👉 Pointers in C Language

🔹 What is a Pointer?

A pointer is a variable that stores the address of another variable.

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📌 Syntax of Pointer

data_type *pointer_name;
Example:
int a = 10;
int *p = &a;

✅ Why Use Pointers?

  • Dynamic memory allocation
  • Efficient array handling
  • Call by reference
  • Accessing hardware and memory directly

📘 Example of Pointer

#include <stdio.h>

int main() {
  int x = 10;
  int *p = &x;

  printf("Value of x: %d\n", x);
  printf("Address of x: %p\n", &x);
  printf("Value using pointer: %d", *p);

  return 0;
}

📂 Types of Pointers in C

1️⃣ Null Pointer

A pointer that does not point to any memory location.

int *p = NULL;

2️⃣ Void Pointer

A pointer that can store the address of any data type.

void *p;
int a = 10;
p = &a;

3️⃣ Wild Pointer

A pointer that is not initialized.

int *p; // wild pointer

4️⃣ Dangling Pointer

A pointer that points to a memory location that has been freed.

int *p = (int*)malloc(sizeof(int));
free(p); // p becomes dangling

5️⃣ Pointer to Pointer

A pointer that stores the address of another pointer.

int a = 10;
int *p = &a;
int **q = &p;

6️⃣ Constant Pointer

Pointer whose value cannot be changed.

int a = 10;
int *const p = &a;

📌 Call by Value in C

In call by value, a copy of the variable is passed to the function. Changes made inside the function do not affect the original variable.

#include <stdio.h>

void change(int x) {
  x = 20;
}

int main() {
  int a = 10;
  change(a);
  printf("%d", a);
  return 0;
}
Output:
10

📌 Call by Reference in C

In call by reference, the address of the variable is passed using pointers. Changes inside the function affect the original variable.

#include <stdio.h>

void change(int *x) {
  *x = 20;
}

int main() {
  int a = 10;
  change(&a);
  printf("%d", a);
  return 0;
}
Output:
20

⚖️ Call by Value vs Call by Reference

Call by Value Call by Reference
Passes value Passes address
Uses normal variables Uses pointers
Original value not changed Original value changes
Less efficient More efficient

🔁 Example: Swapping Numbers

Call by Value (Not Working)
void swap(int a, int b) {
  int temp = a;
  a = b;
  b = temp;
}
Call by Reference (Working)
void swap(int *a, int *b) {
  int temp = *a;
  *a = *b;
  *b = temp;
}

📚 FAQs on Pointers in C

Q1. What is a pointer?

A pointer is a variable that stores the address of another variable.

Q2. Why use call by reference?

To modify original values and improve performance.

Q3. What is a null pointer?

A pointer that points to nothing (NULL).

Q4. What is a dangling pointer?

A pointer pointing to deallocated memory.

Q5. Does C support call by reference directly?

No, C uses pointers to achieve call by reference.

🔑 Key Points to Remember

  • Pointers store addresses
  • * is used to access value
  • & gives address
  • Call by reference uses pointers
Functions in C Arrays in C