/in/hasCircular.c:4:24: error: 'struct ListNode' declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
 int hasCircular(struct ListNode *head)
                        ^~~~~~~~
/in/hasCircular.c: In function 'hasCircular':
/in/hasCircular.c:6:17: error: 'null' undeclared (first use in this function)
     if (head == null || head.next == null) {
                 ^~~~
/in/hasCircular.c:6:17: note: each undeclared identifier is reported only once for each function it appears in
/in/hasCircular.c:6:29: error: 'head' is a pointer; did you mean to use '->'?
     if (head == null || head.next == null) {
                             ^
                             ->
/in/hasCircular.c:9:5: error: unknown type name 'ListNode'; use 'struct' keyword to refer to the type
     ListNode fast = head;
     ^~~~~~~~
     struct 
/in/hasCircular.c:9:21: error: initialization makes integer from pointer without a cast [-Werror=int-conversion]
     ListNode fast = head;
                     ^~~~
/in/hasCircular.c:10:5: error: unknown type name 'ListNode'; use 'struct' keyword to refer to the type
     ListNode slow = head;
     ^~~~~~~~
     struct 
/in/hasCircular.c:10:21: error: initialization makes integer from pointer without a cast [-Werror=int-conversion]
     ListNode slow = head;
                     ^~~~
/in/hasCircular.c:12:48: error: request for member 'next' in something not a structure or union
     while (slow != null && fast != null && fast.next != null) {
                                                ^
/in/hasCircular.c:13:20: error: request for member 'next' in something not a structure or union
         fast = fast.next.next;
                    ^
/in/hasCircular.c:14:20: error: request for member 'next' in something not a structure or union
         slow = slow.next;
                    ^
cc1: all warnings being treated as errors