Dynamic array member of a nested structure

Question

I am having a structure that includes a set of other structures:

 typedef struct{
    int cell_Type;      //user association: 0--> Macro 1--> Femto
    int cell_ID;        //cell ID that the user is associated with
}association;

 //User struct
 typedef struct{
    coord c;                //coordinates
    association id;             //association parameters
    capacity cap;               //capacity parameters   
 }usu;

 //Struct with a vector of users
 typedef struct{
     usu user[NUM_USERS+NUM_HS]; //each user is defined by the association (id), etc...
 }vectusers;

void main(int argc,char *argv[])
{
       //.....

       //memory allocation for the structures
       users_ptr=calloc(1,sizeof(vectusers)); //users pointer

       //.....
}

So far so good, up to this point that there has been the need for an array that I will not know its size until the program is almost finished.

As such, the structure will be:

 typedef struct{
    int cell_Type;      //user association: 0--> Macro 1--> Femto
    int cell_ID;        //cell ID that the user is associated with
    int assigned_RBs;   //counter of how many resources have been assigned
    int *RB_number;     //array with size according to assigned_RBs counter
}association;

My problem is with the malloc() and realloc(). Actually I think I am reserving correctly memory with the malloc:

users_ptr->user[u].id.RB_number = malloc(1*sizeof(int));

but when it comes to realloc(), I do not change the size of the array:

users_ptr->user[index_max_W].id.assigned_RBs++;

size = users_ptr->user[index_max_W].id.assigned_RBs;

users_ptr->user[index_max_W].id.RB_number = realloc((users_ptr->user[index_max_W].id.RB_number),size);

where index_max_w is the index of the user with a max value of a statistic. Both size and index_max_W have been declared as int.

Could someone please help me with the realoc in this case?

Answer 1

The size argument to realloc() is in bytes, just like the argument to malloc(). Your code seems to omit tihs scaling for the realloc(), which will lead to (drastic) under-allocation and thus undefined behavior if you access the memory.

It should be something like:

const size_t new_size = users_ptr->user[index_max_W].id.assigned_RBs;

users_ptr->user[index_max_W].id.RB_number = realloc(users_ptr->user[index_max_W].id.RB_number,
new_size * sizeof *users_ptr->user[index_max_W].id.RB_number);

It's a bit unwieldy due to the nested name of the target pointer. We can simplify that by using a suitable temporary pointer variable to remove the repeated complicated access:

usu *user = &users_ptr->user[index_max_W];
user->id.assigned_RBs++;
user->id.RB_number = realloc(user->id.RB_number,
                             user->id.assigned_RBs * sizeof *user->id.RB_number);