C does not support function overloading, making it difficult to direcly convert your class. It can be done but it would be quite ugly. You could have something similar to this and be quite readable.
Code:
struct node {
int data;
node *left,*right;
int bf;
};
void node(struct node *curNode, int d) {
curNode->data=d;
curNode->left=NULL;
curNode->right=NULL;
curNode->bf=0;
}
void nodeDefault(struct node *curNode) {
curNode->data=0;
curNode->left=NULL;
curNode->right=NULL;
curNode->bf=0;
}
And to use the two node functions your overloaded in your class, just call the node or nodeDefault functions and pass a pointer to an instance of the structure.