Why isn't sizeof for a struct equal to the sum of sizeof of each member in C/C++?

The size of a struct type element taken by sizeof() is not always equal to the size of each individual member. Sometimes the compilers add some padding to avoid alignment issues. So the size may change. The padding is added when a structure member is followed by a member with a larger size or at the end of the structure. Different compiler has different types of alignment constraints. In C standard, the total alignment structure depends on the implementation.

Case 1

In this case the double z is 8-byte long, which is larger than x (4-byte). So another 4-byte padding is added. Also short type data y has 2-byte space in memory so extra 6-bytes are added as padding.

Example Code

#include <stdio.h>
struct myStruct {
   int x; //Integer takes 4 bytes, and padding 4 bytes
   double z; //Size of double is 8-byte, no padding
   short int y; //Size of short is 2-byte, padding 6-bytes
};
main() {
   printf("Size of struct: %d", sizeof(struct myStruct));
}

Output 2

Size of struct: 24

Case 2

In this case the double is inserted at first, and it takes 8-bytes of space. Now the integer x (4-byte) is added. So there is another 4-byte space. When the short y is added, it can be placed into that extra 4-byte space and occupies total 16-bytes of space.

Example Code

#include <stdio.h>
struct myStruct {
   double z; //Size of double is 8-byte, no padding
   int x; //Integer takes 4 bytes, and padding 4 bytes
   short int y; //Size of short is 2-byte, padding 6-bytes
};
main() {
   printf("Size of struct: %d", sizeof(struct myStruct));
}

Output 2

Size of struct: 16

Case 3

In the third case it also takes 16-byte of memory space, but the arrangements are different. As the first member is double then it is placed at first, then the short type data is added. Now when the integer is trying to insert, it can be placed into the remaining 6-byte area. So one padding is present after short but no padding is required after the integer data.

Example Code

#include <stdio.h>
struct myStruct {
   double z; //Size of double is 8-byte, no padding
   short int y; //Size of short is 2-byte, padding 6-bytes
   int x; //Integer takes 4 bytes, and padding 4 bytes
};
main() {
   printf("Size of struct: %d", sizeof(struct myStruct));
}

Output 2

Size of struct: 16

Arjun Thakur

Updated on 30-Jul-2019 22:30:25

• Related Questions & Answers
• Why is not sizeof for a struct equal to the sum of sizeof of each member in C/C++?
• Result of sizeof operator using C++
• Sizeof operator in C
• Why is sizeof() implemented as an operator in C++?
• What is the use of sizeof Operator in C#?
• Deep Copy of Struct Member Arrays in C
• sizeof() function in PHP
• Difference between strlen() and sizeof() for string in C
• What is sizeof operator in C++?
• How to find the size of a variable without using sizeof in C#?
• Find size of array in C/C++ without using sizeof
• Difference between strlen() and sizeof() for string in C Program
• Implement your own sizeof operator using C++
• How will implement Your Own sizeof in C
• How to use sizeof() operator to find the size of a data type or a variable in C#