Why does the treeprint function fail?

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C

This is an awkward question, but if I don't ask I will never know what's going on, so thanks for your help in advance. I'm attempting exercise 6-2 in K&R (binary tree). It requires me to write a program that reads a C program and prints in alphabetical order each group of variable names that are identical in the first 'X' characters, but different somewhere thereafter. My approach is to define a struct that holds an array of words rather than just one word (as the example given in the book), so words that have the same 'X' characters can be stored in the same node. So I tried testing this concept before address the entire. But when I try running the following code, while the addtree function seems to work (according to the debugger), the treeprint doesn't. It prints the variables that have the same first 3 characters before crashing (segmentation error, so it doesn't print the words in the other nodes). Not really sure what's going on. Thanks!

By the way, I find the binary tree a really intriguing concept!

What I have tried:

struct tnode *addtree(struct tnode *p, char *w){
	int cond;
	
	if(p==NULL){
		p=talloc();
		p->np=p->word;
		*(p->np)=strdup(w);
		p->count=1;
		p->left=p->right=NULL;
	}
	else if((cond=strncmp(w,*(p->np),3))==0){
		(p->np)++;
		*(p->np)=strdup(w);
	    p->count++;
	}
	else if(cond<0)
		p->left=addtree(p->left,w);
	else 
		p->right=addtree(p->right,w);
	return p;
}

void treeprint(struct tnode *p){
	if(p!=NULL){
		treeprint(p->left);
		for(int i=0;(p->word[i])!="";i++)
			printf("%4d %s\n",p->count,p->word[i]);
		treeprint(p->right);
	}
}