Linking inorder successor in a binary tree

inorder.c

// Global var to contain the inorder predecessor
node * prev;

void add_inorder(node * nd)
{
    // Base case
    if (nd == NULL)
        return;
    
    // Recursive call for inorder processing
    add_inorder(nd->left);
    
    // The current node has a predecessor
    if (prev != NULL)
    {
        // Set the next pointer
        prev->next = nd;
        prev = nd;
    }
    
    // This is the first node in the inorder processing
    else
    {
        prev = nd;
    }
    
    // Recursive call for inorder processing
    add_inorder(nd->right);
}

void mark_inorder(node * root)
{
    /* assuming the structure to be:
    typedef struct node{
        int val;
        struct node * next;
        struct node * left;
        struct node * right;
    } */

    // Initialising the global variable
    prev = NULL;
    // calling the function to add the values to pointers
    add_inorder(root);
    // Complete the next pointers by adding to the next 
    // pointer of last node in inorder processing, NULL
    prev->next = NULL;
}