C

BASIC STRUCTURE

#include <stdio.h>
int main() {
    // code
    return 0;
}

---

Data Types

Type	Size	Format	Range
char	1 byte	%c	-128 to 127
int	4 bytes	%d	-2,147,483,648 to 2,147,483,647
float	4 bytes	%f	3.4E-38 to 3.4E+38
double	8 bytes	%lf	1.7E-308 to 1.7E+308
short	2 bytes	%hd	-32,768 to 32,767
long	8 bytes	%ld	-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807

---

Input and Output

/* --- INPUT/OUTPUT CHEAT SHEET --- */

// Standard Console I/O
printf("Value: %d\n", num);          // Output formatted data
scanf("%d", &num);                   // Input formatted data (use & for address)
getchar();                           // Read a single character from stdin
putchar('A');                        // Write a single character to stdout
puts("Hello");                       // Print string + automatic newline
fgets(str, 100, stdin);              // Read string safely (prevents overflow)

// String Buffer I/O (In-memory)
sprintf(buffer, "Age: %d", age);     // Write formatted data TO a string
sscanf(buffer, "%d", &age);          // Read formatted data FROM a string

// File Stream I/O
FILE *fp = fopen("data.txt", "r");   // Open for reading ("r"), writing ("w"), or appending ("a")
fprintf(fp, "ID: %d", id);           // Print formatted data to file
fscanf(fp, "%d", &id);               // Read formatted data from file
fputs("No newline", fp);             // Write string to file without adding \n
fgetc(fp);                           // Read single character from file
fclose(fp);                          // Always close your file pointers!

// Essential Format Specifiers
// %d (int), %u (unsigned), %f (float), %lf (double), %c (char), %s (string), %p (pointer)

// Buffer Clearing Trick (Fixes skipped scanf/fgets issues)
while ((getchar()) != '\n' && getchar() != EOF);

---

FORMAT SPECIFIERS

Specifier	Type
%d, %i	Integer
%f	Float
%lf	Double
%c	Character
%s	String
%p	Pointer
%x	Hexadecimal
%o	Octal
%%	Print %

---

Operators

// ARITHMETIC
+ - * / %  ++  --

// RELATIONAL
== != > < >= <=

// LOGICAL
&&  ||  !

// BITWISE
&  |  ^  ~  <<  >>

// ASSIGNMENT
= += -= *= /= %= &= |= ^= <<= >>=

// TERNARY
condition ? true : false

---

Control flow

// IF-ELSE
if (x > 0) { }
else if (x == 0) { }
else { }

// SWITCH
switch(x) {
    case 1: break;
    case 2: break;
    default: break;
}

// FOR LOOP
for (int i = 0; i < 10; i++) { }

// WHILE LOOP
while (condition) { }

// DO-WHILE
do { } while (condition);

// BREAK, CONTINUE
break;      // Exit loop
continue;   // Skip iteration

---

Functions

// DECLARATION
return_type func_name(type param);

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

// CALL
result = add(5, 3);

---

Arrays

int arr[5];                     // Declaration
int arr[] = {1,2,3,4,5};       // Initialization
arr[0] = 10;                    // Access
int len = sizeof(arr)/sizeof(arr[0]);  // Length

// 2D ARRAY
int matrix[3][3] = {{1,2,3}, {4,5,6}, {7,8,9}};

---

Strings

/* --- STRING MANIPULATION (<string.h>) --- */

char str[100] = "Hello";
strlen(str);                 // Returns length (excluding null terminator '\0')
strcpy(dest, src);           // Copy src to dest (risky if dest is small)
strncpy(dest, src, n);       // Copy up to n characters (safer)
strcat(dest, src);           // Append src to end of dest
strncat(dest, src, n);       // Append up to n characters (safer)
strcmp(s1, s2);              // Compare: 0 if equal, <0 if s1 < s2, >0 if s1 > s2
strncmp(s1, s2, n);          // Compare first n characters
strcasecmp(s1, s2);          // Case-insensitive compare (POSIX/Strings.h)

