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?

Answer 1

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.