sizeof() Operator in C: Usage and Examples

The sizeof is a unary operator which returns the size of data (constant, variables, array, structure etc).

sizeof operator on Datatype

When an operand is a Data Type. When sizeof() is used with the data types such as int, float, char… etc it simply returns the amount of memory is allocated to that data types.

Program

#include<stdio.h>
int main()
{
    printf("%d\n",sizeof(char));
    printf("%d\n",sizeof(int));
    printf("%d\n",sizeof(float));
    printf("%d", sizeof(double));
    return 0;
} 

Output

1
4
4
8Press any key to continue . . .

sizeof operator on Variable

Program

#include <stdio.h>
int main()
{
    int a, e[10];
    float b;
    double c;
    char d;
    printf("Size of int=%lu bytes\n",sizeof(a));
    printf("Size of float=%lu bytes\n",sizeof(b));
    printf("Size of double=%lu bytes\n",sizeof(c));
    printf("Size of char=%lu byte\n",sizeof(d));
    printf("Size of integer type array having 10 elements = %lu bytes\n", sizeof(e));
    return 0;
}

Output

Size of int=4 bytes
Size of float=4 bytes
Size of double=8 bytes
Size of char=1 byte
Size of integer type array having 10 elements = 40 bytes
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sizeof operator on operand is an expression

When operand is an expression. When sizeof() is used with the expression, it returns size of the expression. Let see example:

Program

#include<stdio.h>
int main()
{
    int a = 0;
    double d = 10.21;
    printf("%d \n", sizeof(a+d));
    return 0;
} 

Output

8
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As we know from first case size of int and double is 4 and 8 respectively, a is int variable while d is a double variable. final result will be a double, Hence output of our program is 8 bytes.

Need of Sizeof

To find out number of elements in a array. Sizeof can be used to calculate number of elements of the array automatically. Let see Example :

Program

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

Output

Number of elements :11
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To allocate block of memory dynamically.

sizeof is greatly used in dynamic memory allocation. For example, if we want to allocate memory for which is sufficient to hold 10 integers and we don't know the sizeof(int) in that particular machine. We can allocate with the help of sizeof.

Program

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

Important Program

Program 1:

#include <stdio.h>
int main()
{ 
printf( "\n %d %d \n", sizeof('3'), sizeof("3"), sizeof(3) ) ;
 
}

Output

4 2
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Program 2:

In this case x and y will not hold the same value

x = sizeof(pi); this will return 4 in case of 32 bit machine.

y = sizeof(3.14); this will return double precion value at is 8 in case of same 32 bit machine.

#include<stdio.h>
void main()
{
int x,y;
float pi=3.14;
x = sizeof(pi);
y = sizeof(3.14);
if(x==y)
 {
 printf("Size is same \n");
 }
 else{
 printf("Size is not same \n"); 
 }

}

Output

Size is not same
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#include<stdio.h>
void main()
{
int x,y;
float pi=3.14;
x = sizeof(pi);
y = sizeof(3.14);
if(x==y)
 {
 printf("Size is same \n");
 }
 else{
 printf("Size is not same \n"); 
 }
 
  printf("%d\n",x);
  printf("%d\n",y);

}

Output

Size is not same
4
8
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