These weakly defined empty functions prevent the strong definition from being linked in.
For example, libarm.a contains a weak symbol 'red_LED_on', which is expected to be defined (strongly) in the board library. Because archive libraries are being used, this fails (testing with binutils 2.20.1), and only the empty __red_LED_on stub is linked in; the red_LED_on definition in the board library is throw away.
This behavior is documented and it is the intended one; from http://www.sco.com/developers/gabi/latest/ch4.symtab.html:
When the link editor searches archive libraries [see ``Archive File'' in Chapter 7], it extracts archive members that contain definitions of undefined global symbols. The member's definition may be either a global or a weak symbol. The link editor does not extract archive members to resolve undefined weak symbols. Unresolved weak symbols have a zero value.
Empty weak definitions would have to be supplied to the linker only _after_ the strong definitions have been provided.
Leaving undefined weak symbols and testing for NULL-ity at call sites seems to be a more robust approach.
Note that with some ld versions (at least with 2.20.1), ld creates PLT entries for undefined weak symbols and crashes when the PLT-related sections (.plt, .got.plt, and .rel.plt) are discarded...
-- Sebastien Carlier