A guide to pointers in C

Address of a variable

Every variable in C has a memory location where it is stored. This memory location is called the variable's address.

Pointer

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

• We use the * (asterisk) operator to define pointers and the & (address of) operator to get the address of a variable.

• For example:

    int a = 10;
    int *p;
  
    p = &a;  // p is a pointer to an integer, and now points to address of a
  

• Here p is a pointer that stores the address of a.

• We use the -> operator to access the value at the address stored in the pointer.

    int val = *p; // val becomes 10, as p contains address of a which has value 10
  

• Pointers allow us to pass variables by reference instead of by value. This is useful to modify the original variable.

Pointer Operators

Here are short notes on the main pointer operators in C:

& - Address Of Operator

• Returns the memory address of a variable.

• Syntax: &variable_name

• Example

    int a = 10; 
    int *ptr;
  
    ptr = &a; // ptr points to the address of a
  

* Dereference Operator

• Accesses the value at the address stored in a pointer.

• Syntax: *pointer_name

• Example

    int val = *ptr;  // val becomes 10, as ptr points to address of a which has value 10
  

-> - Structure Dereference Operator

• Accesses a member of a structure through a pointer.

• Equivalent to using the . operator, but dereferences the pointer first.

• Example

    struct student *p;
    p->age = 20; // Accesses the age member through pointer p
  

Format Specifiers for pointers

The format specifiers for pointers in C are:

• %p - To print a generic pointer address. This is the preferred way.

• %#x - To print a pointer address with 0x prefix.

• %x - To print a pointer address as an unsigned hexadecimal integer, without the 0x prefix.

• %d would give an incorrect result when used to print a pointer address. It may print some decimal number, but that number does not represent the actual pointer address.

  Example

    #include <stdio.h>
    int main()
    {
        int a = 5;
        int *ptr;
        ptr = &a;
        printf("%p\n", ptr);   
        printf("%x\n", ptr);
        printf("%#x\n", ptr);
        return 0;
    }
  

  OUTPUT:

  0x7fff17337f4c

  17337f4c

  0x17337f4c

Size of pointer variable

The size of a pointer variable depends on the system architecture. It can be 4 bytes or 8 bytes, depending on whether it is a 32-bit or 64-bit system.

• On 32-bit systems, pointers are usually 4 bytes in size

• On 64-bit systems, pointers are usually 8 bytes in size

The size of a pointer variable does not depend on the type of data it is pointing to. A pointer to an int and a pointer to a char will have the same size, depending on the system architecture.