您可以将结构指针转换为字符指针吗？

Question

I have a struct named Characters , a variable of that struct named a , and a pointer to char named pChar and I want to change the value of FirstChar in a to 27 through pChar

struct Characters {
    short CharCount;
    char FirstChar;
    char SecondChar;
    char ThirdChar;
};

struct Characters a = {3, 4, 5, 6};

char* pChar;
pChar = (char*)&a;
*(pChar+2) = 27;

Can I cast the struct a to char* safely? And will the padding be added to the end of the struct? If I used *(pChar+2) will I be guaranteed that it will be referring to FirstChar ?

Answer 1

Can I safely cast the struct a to char* safely?

The cast itself is safe. Accessing the contents of the pointer *(pChar+2) = 27; is not safe. Formally it may invoke undefined behavior because it violates aliasing .

(Using char* is fine as far as the aliasing rules are concerned, but when writing the value 27 to a random byte, there are formally no guarantees of what the result will be. And you might be writing to a padding byte.)

And will the padding be added to the end of the struct?

Padding may be added anywhere in the struct, except at the very beginning.

---

In practice, doing things like this is likely rather safe, even though it is not recommended and formally undefined behavior. I have yet to encounter a system where it wouldn't work safely and deterministically. But you have to ensure that there is no padding:

struct Numbers {
    short CharCount;
    char FirstChar;
    char SecondChar;
    char ThirdChar;
};

static_assert(sizeof(struct Numbers) == 
                sizeof(short) +
                sizeof(char)  +
                sizeof(char)  +
                sizeof(char), 
              "Padding found");

To actually disable padding there is usually some non-standard way such as #pragma pack(1) .

EDIT

The best and most portable way to reliably get the address of a struct member might be this:

#include <stddef.h>

struct Numbers x;
char* ptr = (char*)&x + offsetof(struct Numbers, FirstChar);

Answer 2

You can cast any pointer type into another pointer type and should still work in the sense that it points to the same memory space, if thats what you are asking.

However, its hard to determine whats going to happen if you modify a variable in the structure the way you are doing, mainly because of memory alignment .

You can suppress memory alignment in most compilers by setting #pragma pack(1) .

If you do so, and char is 1 byte long and short is 2 bytes long (it might not always be), only then its guaranteed that modifying pChar[2] will change the value of FirstChar .