Question

I am trying to make a binary search tree, but when I try to insert any value, or more precisely when a NULL pointer gets passed to the function, it just freezes for a little while and then it crashes. Here’s the code:

void create(int co, struct node **leaf){
 if(*leaf==0){
   (*leaf)=malloc(sizeof(**leaf));
   (*leaf)->val=co;
   (*leaf)->left=0;
   (*leaf)->right=0;
   }
 else if(co<(*leaf)->val){
   create(co, &(*leaf)->left);
   }
 else if(co>=(*leaf)->val){
   create(co, &(*leaf)->right);
   }
}

I can’t figure out why it does that. Can you explain?

EDIT: The first call of the function looks like this:

struct node *root;
 root=0;
 for(i=0;i<c;i++){
   create(f[i], &root);
   }

Where c is the number of elements in the array. And this is the definition of the struct:

struct node{
        int val;
        struct node *left;
        struct node *right;
        };

So the problem is not in the code I posted here, the entire code can be found here If I should just rewrite the whole question and just post the whole code here please saz so in the commnets, will try to rectify ASAP.

FOUND MY ANSWER
After I got it to actually get past create safely, I was able to find the last one mistake that screwed up my program. It was *i++;. Apparently ++ doesn’t work well with values being pointed to. After I rewrote it to *i=*i+1; it finally works, so I’ like to thank all the people who helped me and ask one final question: What is the difference between *i++; and *i=i+1;?

Answer 1

I took your struct definition and insert function exactly as they were and took your other code and dumped into a main() function as such:

int main()
{
    struct node *root;
    int i, c = 10;
    root=0;
    for(i=0;i<c;i++){
        create(i, &root);
    }
    return 0;
}

Seems it works just fine. I also tried a number of different ordered elements:

int f[] = {6, 1, 9, 2, 0, 18, 2, -8, 10000, 5};

And again, no crashes and I get the correct ordering…

Did you verify that c, which you use in the condition: i<c has f[] number of elements? You could remove the c by just using: sizeof(f)/sizeof(int).

What input makes this function fail? What is the exact error message it fails with?

When you “walk” your tree, did you check for NULL before printing values?

---

After you posted the whole code, I can see it crashes here:

int *pole, i, count=3;
pole[0]=25;   <----

You didn’t give pole any memory, so you’re deferencing an uninitialized pointer.

pole = malloc(3 * sizeof(int));

fixes that, but there’s more.

Next you’ll die here:

void getorder(struct node *leaf, int *f, int *i){
    if(leaf->left!=NULL){
        getorder(leaf->left, f, i);
     }
     f[*i]=leaf->val;  <-- this will kill you

Because again, you don’t give j any memory:

int *j;
...
*j=0;
getorder(root, f, j);