I list symbol from udpcdialog/lib.a

1) cd busybox-1.13.2/udpcdialog
2) nm lib.a | grep "_IO_cleanup"

get result:

                U _IO_cleanup
00000000  T  __bring_in_IO_cleanup
00000000  d __elf_set___libc_atexit_element__IO_cleanup__

_IO_cleanup not defined in busybox-1.13.2/udpcdialog/lib.a  why ???