Linking behavior and padding bytes

I have two source files I am linking:

src1.c:

#include <stdio.h>

typedef struct my_struct {
    char a;
    short b;
    int c;
} my_struct;

my_struct x;

void add1();

int main()
{
    x.a = 0;
    x.b = 0;
    x.c = 0;
    
    add1();
    
    printf("%x, %x, %x\n", x.a, x.b, x.c);
}

and src2.c:

#include <stdio.h>

typedef struct my_struct {
    int c;
    short b;
    char a;
} my_struct;

extern my_struct x;

void add1(){
    x.a += 1;
    x.b += 1;
    x.c += 1;
}

The output is "1, 0, 10001" due to the nature of the type definitions and alignment. However, this rel开发者_C百科ies on the second byte of the struct to be 0x00 (which is a padding byte in the struct for src1.c).

Is this guaranteed behavior? Are padding bytes typically initialized to 0? Are there cases when they wouldn't be?

---

The two struct types in each source file are not compatible with each other because the members aren't declared in the same order.

This means that my_struct x; and extern my_struct x; are not compatible, so you trigger undefined behavior, which loosely speaking means there are no guarantees at all what your program will do.