Accessing data from an address

Question

I have a struct meta , one of which I insert at the start of my malloc:

void *ptr = malloc(sizeof(meta) + sz);
struct meta meta1;
meta1.size = sz;

//Which is better?
//memmove(ptr, &meta, sizeof(meta));
//ptr = meta;

Later on, I want to access my meta struct. I have tried doing:

*((struct meta*) (ptr - sizeof(m61_meta))).size;

Which, moves back the address to the 'start' of the malloc, casts the pointer to be a meta pointer, then gets size. However, I'm having a compile time error, saying that it is not a struct.

Why am I receiving this error, and if you know of a better way of doing this, that would be great.

EDIT: I am trying to put 'metadata' in my pointers. So basically:

  memory 
[meta|data]
A    B

Where memory is just one malloc. I keep the pointer of data (B) and later on, I try and access the 'meta' of my data. That is,

(ptr - sizeof(m61_meta))

Which is the start of my memory block. I want to access my meta from here. Unfortunately, casting my A pointer to a struct meta pointer fails.

Answer 1

The reason you're getting the compiler error is that the . operator binds more tightly than the * operator, so when you write

*((struct meta*) (ptr - sizeof(m61_meta))).size;

You're trying to take the size member of a pointer. You probably mean

(*((struct meta*) (ptr - sizeof(m61_meta)))).size;

After that, I have no clue what your code is trying to do, but we'll deal with one bug at a time. It's almost certainly a mistake to do ptr - sizeof(... , because pointer arithmetic is already scaled by the size of what's pointed to, so you'll end up skipping over a whole bunch of structures, not just one. Second, you've allocated space for a structure, but not copied anything into it. I'm sure there are more problems.