Difference between int *a and int **a in C

In C, the declarations int *a and int **a represent two different concepts related to pointers. Pointers play a fundamental role in memory management and data manipulation in C programming so it is important to have a clear understanding of them.

What does int * means?

This declares a pointer to an integer. It means that the variable a can store the memory address of the integer variable.

You can use another variable of type integer pointer ‘a’ to point to a single integer variable:

int x = 10;

int *a = &x; // ‘a’ points to the integer variable ‘x’

a stores the address of an integer variable, so you can dereference it using the * operator to access the value of the integer it points to like *a.

Example

C

// C Program to illustrate 'int *'
#include <stdio.h>

int main()
{
    int x = 20;
    int* a = &x;
    // 'a' is a pointer to an integer
    // pointing to 'x'
    printf("The Value of x: %d\n", x);
    printf("The Value pointed to by 'a': %d\n", *a);
    return 0;
}

Output

The Value of x: 20
The Value pointed to by 'a': 20

In this example, int *a; declares a single-level pointer a which points to the integer variable x. a stores the memory address of the x and you can access the value of x using the *a.

Understanding ‘int **’

This declares a pointer to a pointer to an integer. It means that the variable a can store the memory address of the pointer to an integer variable.

You can use a to point to a pointer to an integer:

int x = 10;

int *p = &x; // ‘p’ is a pointer to an integer, pointing to ‘x’

int **a = &p; // ‘a’ is a pointer to a pointer to an integer, pointing to ‘p’

In this case, a points to a pointer p that in turn points to an integer x. You can use a to access the address of p and then use *a to access the value of p which is the address of x.

Example

C

// C Program to illustrate 'int **'
#include <stdio.h>

int main()
{
    int x = 20;
    int* p = &x;
    int** a = &p;
    printf("x: %d\n", x);
    printf("p: %d\n", p);
    printf("*p: %d\n", *p);
    printf("a: %d\n", a);
    printf("*a: %d\n", *a);
    printf("**a: %d\n", **a);

    return 0;
}

Output

x: 20
p: -1569282476
*p: 20
a: -1569282472
*a: -1569282476
**a: 20

In this example, int **a; declares a double-level pointer a which points to a pointer to an integer p. The p itself points to the integer variable x. You can access the value of x indirectly through a by first dereferencing a to get the address of the p and then dereferencing p to access the value of x.

Difference Between ‘int *’ and ‘int **’ in C

Properties	int *a;	int **a;
Pointer Type	Points to an integer variable.	Points to a pointer to an integer.
Usage	Directly points to a value.	Points to a pointer that points to value.
Declaration	int *a;	int **a;
Initialization	int *a = &x; where x is an integer variable.	int **a = p; where p is a pointer to an integer.
Dereferencing	Access the value using *a.	Access the value using the **a.
Typical Use Cases	Commonly used for single values, arrays, and dynamic memory allocation.	Used when working with the multi-dimensional arrays, dynamic memory allocation, or functions that modify pointers.
Example	int x = 10; int *a = x;	int x = 10; int *px = x; int **a = px;