Values of padding bytes in C - c++

Question

I've spotted a strange behavior in this small C program. I have 2 structures, both with padding bytes, but in different places.
The first structure has padding bytes with indices [1:3], and the output is expected: static variables are zeroed-out, so padding values are all 0, local variables on stack are left with garbage values in padding bytes. Example output:
Char is first, then int:
aa 60 8e ef ff ff ff ff
aa 00 00 00 ff ff ff ff
But in the second structure, something strange happens. Padding bytes in this structure are with indices [5:7], so I expected some garbage values in non-static variable, but every time the output is:
Int is first, then char:
ff ff ff ff aa 7f 00 00
ff ff ff ff aa 00 00 00
Why the padding is always 7f 00 00?
The complete program:
#include "stdio.h"
#include "stdint.h"
#include "stddef.h"
// 0 1 2 3 4 5 6 7
// |a|#|#|#|b|b|b|b|
typedef struct
{
uint8_t a;
uint32_t b;
} S1;
// 0 1 2 3 4 5 6 7
// |a|a|a|a|b|#|#|#|
typedef struct
{
uint32_t a;
uint8_t b;
} S2;
void print_bytes(void* mem, size_t num_bytes)
{
for (size_t i = 0; i < num_bytes; i++)
printf("%02x ", *((unsigned char*)mem + i));
putc('\n', stdout);
}
int main()
{
S1 var1 = { .a = 0xAA, .b = 0xFFFFFFFF };
static S1 var1_s = { .a = 0xAA, .b = 0xFFFFFFFF };
printf("Char is first, then int:\n");
print_bytes(&var1, sizeof(S1));
print_bytes(&var1_s, sizeof(S1));
S2 var2 = { .a = 0xFFFFFFFF, .b = 0xAA };
static S2 var2_s = { .a = 0xFFFFFFFF, .b = 0xAA };
printf("\nInt is first, then char:\n");
print_bytes(&var2, sizeof(S2));
print_bytes(&var2_s, sizeof(S2));
}

Answer 1

There's no problem if I run your program. Last bytes are random. It likely depends on your system.