A pointer “addr” in a struct was allocated memory but when I call it in another function, it can't work

Question

At the beginning of the C file, I define two structures like this.

typedef struct
{
    char name[LEN];
    double price;
} book;

typedef struct
{
    void *addr;
    int num;
} Array;

Array book;

Later, I use malloc(sizeof(void *)*len) to allocate memory to the pointer addr . The void pointer points to a book structure which is also allocated memory in heap.

After doing this, when I call it in another function in this way:

void print_view_of_books(Array books)
{
   int j;
   int limit = books.num;
   for(j=0; j<limit; j++)
   {
      book * bk = (book *)books.addr[j]; 
      printf("Book title: %s\n", bk->name);
      printf("Book price: %lf\n", bk->price);
      puts("----------------------");
   }

}

My compiler says

error: operand of type 'void' where arithmetic or pointer type is required. 1 error generated.

I find there's a problem with the expression books.addr[j] and if I substitute it simply with books.addr , it will work.

Can somebody instruct me how to fix the problem please?

Answer 1

This is a problem of operator precedence.

Changing book * bk = (book *)books.addr[j]; to book * bk = ((book *)books.addr)[j]; should fix the problem.

Answer 2

error: operand of type 'void' where arithmetic or pointer type is required. 1 error generated.

You are using pointer arithmetic on a void pointer which is not allowed ( addr[ j ] ).

First you have to cast the void pointer

((book *)(books.addr))

And then dereference it

((book *)(books.addr))[j]