C Cheatsheet - C Language Syntax & Pointer Reference

This reference is for developers writing C for embedded, OS, or performance-critical work, where unlike managed languages you own memory and pointer lifetimes. It covers data types and constants, control flow, pointers and arrays (where the footguns live), functions, manual allocation and freeing, file I/O, and the preprocessor. Entries are grouped by the build block you are working in, with notes flagging where behavior is easy to get wrong. After reading you should be able to decide when to use a pointer vs an array, manage malloc/free without leaks, and encode compile-time logic with #ifdef.

Languages·58 commands·Last updated 2026-07-21

Data Types & Variables 8

int x = 10;

Integer variable

float pi = 3.14f;

Single-precision float

double e = 2.718;

Double-precision float

char c = 'A';

Character variable

char str[] = "Hello";

String (char array)

const int MAX = 100;

Constant

enum {RED, GREEN, BLUE};

Enum type

typedef int Integer;

Type alias

Control Flow 8

if (x > 0) { } else if (x < 0) { } else { }

Conditional branch

switch (x) { case 1: break; default: break; }

Multi-way branch

for (int i = 0; i < n; i++) { }

for loop

while (condition) { }

while loop

do { } while (condition);

do-while loop

break;

Break out of loop/switch

continue;

Skip current iteration

goto label;

Jump (use with care)

Pointers & Arrays 8

int *p;

Pointer declaration

int x = 10; int *p = &x;

Take an address

*p = 20;

Dereference

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

Array initialization

int *p = arr;

Array name is the base address

*(p + i)

Pointer arithmetic (equals arr[i])

int **pp;

Double pointer

void *vp;

Generic pointer

Functions 6

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

Function definition

void swap(int *a, int *b);

Function declaration (prototype)

int (*fp)(int, int) = add;

Function pointer

static int count = 0;

Static local variable

extern int globalVar;

Extern variable declaration

inline int square(int x) { return x * x; }

Inline function

Memory Management 7

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

Allocate memory dynamically

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

Allocate and zero-initialize

p = (int *)realloc(p, sizeof(int) * 20);

Reallocate memory

free(p);

Free memory

memset(p, 0, size);

Initialize memory

memcpy(dest, src, size);

Copy memory

memmove(dest, src, size);

Safe memory copy (overlap-safe)

Structs & Unions 6

struct Point { int x; int y; };

Struct definition

struct Point p = {1, 2};

Struct initialization

p.x = 10;

Access a member

struct Point *pp = &p; pp->x = 10;

Pointer access to a member

union Data { int i; float f; };

Union (shared memory)

typedef struct { int x; int y; } Point;

Struct alias

File I/O 8

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

Open a file (r/w/a/rb/wb)

fclose(fp);

Close a file

fscanf(fp, "%d", &x);

Formatted read

fprintf(fp, "%d\n", x);

Formatted write

fread(buf, size, count, fp);

Binary read

fwrite(buf, size, count, fp);

Binary write

fgets(buf, size, fp);

Read a line

fseek(fp, offset, SEEK_SET);

Seek within a file

Preprocessor 7

#include <stdio.h>

Include a system header

#include "myheader.h"

Include a custom header

#define MAX 100

Macro definition

#define SQUARE(x) ((x) * (x))

Macro function

#ifdef DEBUG ... #endif

Conditional compilation

#ifndef HEADER_H ... #define HEADER_H ... #endif

Header guard

#pragma once

Modern header guard

Tips

Always check that malloc's return value is not NULL.
Set the pointer to NULL after free to avoid dangling pointers.
An array name decays to a pointer in most expressions, but sizeof(arr) is still the whole array size.

Official References

Each command links to its official documentation below, so you can verify the latest usage and read deeper.

Official Docs

Maintained by LaoHand

Publicly updated on Jul 21, 2026, continuously proofread against official docs.

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