Set an element of an array of *char as NULL in C - arrays

Question

I have the task of going through a binary search tree and collecting the keys in every node and storing them in an array. After this, I have to set the last element of this array as NULL. This is all done inside a function, which then returns this array of *char.
I believe the part about collecting the keys and storing them in the array is working well, as I've tested that enough. However, I'm not able to define the last element of the array as NULL correctly. I thought it should be done like this:
list_keys[tree_size(tree)] = NULL;
However, after some printf's, I realized that this was cleaning my tree. When I print tree_size(tree) before this line, it gives me the size of the tree correctly. However, when I do it after that line, it gives me 0. I don't believe that the problem is in the function tree_size() because when I attempt to access an element of the tree before that line, it works well, but after the line, I get a Segmentation fault (core dumped) error, probably due to trying access something that doesn't exist anymore.
I have no idea what's wrong, so any help is appreciated. Thanks in advance.
EDIT:
The tree is of the type tree_t and it's defined as:
struct tree_t {
struct entry_t *entry;
struct tree_t *right_child;
struct tree_t *left_child;
};
The tree_size() is defined as:
int tree_size(struct tree_t *tree) {
if (tree->entry != NULL) {
//if two children exist
if (tree->left_child != NULL && tree->right_child != NULL) {
return 1 + tree_size(tree->left_child) + tree_size(tree->right_child);
}
//there's a right child and there isnt a left child
if (tree->right_child != NULL && tree->left_child == NULL) {
return 1 + tree_size(tree->right_child);
}
//there's a left child and there isnt a right child
if (tree->left_child != NULL && tree->right_child == NULL) {
return 1 + tree_size(tree->left_child);
}
//there are no children
if (tree->left_child == NULL && tree->right_child == NULL) {
return 1;
}
}
//if the entry is empty
return 0;
}
Basically, what I'm doing right now is:
First, I define list_keys and allocate memory:
char **list_keys;
list_keys = (char *)malloc((tree_size(tree)+1)*sizeof(char));
Then, I call an auxiliary function tree_get_keys_aux(tree, list_keys, 0) that will do the initial part I mentioned. It is defined as:
void tree_get_keys_aux(struct tree_t *tree, char **list_keys, int index) {
//N
list_keys[index] = (char *)malloc((strlen(tree->entry->key))*sizeof(char)); //allocating memory for each string that I want to add to the list
strcpy(list_keys[index],tree->entry->key); //copying the content
index = index + 1;
//L
if (tree->left_child != NULL) {
tree_get_keys_aux(tree->left_child, list_keys, index);
}
//R
if (tree->right_child != NULL) {
tree_get_keys_aux(tree->right_child, list_keys, index);
}
return;
}
Then, I do the line that's bringing me problems,
list_keys[tree_size(tree)] = NULL;
And lastly,
return list_keys;

Answer 1

Firstly,
list_keys = (char *)malloc((tree_size(tree)+1)*sizeof(char));
is wrong. The elements are char*, so you have to allocate for that or you will cause out-of-range access.
It should be:
list_keys = malloc((tree_size(tree)+1)*sizeof(char*));
or
list_keys = malloc((tree_size(tree)+1)*sizeof(*list_keys));
See also: c - Do I cast the result of malloc? - Stack Overflow
Secondly,
list_keys[index] = (char *)malloc((strlen(tree->entry->key))*sizeof(char)); //allocating memory for each string that I want to add to the list
strcpy(list_keys[index],tree->entry->key); //copying the content
is wrong. You have to allocate one more element for terminating null-character.
It should be:
list_keys[index] = malloc((strlen(tree->entry->key) + 1)*sizeof(char)); //allocating memory for each string that I want to add to the list
strcpy(list_keys[index],tree->entry->key); //copying the content
Thirdly, tree_get_keys_aux is not recoginizing number of elements in left child and data from left child will be overwritten by data from right child.
To avoid this overwriting, you can use tree_size to determint the tree size and advance index according to that.
//L
if (tree->left_child != NULL) {
tree_get_keys_aux(tree->left_child, list_keys, index);
index += tree_size(tree->left_child); // add this
}
//R
if (tree->right_child != NULL) {
tree_get_keys_aux(tree->right_child, list_keys, index);
}