C struct type confusing

Question

struct payload{
    struct metadata meta;
    char data_pointer[0];
};

struct payload* mypayload = (struct payload*) malloc(sizeof(struct payload) + 50);

What is the type of mypayload here? The address, pointer, or something else

Answer 1

malloc returns an untyped chunk of memory, and the assignment is how you tell the compiler what type it is. In this case struct payload * which is a pointer (an address) of the memory that was allocated or NULL if the allocation failed.

You don't need the cast (struct payload*).

If you want data_pointer to be a flexible array member, then you leave the size unspecified char data_pointer[];. An array of size 0 is undefined behavior.

Answer 2

What is the type of mypayload here?

mypayload is a pointer, a pointer to a struct payload. @kaylum

---

A struct array member with size 0, like char data_pointer[0]; is not defined by C although some implementations had allowed it as a pre-cursor to a flexible array member (FAM).

Since C99 use char data_pointer[]; to define the last member as a flexible array member.

To best allocate for a flexible array member, compute the sum of 2 sizes:

• sizeof mypayload[0]: Size of the object up to, but not including the FAM member. This will include any padding before the FAM member.

• sizeof mypayload->data_pointer[0]: Size of the FAM referenced data times the count, 50, of array elements desired.

• Tip: a type is not needed in the sizeof code. Easier to code right, review and maintain to use the size of the referenced object.

• Tip: Cast not needed.

• Tip: Check for allocation success.

Example:

struct payload{
  struct metadata meta;
  char data_pointer[];  // No 0
};

struct payload* mypayload = malloc(
    sizeof mypayload[0] + 
    sizeof mypayload->data_pointer[0] * 50);
if (mypayload == NULL) {
  ; // TBD code to handle out-of-memory
} 

Answer 3

mypayload is a pointer. It's a pointer to the memory allocated by malloc(). And you have it configured as a pointer to a payload.