Thread: sizeof union question

I read that the sizeof a union is the largest of it's members. However, in the code below the union is larger than any of it's members. I only see this when using amd64 linux. I am compiling as gcc -o union.o union.c

Code:

#include <stdio.h>

typedef union _NESTEDU
{
	long	l;
	int	f;
} NESTEDU;

typedef struct _NOTHING
{
	int a;
	int b;
	int c;
} NOTHING;

typedef union _UNION
{
	int	i;
	char	c;
	NESTEDU	n;
	NOTHING k;
} UNION;

int main()
{
	printf("struct/union sizeof testing\n");
	printf("sizeof() int = %d\n", sizeof(int));
	printf("sizeof() char = %d\n", sizeof(char));
	printf("sizeof() UNION = %d\n", sizeof(UNION));
	printf("sizeof() NESTEDU = %d\n", sizeof(NESTEDU));
	printf("sizeof() NOTHING = %d\n", sizeof(NOTHING));

	return(0);
}

And the output:

Code:

struct/union sizeof testing
sizeof() int = 4
sizeof() char = 1
sizeof() UNION = 16
sizeof() NESTEDU = 8
sizeof() NOTHING = 12

I am new to C so I may be missing something fundamental. Thanks for your help!