Behaviour of sizeof operator in C

Question

I am getting unusual behaviour with my code, which is as follows

#include<stdio.h>
struct a
{
    int x;
    char y;
};
int main()
{   
   struct a str;
   str.x=2;
   str.y='s';
   printf("%d %d %d",sizeof(int),sizeof(char),sizeof(str));
   getch();
   return 0;
}

For this piece of code I am getting the output:

4 1 8

As of my knowledge the structure contains an integer variable of size 4 and a char variable of size 1 thus the size of structure a should be 5. But how come the size of structure is 8. I am using visual C++ compiler. Why this behaviour?

Answer 1

It is called Structure Padding

Having data structures that start on 4 byte word alignment (on CPUs with 4 byte buses and processors) is far more efficient when moving data around memory, and between RAM and the CPU.

You can generally switch this off with compiler options and/or pragmas, the specifics of doing so will depend on your specific compiler.

Hope this helps.