The first four code vectors of a library must be the following entries:

OPEN
    is the entry point called by the function openlibrary().  in most
    libraries, OPEN increments the library variable lib_opencnt. this
    variable is also used by CLOSE and EXPUNGE.

CLOSE
    is the entry point called by the function closelibrary(). it
    decrements the library variable lib_opencnt and may do a delayed
    EXPUNGE.

EXPUNGE
    prepares the library for removal from the system. This often includes
    deallocating memory resources that were reserved during
    initialization. EXPUNGE not only frees the memory allocated for data
    structures, but also the areas reserved for the library node itself.

RESERVED
    is a fourth function vector reserved for future use. It must always
    return zero.