// Searching & Tokenizing
strchr(str, 'c');            // Pointer to first occurrence of 'c' (NULL if not found)
strrchr(str, 'c');           // Pointer to LAST occurrence of 'c'
strstr(str, "sub");          // Pointer to first occurrence of substring "sub"
strspn(str, "abc");          // Length of initial segment containing only 'a', 'b', or 'c'
strcspn(str, "123");         // Length of initial segment NOT containing '1', '2', or '3'
strtok(str, ",");            // Tokenize string by delimiter (updates state with NULL)

// Character Logic (<ctype.h>) - Useful for loops
isalpha(c);                  // Non-zero if alphabetic
isdigit(c);                  // Non-zero if 0-9
isspace(c);                  // Non-zero if whitespace (space, tab, \n)
toupper(c);                  // Convert to uppercase
tolower(c);                  // Convert to lowercase

// Memory Operations (also in <string.h>)
memset(str, 0, sizeof(str)); // Fill memory with a constant (e.g., zeroing an array)
memcpy(dest, src, n);        // Copy raw memory blocks (faster than strcpy)
memcmp(s1, s2, n);           // Compare raw memory blocks

---

Pointers

/* --- POINTERS & MEMORY (<stdlib.h>) --- */

int *ptr;                // Pointer to an integer
ptr = &var;              // Store address of var in ptr
*ptr = 10;               // Dereference: change value at address to 10
int **dptr = &ptr;       // Pointer to a pointer (Double Pointer)

// Array & Pointer Equivalence
arr[i] == *(arr + i);    // Standard indexing vs pointer arithmetic
*(ptr + 1);              // Moves to the next memory block (size of int)

// Dynamic Memory Allocation (The Heap)
ptr = (int*)malloc(n * sizeof(int));  // Allocate memory for n integers
ptr = (int*)calloc(n, sizeof(int));  // Allocate AND zero-initialize memory
ptr = realloc(ptr, new_size);        // Resize previously allocated block
free(ptr);                           // Deallocate memory (prevent leaks!)
ptr = NULL;                          // Best practice: nullify after freeing

// Constants and Pointers
const int *p = &x;       // Data is constant (cannot change *p)
int * const p = &x;      // Pointer is constant (cannot change p)

// Special Pointers
void *vptr;              // Generic pointer (can point to any type)
int (*f)(int);           // Function pointer (points to a function)
if (ptr == NULL)         // Null check: always verify before dereferencing

---

Memory Management

/* --- DYNAMIC MEMORY MANAGEMENT (<stdlib.h>) --- */

// Allocation Functions
void *ptr = malloc(size);          // Allocate 'size' bytes (contains garbage values)
void *ptr = calloc(n, size);       // Allocate n * size bytes (all bits set to 0)
void *ptr = realloc(old_ptr, s);   // Resize block; may move to a new address
free(ptr);                         // Release memory back to the system

// Safety & Best Practices
if (ptr == NULL) {                 // ALWAYS check if allocation failed
    perror("malloc failed");
    exit(1);
}

// Avoiding Memory Leaks & Errors
free(ptr);                         // 1. Deallocate when finished
ptr = NULL;                        // 2. Prevent "Dangling Pointer" (pointing to freed memory)

// Memory Operations (<string.h>)
memset(ptr, 0, n * sizeof(int));   // Manually zero out a block
memcpy(dest, src, n);              // Copy n bytes from src to dest
memmove(dest, src, n);             // Safer copy if memory regions overlap

// Common Pitfalls
// 1. Memory Leak: Forgetting to free() before losing the pointer.
// 2. Double Free: Calling free() on the same pointer twice (causes crashes).
// 3. Buffer Overflow: Writing past the allocated size of the block.

---

Structures

// DEFINITION
struct Person {
    char name[50];
    int age;
};

// DECLARATION
struct Person p1;
p1.age = 25;

// WITH TYPEDEF
typedef struct {
    int x, y;
} Point;

union Data {
    int i;
    float f;
    char str[20];            // Only ONE member can be held at a time
};
// sizeof(union Data) is the size of its largest member (20 bytes)

Point p2 = { .y = 50, .x = 10 };        // Initialize by name, any order

---

File I/O

FILE *fp;

// OPEN MODES: "r" "w" "a" "r+" "w+" "a+"
fp = fopen("file.txt", "r");

