How to iterate over a dynamic array inside a dynamic array in C?

I have the following structs:

typedef struct {
    char name[25];
} Piece;

typedef struct {
   int num;   //number of positions *piece will have
   Piece *piece;
} Category;

I'm creating a function that fills the structs with information from a text file. How can I iterate this dynamic structures?

On static variables, for example Category category[50]; I would declare an iterator (int i=0;) and move around the array doing category[i] = ....

What would be the equivalent with this dynamic structures?

This is a schema on how my function works:

void foo (Category *category) {

    int num_of_categories = 5;   //It won't be 5 always 

    category = (Category*)malloc(sizeof(Category)*num_of_category);

    while () {
        category->num = ...;

        category->piece = (Piece*)malloc(sizeof(Piece)*category->num);

        while () {
           category->piece->name = ...;

           //How to go to the next piece position?
        }
        //How to go to the next category position?
    }