C中sizeof运算符的行为

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.

Answer 2

The compiler inserts padding for optimization and aligment purposes. Here, the compiler inserts 3 dummy bytes between (or after) your both members.

You can handle the alignment with #pragma directive.

Answer 3

Mostly to illustrate how this padding actually works, I've amended your program a little.

#include<stdio.h>
struct a
{
  int x;
  char y;
  int z;
};
int main()
{   
   struct a str;
   str.x=2;
   str.y='s';
   str.z = 13;

   printf ( "sizeof(int) = %lu\n", sizeof(int));
   printf ( "sizeof(char) = %lu\n", sizeof(char));
   printf ( "sizeof(str) = %lu\n", sizeof(str));

   printf ( "address of str.x = %p\n", &str.x );
   printf ( "address of str.y = %p\n", &str.y );
   printf ( "address of str.z = %p\n", &str.z );

   return 0;
}

Note that I added a third element to the structure. When I run this program, I get:

amrith@amrith-vbox:~/so$ ./padding 
sizeof(int) = 4
sizeof(char) = 1
sizeof(str) = 12
address of str.x = 0x7fffc962e070
address of str.y = 0x7fffc962e074
address of str.z = 0x7fffc962e078
amrith@amrith-vbox:~/so$

The part of this that illustrates padding is highlighted below.

address of str.y = 0x7fffc962e074
address of str.z = 0x7fffc962e078

While y is only one character, note that z is a full 4 bytes along.