The file /usr/include/socketbits.h has an inline
routine "__cmsg_nxthdr" that includes "return NULL;"
in its body. Under gc++ this produces: "warning: ANSI C++
forbids implicit conversion from `void *' in return"

The warning is only of minor annoyance, but it can be made
to completely go away for all platforms by chaning
"return NULL;" to "return 0;". C and C++ both say that
trying to "assign" an explicit integral constant zero
to any pointer produces the valid NULL pointer.