Why use address of first element of struct, rather than struct itself?

Question

I've just come upon yet another code base at work where developers consistently use the address of the first element of structs when copying/comparing/setting, rather than the struct itself. Here's a simple example.

First there's a struct type:

typedef struct {
    int a;
    int b;
} foo_t;

Then there's a function that makes a copy of such a struct:

void bar(foo_t *inp)
{
    foo_t l;
    ...
    memcpy(&l.a, &inp->a, sizeof(foo_t));
    ...
}

I wouldn't myself write a call to memcpy in that way and I started out with suspecting that the original developers simply didn't quite grasp pointers and structs in C. However, now I've seen this in two unrelated code bases, with no common developers so I'm starting to doubt myself.

Why would one want to use this style?

None of the answers below actually added anything to my knowledge of C, which sort of was what I was hoping for ;) — Magnus
This is pretty useful specifically when multiple types of struct start with the same first element, as a sort of pseudo-inheritance. The CPython codebase uses this heavily with PyObject; all Python objects start with a PyObject member, and they're usually passed around with PyObject * pointers so you don't have to hardcode the actual type of the object you're working with. — user2357112 supports Monica
The memcpy becomes safe if this is the final argument: sizeof(foo_t) - offsetof(foo_t, a). — Darren Stone
I did stuff like this a fair bit when I was first getting the hang of C structs and arrays. I'd guess your belief as to why it was happening was pretty much spot-on. — Fake Name
I would start by not having a single letter variable named l. — Guilherme Bernal

ouah ouah · Accepted Answer · 2013-11-04T20:47:48

Instead of that:

memcpy(&l.a, &inp->a, sizeof(foo_t));

you can do that:

memcpy(&l, inp, sizeof(foo_t));

While it can be dangerous and misleading, both statements actually do the same thing here as C guarantees there is no padding before the first structure member.

But the best is just to copy the structure objects using a simple assignment operator:

l = *inp;

Why would one want to use this style?

My guess: ignorance or bad discipline.

Why use address of first element of struct, rather than struct itself?

6 Answers