foo.c:1:1: error: unknown type name 'Item'
    1 | Item *create(double p[], int e[],int len)
      | ^~~~
foo.c: In function 'create':
foo.c:3:5: error: unknown type name 'Item'
    3 |     Item* head = new Item;
      |     ^~~~
foo.c:3:18: error: 'new' undeclared (first use in this function)
    3 |     Item* head = new Item;
      |                  ^~~
foo.c:3:18: note: each undeclared identifier is reported only once for each function it appears in
foo.c:3:22: error: expected ',' or ';' before 'Item'
    3 |     Item* head = new Item;
      |                      ^~~~
foo.c:4:9: error: request for member 'next' in something not a structure or union
    4 |     head->next = NULL;
      |         ^~
foo.c:4:18: error: 'NULL' undeclared (first use in this function)
    4 |     head->next = NULL;
      |                  ^~~~
foo.c:1:1: note: 'NULL' is defined in header '<stddef.h>'; did you forget to '#include <stddef.h>'?
  +++ |+#include <stddef.h>
    1 | Item *create(double p[], int e[],int len)
foo.c:6:5: error: unknown type name 'Item'
    6 |     Item *tail = head;
      |     ^~~~
foo.c:11:9: error: unknown type name 'Item'
   11 |         Item *item = new Item;
      |         ^~~~
foo.c:11:26: error: expected ',' or ';' before 'Item'
   11 |         Item *item = new Item;
      |                          ^~~~
foo.c:12:13: error: request for member 'next' in something not a structure or union
   12 |         tail->next = item;
      |             ^~
foo.c:14:13: error: request for member 'p' in something not a structure or union
   14 |         item->p = p[i];
      |             ^~
foo.c:15:13: error: request for member 'e' in something not a structure or union
   15 |         item->e = e[i];
      |             ^~
foo.c:16:13: error: request for member 'next' in something not a structure or union
   16 |         item->next = NULL;
      |             ^~
foo.c:17:17: error: request for member 'next' in something not a structure or union
   17 |         if (head->next == NULL) {
      |                 ^~
foo.c:18:17: error: request for member 'next' in something not a structure or union
   18 |             head->next = item;
      |                 ^~
foo.c: At top level:
foo.c:26:18: error: unknown type name 'Item'
   26 | double calculate(Item* head, double x) {
      |                  ^~~~
foo.c:37:1: error: unknown type name 'Item'
   37 | Item* add(Item *head1, Item *head2) {
      | ^~~~
foo.c:37:11: error: unknown type name 'Item'
   37 | Item* add(Item *head1, Item *head2) {
      |           ^~~~
foo.c:37:24: error: unknown type name 'Item'
   37 | Item* add(Item *head1, Item *head2) {
      |                        ^~~~
foo.c:112:1: error: unknown type name 'Item'
  112 | Item* sub(Item *head1, Item *head2) {
      | ^~~~
foo.c:112:11: error: unknown type name 'Item'
  112 | Item* sub(Item *head1, Item *head2) {
      |           ^~~~
foo.c:112:24: error: unknown type name 'Item'
  112 | Item* sub(Item *head1, Item *head2) {
      |                        ^~~~

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