Thread: What does this struct look like in memory?

Code:

struct student {
	char id;
	char *name;
	int grade;
};

int main ()
{
	struct student school[2];
	
	school[0].id = 1;
	school[0].grade = 92;
	school[0].name = "roberto clemente";
	
	school[1].id = 2;
	school[1].grade = 76;
	school[1].name = "bobby hillenbrand";
	
	return 0;
}

When I messed around with it myself, printing addresses, etc., I found that school[1] was 24 bytes after school[0] and that, within each struct, each element was 8 bytes away from the next... even though I have different data types such as char, char*, and int.

This leads me to believe that, within the structure, each element is actually a pointer to the value (obviously I would expect this of the element char *name, but not the others)... Is this incorrect? Because it seems awful weird.

Ah I see.
Then in this case, sizeof(struct whatever) probably doesn't take into consideration the types of data in the struct but rather how many elements there are?

Originally Posted by quzah

sizeof( struct foo ) takes the size of the space in memory that the structure takes due to alignment of the number of elements.

If they have padding, then you'll see bigger than if you just add up: sizeof( char ) + sizeof( char* ) + sizeof( int ).

Exactly!

Originally Posted by steez

Ah I see.
Then in this case, sizeof(struct whatever) probably doesn't take into consideration the types of data in the struct but rather how many elements there are?

The sizeof struct objects is determined by two kinds of padding:

- Internal padding to align every struct member on its natural boundary, and
- Tail padding to make sizeof struct a multiple of the sizeof its most restrictive type.

id can be located anywhere; name and grade will be located at an address divisble by 4, and
the size of the struct is a multiple of 4 as that's the sizeof its most restrictive member type.