Reading Files

FILE *f = fopen("data.txt", "r");
if (!f) { perror("open"); return 1; }
char line[256];
while (fgets(line, sizeof(line), f)) printf("%s", line);
fclose(f);

Writing to Files

FILE *f = fopen("out.txt", "w");
fprintf(f, "value: %d\n", 42);
fputs("hello\n", f);
fclose(f);

File Modes

"r"	Read (file must exist)
"w"	Write (truncates or creates)
"a"	Append (creates if needed)
"rb", "wb"	Binary read / write
"r+"	Read and write (file must exist)

Preprocessor

Directives

#include        // system header
#include "myheader.h"    // local header
#define PI 3.14159
#define MAX(a, b) ((a) > (b) ? (a) : (b))

Conditional Compilation

#ifdef DEBUG
    printf("debug: x = %d\n", x);
#endif
#ifndef HEADER_H   /* include guard */
#define HEADER_H   /* ... */ #endif

Common Macros

__FILE__	Current source file name
__LINE__	Current line number
__func__	Current function name (C99+)
__DATE__	Compilation date string
NULL	Null pointer constant
sizeof(x)	Size of type or variable in bytes

C QUICK REFERENCE