Question

This is the code that I have to dynamically declaring an array inside a structure. I am dynamically allocating the array list with a size called ‘capacity’. At a later point in my program, I want to increase the size of my array and re-allocate it. How do I go about it?

struct mystruct {
 int x;
 struct y **list;
};

wrapper function to declare the array present inside the structure

struct mystruct *mystruct_init()
{
    struct mystruct *mystruct = calloc(1, sizeof(*mystruct));

    // loop through and allocate memory for each element in list
    mystruct->list = calloc(1, sizeof(struct y *) * mystruct->list_length);

    for (int i = 0; i < capacity; i++)
        mystruct->list[i] = calloc(1, sizeof(struct y));

    return mystruct;
}

calling the wrapper function

struct mystruct *h1 = mystruct_init();

My question is, how do I use the realloc function to increase the size of list (double the value of capacity)? It would be really nice if someone could help me out.

Answer 1

assume you have int oldsize:

struct y **newlist=realloc(h1->list,oldsize*2*sizeof(struct y*));
if (!newlist) return -1;//error
h1->list=newlist;
int i;
for (i=oldsize;i<2*oldsize;i++) h1->list[i]=calloc(1,sizeof(struct y));