C - The sizeof Operator

The sizeof operator is a compile−time unary operator. It is used to compute the size of its operand, which may be a data type or a variable. It returns the size in number of bytes.

It can be applied to any data type, float type, or pointer type variables.

sizeof(type or var);

When sizeof() is used with a data type, it simply returns the amount of memory allocated to that data type. The outputs can be different on different machines, for example, a 32-bit system can show a different output as compared to a 64-bit system.

Example 1: Using the sizeof Operator in C

Take a look at the following example. It highlights how you can use the sizeof operator in a C program −

#include <stdio.h>

int main(){

   int a = 16;

   printf("Size of variable a: %d \n",sizeof(a));
   printf("Size of int data type: %d \n",sizeof(int));
   printf("Size of char data type: %d \n",sizeof(char));
   printf("Size of float data type: %d \n",sizeof(float));
   printf("Size of double data type: %d \n",sizeof(double));    

   return 0;
}

Output

On running this code, you will get the following output −

Size of variable a: 4
Size of int data type: 4
Size of char data type: 1
Size of float data type: 4
Size of double data type: 8

Example 2: Using sizeof with Struct

In this example, we declare a struct type and find the size of the struct type variable.

#include <stdio.h>

struct employee {
   char name[10];
   int age;
   double percent;
};

int main(){
   struct employee e1 = {"Raghav", 25, 78.90};
   printf("Size of employee variable: %d\n",sizeof(e1));   
   return 0;
}

Output

Run the code and check its output −

Size of employee variable: 24

Example 3: Using sizeof with Array

In the following code, we have declared an array of 10 int values. Applying the sizeof operator on the array variable returns 40. This is because the size of an int is 4 bytes.

#include <stdio.h>

int main(){

   int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
   printf("Size of arr: %d\n", sizeof(arr));
}

Output

Run the code and check its output −

Size of arr: 40

Example 4: Using sizeof to Find the Length of an Array

In C, we don’t have a function that returns the number of elements in a numeric array (we can get the number of characters in a string using the strlen() function). For this purpose, we can use the sizeof() operator.

We first find the size of the array and then divide it by the size of its data type.

#include <stdio.h>

int main(){

   int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
   int y = sizeof(arr)/sizeof(int);

   printf("No of elements in arr: %d\n", y);
}

Output

When you run this code, it will produce the following output −

No of elements in arr: 10

Example 5: Using sizeof in Dynamic Memory Allocation

The sizeof operator is used to compute the memory block to be dynamically allocated with the malloc() and calloc() functions.

The malloc() function is used with the following syntax −

type *ptr = (type *) malloc(sizeof(type)*number);

The following statement allocates a block of 10 integers and stores its address in the pointer −

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

When the sizeof() is used with an expression, it returns the size of the expression. Here is an example.

#include <stdio.h>

int main(){

   char a = 'S';
   double b = 4.65;

   printf("Size of variable a: %d\n",sizeof(a));
   printf("Size of an expression: %d\n",sizeof(a+b));

   int s = (int)(a+b);
   printf("Size of explicitly converted expression: %d\n",sizeof(s));

   return 0;
}

Output

Run the code and check its output −

Size of variable a: 1
Size of an expression: 8
Size of explicitly converted expression: 4

Example 6: The Size of a Pointer in C

The sizeof() operator returns the same value irrespective of the type. This includes the pointer of a built−in type, a derived type, or a double pointer.

#include <stdio.h>

int main(){

   printf("Size of int data type: %d \n", sizeof(int *));
   printf("Size of char data type: %d \n", sizeof(char *));
   printf("Size of float data type: %d \n", sizeof(float *));
   printf("Size of double data type: %d \n", sizeof(double *));
   printf("Size of double pointer type: %d \n", sizeof(int **));
}

Output

Run the code and check its output −

Size of int data type: 8
Size of char data type: 8
Size of float data type: 8
Size of double data type: 8
Size of double pointer type: 8