C - Dereferencing pointer to incomplete type - insert string into Binary Search Tree -

i dereferencing pointer incomplete type on line 58: rootnode->_left = null. ideas?

also there lot of code commented out single out error have question format of adt: usual binary search tree structures out there have node class , bst functions such insert take , return node. here have use separate tree structure has root structure tnode. has been problematic me example in addstringtotree function returns , takes tree parameter. don't know how recurse on in usual way nodes. created helper function solution not sure ideal.

#include <stdio.h> #include <stdlib.h> #include <ctype.h> #include <string.h> #include <stdbool.h>  typedef struct tnode { struct tnode* _left; struct tnode* _right; struct tnode* _key; } tnode;  typedef struct tree { tnode* _root; } tree;  tree* makeemptytree(); void destroytree(tree* root); tree* addstringtotree(tree* t, char* value); tnode* addstringtotreehelper(tnode* node, char* value); bool lookupintree(tree* t, char* value); void traverse(tnode* root);  struct tree* wordtree; struct tnode* wordroot;   int main() {  if(wordtree = makeemptytree()) {     printf("tree initialized.\n");  ///     traverse(wordtree->_root);     addstringtotree(wordtree, "peter");      //printf(wordtree->_root->_key);     //traverse(wordtree->_root);  } else {     printf("error initializing tree.\n"); }  return 0; }  tree* makeemptytree() {  struct tree* thetree = malloc(sizeof(struct tree*));    // allocate memory tree thetree->_root = null; return thetree; }   tree* addstringtotree(tree* t, char* value) {  if(t->_root == null) {     struct tnode* rootnode = malloc(sizeof(struct tnode*));     rootnode->_left = null;     //rootnode = (tnode*)malloc(sizeof(struct tnode));     //strcpy(rootnode->_key, value);     // rootnode->_left = null;     // rootnode->_right = null;     //printf(rootnode->_key); } else {     //addstringtotreehelper(root, value); }  return t; }  tnode* addstringtotreehelper(tnode* node, char* value) { // node = malloc(sizeof(tnode));  // going on  if(strcmp(value, node->_key) < 0) {     node->_left = addstringtotreehelper(node->_left, value); } else if(strcmp(value, node->_key) > 0) {     node->_right = addstringtotreehelper(node->_right, value); } return node; }  void traverse(tnode* root) { // if(root != null) { //  traverse(root->_left); //  printf("%s\n", root->_key); //  traverse(root->_right);  // } else { //  printf("empty tree\n"); // } } 

2nd edit

wow silly typo. guys. _key variable of tnode should of type char* instead of struct tnode* rolls eyes

struct tnode* rootnode = malloc(sizeof(struct tnode*));

should be

struct tnode* rootnode = malloc(sizeof(struct tnode));

you're allocating enough memory pointer , pointing it.

edit:

that should tnode* rootnode not tnode* rootnode.