Moving pointer to desired location

Question

I am offsetting my pointer as shown in the below code to copy to another structure.

#include <stdio.h>

struct a
{
    int i;
};
struct test
{
    struct a *p;
    int x,y,z;
};

int main()
{
  struct test *ptr = malloc(sizeof(struct test));
  struct test *q = malloc(sizeof(struct test));
  ptr->x = 10;
  ptr->y = 20;
  ptr->z = 30;
  memcpy(&(q->x),&(ptr->x),sizeof(struct test)-sizeof(struct a*));

  printf("%d %d %d\n",q->x,q->y,q->z);
  return 0;
}

Is there a better way to do my memcpy() ? My question is what if I am in-cognizant of the members of the structure and want to just move my pointer by sizeof(struct a*) and copy rest of the structure?

Edits:

I want to copy some part of the structure but I don't know the members in it, but I know I want to skip some type of variable as shown in the example (struct a*) and copy rest of the structure.

The question is not very clear did you mean copying by traversing through the structure? — Ginu Jacob, Jun 10 '15 at 11:34
@GinuJacob Yeah.. I meant that I want to copy some part of the structure based on the type — Gopi, Jun 10 '15 at 11:35
Question unclear. Do you mean that you want to copy all members of the struct, except `p`? — Peter, Jun 10 '15 at 11:35
Use `offsetof(struct test, x)` (C99, gcc), not `sizeof(stuct a)`. They might not be guaranteed equal, because of padding/alignment. — too honest for this site, Jun 10 '15 at 11:35
I want to bring to your notice that `memcpy()` has no cognizance of the padding that structs may contain. — Arjun Sreedharan, Jun 10 '15 at 11:38
@Olaf Yeah that's what I was asking for..Can we move something like what you have shown ? Does that take care of padding and all? — Gopi, Jun 10 '15 at 11:38
I didn't really understand your question. Why doesn't `q->x = ptr->x;` suffice? — Arjun Sreedharan, Jun 10 '15 at 11:39
@ArjunSreedharan: Because "*... I don't know the members in it.*" — alk, Jun 10 '15 at 11:49
@alk Thanks for pitching in.. Can we do what I am trying to do? Just going by the pointer and the offset by size? I have 2 things 1. Pointer 2. type. I want to copy out the structure from `ptr + offset(type)` — Gopi, Jun 10 '15 at 11:51

score 5 · Answer 1 · edited May 23 '17 at 11:52

5

Use offsetof(struct test, x) (C99, gcc), not sizeof(stuct a *). They are not guaranteed equal, because of padding/alignment. Caution: As a result of padding, using sizeof(..) can result in undefined behaviour, as there are copied too many chars.

offsetof(<type>, <member>) returns the offset of from the start of . So, sizef(struct test) - offsetof(struct test, x) yields the number of chars to copy all fields, starting with x.

Just read here for more details

edited May 23 '17 at 11:52

Community

1
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answered Jun 10 '15 at 11:41

too honest for this site

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Thanks for your inputs – Gopi Jun 10 '15 at 11:53
@Gopi: I added another aspect, which renders your solution dangerous. – too honest for this site Jun 10 '15 at 12:16

Serge Ballesta · Answer 2 · 2015-06-10T11:50:46.670

4

I think the best way would be to use offsetof

memcpy(&(q->x),&(ptr->x),sizeof(struct test)-offsetof(struct test, x));

Because if first element was a little different, you could have alignment problems, whereas offsetof takes care of alignment for you.

Of course, alignement problems on a pointer should not occur on common architecture, but I think that offsetof is better practice anyway

edited Jun 10 '15 at 11:50

answered Jun 10 '15 at 11:41

Serge Ballesta

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Moving pointer to desired location

2 Answers2