Man page 9 : parport_find_number

parport_find_number - find a parallel port by number

struct parport * parport_find_number(int  number );

number
    parallel port number

This returns the parallel port with the specified number, or NULL if there is none.

There is an implicit parport_get_port done already; to throw away the reference to the port that parport_find_number gives you, use parport_put_port.