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C - Pointers Basics

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Pointers Basics

Code Properties

  • Language: C

Overview

Pointers are one of the most powerful and distinctive features of C. A pointer is a variable that stores the memory address of another variable, enabling direct memory manipulation, efficient data structures, and dynamic memory allocation.

Code

Pointer Declaration and Initialization

#include <stdio.h>
 
int main(void) {
    int value = 42;
    int *ptr = &value;  // ptr holds the address of value
    
    printf("Value: %d\n", value);           // 42
    printf("Address of value: %p\n", (void*)&value);
    printf("Pointer ptr: %p\n", (void*)ptr);
    printf("Dereferenced ptr: %d\n", *ptr); // 42
    
    return 0;
}

Pointer Operators

#include <stdio.h>
 
int main(void) {
    int x = 10;
    int *p = &x;  // & = address-of operator
    
    // * = dereference operator (get value at address)
    *p = 20;      // modifies x through the pointer
    
    printf("x = %d\n", x);  // 20
    
    return 0;
}

Pointer Arithmetic

#include <stdio.h>
 
int main(void) {
    int arr[] = {10, 20, 30, 40, 50};
    int *p = arr;  // points to first element
    
    printf("*p     = %d\n", *p);       // 10
    printf("*(p+1) = %d\n", *(p + 1)); // 20
    printf("*(p+2) = %d\n", *(p + 2)); // 30
    
    p++;  // move to next element
    printf("After p++: *p = %d\n", *p); // 20
    
    // pointer difference
    int *start = arr;
    int *end = &arr[4];
    printf("Elements between: %td\n", end - start); // 4
    
    return 0;
}

Pointers and Arrays

#include <stdio.h>
 
void print_array(int *arr, int size) {
    for (int i = 0; i < size; i++) {
        printf("%d ", arr[i]);  // arr[i] is equivalent to *(arr + i)
    }
    printf("\n");
}
 
int main(void) {
    int numbers[] = {1, 2, 3, 4, 5};
    int size = sizeof(numbers) / sizeof(numbers[0]);
    
    // array name decays to pointer to first element
    print_array(numbers, size);
    
    // equivalent pointer notation
    int *p = numbers;
    for (int i = 0; i < size; i++) {
        printf("%d ", *(p + i));
    }
    printf("\n");
    
    return 0;
}

Pointer to Pointer

#include <stdio.h>
 
int main(void) {
    int value = 100;
    int *p = &value;     // pointer to int
    int **pp = &p;       // pointer to pointer to int
    
    printf("value = %d\n", value);
    printf("*p = %d\n", *p);
    printf("**pp = %d\n", **pp);
    
    // modify through double pointer
    **pp = 200;
    printf("value after **pp = 200: %d\n", value);
    
    return 0;
}

NULL Pointers

#include <stdio.h>
#include <stdlib.h>
 
int main(void) {
    int *ptr = NULL;  // null pointer initialization
    
    // always check for NULL before dereferencing
    if (ptr != NULL) {
        printf("Value: %d\n", *ptr);
    } else {
        printf("Pointer is NULL\n");
    }
    
    // common pattern with dynamic allocation
    ptr = malloc(sizeof(int));
    if (ptr == NULL) {
        fprintf(stderr, "Memory allocation failed\n");
        return EXIT_FAILURE;
    }
    
    *ptr = 42;
    printf("Allocated value: %d\n", *ptr);
    
    free(ptr);
    ptr = NULL;  // good practice: set to NULL after freeing
    
    return EXIT_SUCCESS;
}

Function Pointers

#include <stdio.h>
 
// function pointer type definition
typedef int (*operation_fn)(int, int);
 
int add(int a, int b) { return a + b; }
int subtract(int a, int b) { return a - b; }
int multiply(int a, int b) { return a * b; }
 
int calculate(int a, int b, operation_fn op) {
    return op(a, b);
}
 
int main(void) {
    // direct function pointer usage
    int (*op)(int, int) = add;
    printf("add(5, 3) = %d\n", op(5, 3));
    
    // using typedef
    operation_fn operations[] = {add, subtract, multiply};
    char *names[] = {"add", "subtract", "multiply"};
    
    for (int i = 0; i < 3; i++) {
        printf("%s(10, 4) = %d\n", names[i], calculate(10, 4, operations[i]));
    }
    
    return 0;
}

Details

Pointer Types and Sizes

TypeDescription
int *Pointer to int
char *Pointer to char (often used for strings)
void *Generic pointer (no type information)
const int *Pointer to constant int (value cannot be changed)
int * constConstant pointer to int (address cannot be changed)
const int * constConstant pointer to constant int

Common Pitfalls

IssueDescription
Dangling pointerPointer to freed/deallocated memory
Wild pointerUninitialized pointer with garbage address
Memory leakLosing reference to allocated memory
Buffer overflowWriting beyond allocated memory

Void Pointers

#include <stdio.h>
 
void print_value(void *ptr, char type) {
    switch (type) {
        case 'i': printf("%d\n", *(int*)ptr); break;
        case 'f': printf("%f\n", *(float*)ptr); break;
        case 'c': printf("%c\n", *(char*)ptr); break;
    }
}
 
int main(void) {
    int i = 42;
    float f = 3.14f;
    char c = 'A';
    
    print_value(&i, 'i');
    print_value(&f, 'f');
    print_value(&c, 'c');
    
    return 0;
}

Appendix

Note created on 2025-12-31 and last modified on 2025-12-31.

See Also

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