structpadding.c

// gcc structpadding.c -std=c99 -Wall

#include <stdio.h>
#include <stdint.h>
#include <stdbool.h>

typedef struct Foo1 {
	uint64_t a;
} Foo1;

typedef struct Foo2 {
	uint64_t a;
	bool     b;
} Foo2;

int main(void) {
	Foo1 foo1Array[2];
	Foo2 foo2Array[2];

	printf("sizeof(Foo1):      %2ld\n", sizeof(Foo1));      //  8
	printf("sizeof(Foo2):      %2ld\n", sizeof(Foo2));      // 16 (not 9)
	printf("sizeof(bool):      %2ld\n", sizeof(bool));      //  1
	printf("sizeof(foo1Array): %2ld\n", sizeof(foo1Array)); // 16
	printf("sizeof(foo2Array): %2ld\n", sizeof(foo2Array)); // 32

	return 0;
}