Pointers

Theory

Pointers are variables that store memory addresses. They are one of C's most powerful features, enabling direct memory manipulation, dynamic allocation, and efficient array/function operations.

Declaring a pointer uses the * symbol:

int *ptr;    // pointer to an integer
char *cp;    // pointer to a character
float *fp;   // pointer to a float

The address-of operator & gets the memory address of a variable:

int x = 42;
int *ptr = &x;  // ptr stores the address of x

The dereference operator * accesses the value at the address stored in a pointer:

printf("%d", *ptr);  // prints 42 (the value of x)
*ptr = 100;          // changes x to 100

Pointer arithmetic allows incrementing/decrementing pointers. Adding 1 to an int* advances by sizeof(int) bytes (usually 4).

int arr[] = {10, 20, 30};
int *p = arr;       // points to arr[0]
printf("%d", *p);   // 10
p++;                // now points to arr[1]
printf("%d", *p);   // 20

Pointers and arrays are closely related. An array name is a pointer to its first element. arr[i] is equivalent to *(arr + i).

Passing pointers to functions allows modifying variables in the calling scope (simulating pass-by-reference):

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

Pointers to pointers (int **ptr) store the address of another pointer. Used in multi-dimensional arrays and dynamic arrays of strings.

Dynamic memory allocation uses functions from <stdlib.h>:

• malloc(size) — allocates memory on the heap, returns void*
• calloc(count, size) — allocates and zero-initializes
• realloc(ptr, new_size) — resizes existing allocation
• free(ptr) — deallocates memory

int *arr = (int*)malloc(5 * sizeof(int));
free(arr);  // always free!

NULL pointer (NULL or 0) points to nothing. Always check for NULL after allocation and before dereferencing.

Function pointers store the address of a function, enabling callback mechanisms:

int (*funcPtr)(int, int) = &add;
int result = funcPtr(3, 5);

Practical Examples

Exercises

#include <stdio.h>\n\nint main() {\n  int num = 42;\n  int *ptr = #\n  // Your code here\n  return 0;\n}

num = 42\nAddress of num: 0x...\nptr points to: 0x...\nDereferenced: 42\nAfter *ptr = 99: num = 99

How many numbers? 5\nEnter 5 numbers: 10 20 30 40 50\nSum: 150

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