Question

Using this guide I was told that arrays are passed by reference. This holds when a struct looks like this:

struct Person{
 char* name;
 int id;
}

But is doesn’t when the struct looks like:

struct Person{
 char name[20];
 int id;
}

When using the seconds struct, The name array is copied by value:

struct Person p1 = {"John", 1234};
struct Person p2 = p1;
p2.name[0] = 'L';

// p1.name[0] is still 'K'

Why is this happening?

Answer 1

I was told that arrays are passed by reference.

That’s not quite true. Arrays aren’t passed at all, no function can take an array as an argument. When you have a function declared

void foo(int bar[]);

the type of argument that foo takes is actually int *, and when you call it

int arr[23];
/* fill arr with meaningful values */
foo(arr);

a pointer to the first element of arr is passed by value. All function arguments are passed by value in C, without exception.

Thus when you pass a struct Person to a function, that is passed by value, hence the members of the struct are copied. If one of the members is an array, that is copied too, since it is a part of the struct.