Causes of Segmentation Fault

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

structure data_t{
  int num;
  structure data_t*next,*prev;
};

void insert(struct data_t*p, struct data_t*w){
    p->prev->next=w;
    w->prev=p->prev;
    p->prev=w;
    w->next=p;
}

void remove_list(struct data_t*p, int index){
  structure data_t*a;
  inti = 0;
  for(a=p->next;a!=p;a=a->next){
    if(i==index){
      a->next->prev=a->prev;
      a->prev->next=a->next;
      break;
    }
    i+=1;
  }
}


void printall(struct data_t*p){
  structure data_t*a;
  for(a=p->next;a!=p;a=a->next){
  printf("[p:%p, bp:%p, fp:%p, data:%d]\n", a, a->prev, a->next, a->num);
  }
  printf("\n");
}


int main() {
  structure data_t*p, *w, *head;
  int d, count = 0;
  head->next=head;
  head->prev=head;
  printf("State Input\n";
  for(;scanf("%d", &d)!=EOF; count++){
    w=(struct data_t*)malloc(sizeof(struct data_t));
    w->num=d;
    insert(head,w);
  }
  printf("State Remove\n";
  while(scanf("%d", &d)!=EOF){
    if(count<=d){
      printf("List does not have the index number: %d\n", d);
    } else if (count >d) {
      remove_list(head,d);
      count-=1;
    }
  }
  printall(head);
  return 0;
}