问我无法通过引用传递我的节点

EN

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

struct node{
    int data;
    struct node* next;
};

void add(struct node** head,int data){
    struct node * tmp = *head;

    while(tmp != NULL){
        tmp = tmp->next;
    }
    tmp = (struct node*) malloc(sizeof(struct node));
    tmp->data= data;
    tmp->next=NULL;
}

int main()
{
    struct node n1;

    n1.data=5;
    n1.next=NULL;

    add(&(n1.next),15);
    printf("%d",n1.next->data);

    return 0;
}