dynamic array with pointers to struct nodes

Question

I am having trouble with some code in C and i would really need your help . Well i have these 2 structs (they are asked to be like this so we cann't change them)

struct node{
    struct list_node **table;
    int table_size;
};

struct list_node{
    int num;
    struct list_node *ptr;
};

and as you can see in the first one we have an array , with pointers in the nodes of our list. in main , we create the needed memory space to begin like this

struct node *nodeT;
struct list_node *root, *curr, **temp;

root = (struct list_node*)malloc(sizeof(struct list_node));       // root node of list

nodeT = (struct node*)malloc(sizeof(struct node));                // single node
nodeT->table = (struct list_node**)malloc(sizeof(struct list_node*)); 
nodeT->table_size = 0;

and then I creat the list

for(i=0 ; i<X ; i++){     // X is important for the errors and i'll explain why
  curr = (struct list_node*)malloc(sizeof(struct list_node));
  curr -> num = (i+1);
  curr -> ptr = root;
  root = curr;
}

now , i run through the list and i expand the array in the first struct for everysingle list node i find , to enter a pointer to the proper node.

for(curr=root ; curr!=NULL ; curr=curr->ptr){               

  nodeT->table[nodeT->table_size] = curr;
  nodeT->table_size++;

  temp = (struct list_node**)realloc(nodeT->table , nodeT->table_size*sizeof(struct list_node *));
  if(temp!=NULL)
      nodeT->table = temp;
  else{
     printf("Memory error\n");
    return 1;
  }
}

i use this struct list_node **temp to keep safe nodeT and after checking , if everything is ok , i put temp in nodeT again , otherwise i stop the programm . In the end i print the contents of the list throught the pointers of array like this

for(i=0 ; i<nodeT->table_size ; i++)
   printf("-> %d ", nodeT->table[i]->num);
printf("\n");

and i exit the programm. the paradox in this , is that for X 1-4 everything works fine, but for 5+ there is a problem and i get a message

" ** glibc detected * ./dynamix_table_realloc: realloc(): invalid next size: 0x0000000000819050 * "

and about 20 more lines , that dont realy help me. I hope you will , and thats why i posted this. Thanks in advance!

Answer 1

You did not allocate enough memory at here:

temp = (struct list_node**)realloc(nodeT->table , nodeT->table_size*sizeof(struct list_node *));

It should be:

temp = (struct list_node**)realloc(nodeT->table , (nodeT->table_size+1)*sizeof(struct list_node *));

You use realloc() to add space for the next element, but after nodeT->table_size++, the value of nodeT->table->size is the index of next element, because the C array index is zero-based, so the number of elements should be nodeT->table_size + 1.

This is a typical off-by-one error.