# C Quick Reference *Syntax, pointers, memory management, standard library essentials* > Source: C Standard Library (cppreference.com) · MIT ## 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 | Command | Description | |---------|-------------| | `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) | Command | Description | |---------|-------------| | `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 | Command | Description | |---------|-------------| | `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 | Command | Description | |---------|-------------| | `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) | Command | Description | |---------|-------------| | `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 | Command | Description | |---------|-------------| | `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 | Command | Description | |---------|-------------| | `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 | Command | Description | |---------|-------------| | `"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 | Command | Description | |---------|-------------| | `__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 |