// READ/WRITE
fprintf(fp, "%d", num);         // Write
fscanf(fp, "%d", &num);         // Read
fgetc(fp);                      // Read char
fputc('A', fp);                 // Write char
fgets(str, 100, fp);            // Read line
fputs("text", fp);              // Write string

fclose(fp);                     // Close file

/* --- ADVANCED FILE I/O --- */

// 1. ERROR CHECKING
if ((fp = fopen("file.txt", "r")) == NULL) {
    perror("Error opening file");   // Prints system error message
}

// 2. BINARY READ/WRITE
// fwrite(data_ptr, size_of_element, count, fp);
fwrite(&struct_var, sizeof(struct_var), 1, fp);
fread(&struct_var, sizeof(struct_var), 1, fp);

// 3. FILE POSITIONING (SEEKING)
fseek(fp, 0, SEEK_END);             // Move to end of file
long size = ftell(fp);              // Get current position (useful for file size)
rewind(fp);                         // Move cursor back to start (offset 0)
// Origin options: SEEK_SET (start), SEEK_CUR (current), SEEK_END (end)

// 4. BUFFERING & STATUS
fflush(fp);                         // Force write buffered data to disk
feof(fp);                           // Returns non-zero if End-Of-File reached
ferror(fp);                         // Returns non-zero if error occurred
clearerr(fp);                       // Resets error and EOF indicators

// 5. FILE SYSTEM OPS
remove("old_file.txt");             // Delete a file
rename("old.txt", "new.txt");       // Rename a file

---

Preprocessor Directive

#include <stdio.h>              // Search system directories
#include "my_file.h"            // Search local directory first

#define PI 3.14159              // Constant macro
#define SQUARE(x) ((x) * (x))   // Function-like macro (use parentheses!)
#define MAX(a,b) ((a)>(b)?(a):(b))

// 1. CONDITIONAL COMPILATION
#if defined(WIN32) || defined(_WIN64)
    #define PLATFORM "Windows"
#elif defined(__linux__)
    #define PLATFORM "Linux"
#else
    #define PLATFORM "Unknown"
#endif

// 2. HEADER GUARDS (Prevents double inclusion)
#ifndef MY_HEADER_H
#define MY_HEADER_H
    // Header content here
#endif

// 3. PREDEFINED MACROS (Useful for debugging)
__FILE__      // Current file name (string)
__LINE__      // Current line number (int)
__DATE__      // Compilation date (string)
__TIME__      // Compilation time (string)
__func__      // Current function name (string, C99)

// 4. STRINGIFICATION & CONCATENATION
#define TO_STR(x) #x            // Converts x to "x"
#define CONCAT(a, b) a##b       // Joins a and b into one token

// 5. ERROR & PRAGMA
#error "ErrorMessage"           // Stops compilation with a message
#pragma once                    // Modern alternative to header guards
#undef PI                       // Removes macro definition

---

Math Functions (#include <math.h>)

pow(x, y)       // x^y
sqrt(x)         // Square root
abs(x)          // Absolute value
ceil(x)         // Round up
floor(x)        // Round down
sin(x) cos(x) tan(x)
log(x) log10(x)

---

Common Header Files

<stdio.h>       // I/O functions
<stdlib.h>      // Memory, conversion, random
<string.h>      // String operations
<math.h>        // Math functions
<ctype.h>       // Character functions
<time.h>        // Time/date functions
<stdbool.h>     // Boolean type
<limits.h>      // Type limits (INT_MAX, etc.)

---

TYPEDEF & ENUM

// TYPEDEF
typedef unsigned long ulong;
typedef int* intptr;

// ENUM
enum Day {SUN, MON, TUE, WED, THU, FRI, SAT};
enum Day today = MON;

---

USEFUL SNIPPETS

// SWAP TWO NUMBERS
temp = a; a = b; b = temp;

// TERNARY OPERATOR
max = (a > b) ? a : b;

// SIZEOF OPERATOR
size = sizeof(int);  // Returns 4

// COMMAND LINE ARGUMENTS
int main(int argc, char *argv[]) { }

---

COMPILATION & EXECUTION

gcc program.c -o program    # Compile
./program                   # Run
gcc -Wall -g program.c      # Debug mode