Struct Sizes conflict

Question

I am creating two structs in C that have the same number of variables: 3 integers and 3 chars. When i look at the size of an instance of each of the structs using the sizeof() operator i get two different values. I wonder Why is this happening, here is my code: I tried rearranging the variable with the same result.

typedef struct a
{
int x;
char a;
int y;
char b;
int z;
char c;
}s1;

typedef struct b
{
int x;
int y;
int z;
char a;
char b;
char c;
}s2;

int main()
{
printf("%d %d\r\n", sizeof (s1), sizeof (s2));
return 0;
}

This is almost certainly due to *word alignment,* which is going to be different between your two structs because the order of the members is different. — Robert Harvey, May 16 '18 at 21:25
Also note that the correct [`printf`](http://en.cppreference.com/w/c/io/fprintf) format for printing a `size_t` (which is what `sizeof` gives you) is `"%zu"` for decimal output. — Some programmer dude, May 16 '18 at 21:36
@Someprogrammerdude: I see the `u` in that chart, but what does the `z` do? — Robert Harvey, May 16 '18 at 21:38
@RobertHarvey It's the modifier for `size_t` argument types. See to the right in the table. — Some programmer dude, May 16 '18 at 21:41
@Someprogrammerdude: Ah, it's a *length modifier.* (in the column headings of that table, not the row headings) — Robert Harvey, May 16 '18 at 21:46

score 1 · Answer 1 · answered May 16 '18 at 21:33

Very simply putting, the processors align the variables based on the word size. The integers in your case are aligned at 4 byte boundaries, so when you interlace your characters and integers, 3 bytes after every character is not utilized. Refer to: https://www.geeksforgeeks.org/structure-member-alignment-padding-and-data-packing/

Struct Sizes conflict

1 Answers1