How to Use Pointer in C: Syntax, Example, and Common Pitfalls

In C, a pointer is a variable that stores the memory address of another variable. You declare a pointer using the * symbol and assign it the address of a variable using the & operator. You can access or modify the value at that address by dereferencing the pointer with *.

Syntax

To declare a pointer, use the syntax: type *pointerName;. The type specifies the data type the pointer points to. Use & to get the address of a variable and assign it to the pointer. Use * to access the value stored at the pointer's address.

• type: data type of the variable the pointer points to (e.g., int).
• *pointerName: declares a pointer variable.
• &variable: gets the address of variable.
• *pointer: accesses the value at the pointer's address (dereferencing).

c

int *p; // declares a pointer to int
int x = 10;
p = &x; // pointer p stores address of x
int y = *p; // y gets value at address stored in p (10)

Example

This example shows how to declare a pointer, assign it the address of a variable, and use it to read and change the variable's value.

c

#include <stdio.h>

int main() {
    int num = 5;
    int *ptr = &num; // pointer stores address of num

    printf("Value of num: %d\n", num); // prints 5
    printf("Value via pointer: %d\n", *ptr); // prints 5

    *ptr = 20; // change value at address pointed by ptr

    printf("New value of num: %d\n", num); // prints 20
    return 0;
}

Output

Value of num: 5
Value via pointer: 5
New value of num: 20

Common Pitfalls

Common mistakes when using pointers include:

• Using an uninitialized pointer (it points to random memory).
• Dereferencing a NULL or invalid pointer causes crashes.
• Mixing pointer types can cause incorrect data access.
• Forgetting to use & when assigning address to pointer.

Always initialize pointers before use and ensure they point to valid memory.

c

#include <stdio.h>

int main() {
    int *p = NULL; // initialize pointer to NULL
    // printf("%d", *p); // WRONG: dereferencing NULL pointer causes error

    int x = 10;
    p = &x; // correct initialization
    printf("%d", *p); // prints 10
    return 0;
}

Output

10

Quick Reference

Pointer basics cheat sheet:

Concept	Description	Example
Declare pointer	Create a pointer variable	int *p;
Get address	Use & to get variable address	p = &x;
Dereference	Access value at pointer	int val = *p;
Change value	Modify value via pointer	*p = 100;
NULL pointer	Pointer with no address	int *p = NULL;

Key Takeaways

A pointer stores the memory address of a variable using the * symbol.

Use & to assign a variable's address to a pointer and * to access or change the value at that address.

Always initialize pointers before dereferencing to avoid crashes or undefined behavior.

Pointers must match the data type of the variable they point to for correct access.

NULL pointers indicate no valid address and should be checked before use.