Sizeof in C

A pointer in C is defined as a variable that stores the information of the address of another variable. A pointer can be incremented or decremented, which means we can point to a different memory location using the previous one.

In C language, pointers are used for faster execution and in memory management.

& operator

Let us declare a variable, say x of int type.

int x = 10;

Now, if we print the value of x, it will print 10. But if we want to know at what memory location, the x is stored. We will use & operator. This prints the address of a variable.

For example

#include <stdio.h>
int main() {
int x = 10; // The value of x
printf("The value of x is %d\n", x);
printf("The address of x is %d", &x);
return 0;
}

Output

Note - The address can be printed differently on different machines.

How does a pointer work in C language?

We declare pointers as follows -

data_type *pointer_variable;

Example

int *x // An integer pointer variable

char *ch // A character pointer variable

Now we have declared a variable. Let us initialize the variable.

int *x = &y // Here the address of y is stored in x.

Here, x will now point to the memory location of y.

Code

#include<stdio.h> 
int main(){ 
int number=50; // declare a variable whose value is 50
int *p;    // declare pointer variable p
p=&number; //stores the address of number variable in pointer  
printf("Address of p variable is %x \n",p); // p contains the address of the number therefore printing p gives the address of number.
printf("Value of p variable is %d \n",*p); // As we know that * is used to dereference a pointer therefore if we print *p, we will get the value stored at the address contained by p.
return 0; 
}

Output

Sizeof() function in C

The function sizeof() in terms of a variable returns the number of bytes taken by a variable in the memory. It is a compile-time unary operator that returns the size of the operand.

Note: a pointer variable is not fixed as it is dependent on computer architecture.

The sizeof() function is used for different purposes –

When the operand is a data type.

The sizeof() function calculates the size of different data types as an operand. Data types may include int, float, char, etc.

Example

#include <stdio.h>
int main()
{
      printf("%lu\n", sizeof(char)); // calculate the size of char
      printf("%lu\n", sizeof(int)); // calculate the size of int
      printf("%lu\n", sizeof(float)); // calculate the size of float
      printf("%lu", sizeof(double)); // calculate the size of double
      return 0;
}

Output

1

4

4

8

To calculate the size of the expression

The sizeof() function is used to calculate the size of the expression. Let us see the following example –

Example

#include <stdio.h>
int main()
{
      int a = 0; // Declare an int
      double d = 10.21; // Declare a double
      printf("%lu", sizeof(a + d)); // Output is a double
      return 0;
}

Output

8

Explanation

From the above example, it is seen that the size of an integer is 4 bytes. And the size of the double is 8 bytes. Hence, the output of (a+b) will be 8                bytes.

Note: The sizeof() is a compile time operator so it does not evaluate any code inside its (). For example,

Code

#include<stdio.h>
int main() {
int y;
int x = 11;
y = sizeof(x++); //value of x doesn't change
printf("y = %d, x= %d",y,x);
}

Output

y = 4, x= 11

Explanation

In the above code, the value of y is four which is the size of the x integer. When we print the value of x, we get the same as 11 even after increment because it does not evaluate any code.

Uses of Sizeof()

It calculates the number of elements in the array.

The number of elements in the array is calculated by the formula –

sizeof(arr) / sizeof(arr[0])

Example

#include <stdio.h>
int main()
{
      int arr[] = { 1, 2, 3, 4, 7, 98, 0, 12, 35, 99, 14 }; // Create the array
      printf("The Number of elements in the array is:%lu ", sizeof(arr) / sizeof(arr[0]));
      return 0;
}

Output

The number of elements in the array is: 11

For dynamic memory allocation, if we want to allocate memory that can hold ten integers, it can be known by the sizeof() operator. Here, we use the function because we don’t know what integer size is decided on by the compiler.

Example

int* ptr = (int*)malloc(10 * sizeof(int));

 Itallocated space for 20 integers using the sizeof() function.