在C中，当我们将int转换为struct *时，内存中会发生什么？

Question

typedef struct block
{
   size_t size;
   struct block* next;
} node;

static char arr[1000];

What happens with arr

when I do

node* first_block = (node*)arr;

?

I understand that this is equal to

node* first_block = (node*)&arr[0];

but

sizeof(node) = 8;
sizeof(arr[0])= 1;

so first element override next seven elements in the arr, because it's struct now ? Can you explain me this cast, please ?

Answer 1

When you write

node* first_block = (node*)arr;

you are not changing anything in memory, you get a pointer to the area in memory, the type of the pointer determines how the area is treated in regard to pointer arithmetic.

first_block->next will be a pointer that is determined by the characters in the array.

as a comparison say you have a char* pointer to the same array

(if arr is declared at global scope it will contain 0's)

char* q = arr;
node* p = (node*)arr;


                arr[1000]
          +----------------------+
  q  ->   |   |   |          |   |
          | 0 | 0 | ...      | 0 |
  p ->    |   |   |          |   |
          +----------------------+

when you do

q = q + 1;  

// you move the character pointer one char forward since q is a char pointer

when you do

p = p + 1;  

// you move the node pointer sizeof(node) chars forward since p is a node pointer

when you do *q you get the character value of where q points to, *p gives you the node value from the char arr, ie the characters are interpreted as a node struct.

Answer 2

It's related to pointer arithmetic, when you have a pointer type *pointer , and you do this

type *next = pointer + 1;

or

type *next = &pointer[1];

you are effectilvely doing this

type *next = ((char *)pointer + sizeof(type));

Answer 3

nothing happens to the data when the code does a 'cast'. However,

the compiler will treat that data differently for the duration/scope of the cast.

the duration/scope of the cast is (almost always) only the statement that contains the cast.

Answer 4

Memory does not know about types, so nothing happens to it when casting. Types in C just describe how the bytes in memory are interpreted.

You can treat a sequence of 8 bytes as 8 ASCII characters ( char[8] ) or a struct node . Casting just changes the view of the data, but the data is not affected.