Confusing MACRO and enum definition

Question

I was browsing some Route netlink source code.

I wanted to figure out what was the value of RTNLGRP_NEIGH

Source: http://lxr.free-electrons.com/source/include/li

Answer 1

The value of RTNLGRP_NEIGH will be 3 (it is the fourth enumeration constant: RTNLGRP_NONE has the value 0, RTNLGRP_LINK has the value 1, and RTNLGRP_NOTIFY has the value 2).

The #define stuff is somewhat weird — it is the sort of thing that's apt to make people want to stop you using the C pre-processor.

The idea is that it gives you a macro for RTNLGRP_NEIGH that can be tested, but the expansion of the macro is the enumeration constant (spelled the same). There isn't an infinite loop in the expansions because once a macro has been expanded, it is not expanded again while the replacement text is being rescanned.

So, the upshot is that you can write:

#ifdef RTNLGRP_NEIGH
   …code using RTNLGRP_NEIGH…
#endif