Demonstration of how struct padding byte values are unspecified (YMMV depending on compiler)

hole-struct.c

// clang -Wall -O0 hole-struct.c -o hole-struct
// gcc -Wall -O0 hole-struct.c -o hole-struct

#include <stdio.h>
#include <string.h>

struct hole_struct {
	char a;
	// char hole1[3];
	int b;
	char c;
	// char hole2[3];
	int d;
};

static __attribute__((noinline)) void print_bytes(const void *bytes, size_t len)
{
	const unsigned char *c = bytes;
	while (len--)
		printf("%02x%s", *c++, len ? " " : "");
	printf("\n");
}

static __attribute__((noinline)) void fill_stack(int c)
{
	char memory[100];
	memset(memory, c, sizeof memory);
	__asm__ __volatile__ ("" ::: "memory");
}

static __attribute__((noinline)) void init_struct(int a, int b, int c, int d)
{
	struct hole_struct s = { .a = a, .b = b, .c = c, .d = d };
	print_bytes(&s, sizeof s);
}

int main()
{
	fill_stack(0x55);
	init_struct(1, 2, 3, 4); // 01 55 55 55 02 00 00 00 03 55 55 55 04 00 00 00
	fill_stack(0xaa);
	init_struct(1, 2, 3, 4); // 01 aa aa aa 02 00 00 00 03 aa aa aa 04 00 00 00
}