What is the problem with this? 
 expected expression before ‘User’
  408 |                 check_and_add_user(User * user_list, int client_fd);
      |                                    ^~~~
error: too few arguments to function ‘check_and_add_user’
  408 |                 check_and_add_user(User * user_list, int client_fd);
      |                 ^~~~~~~~~~~~~~~~~~
 note: declared here
  256 | void check_and_add_user(User *user_list, int client_fd)