defines inside enum [duplicate]

Answer 1

Another guess: this macro could allow renaming an entry of the enum without breaking other code. Change

enum {
        RTM_BASE        = 16,
#define RTM_BASE        RTM_BASE

to

enum {
        RTM_BASE_NEW_NEW_NEW        = 16,
#define RTM_BASE        RTM_BASE_NEW_NEW_NEW

A user could still use RTM_BASE.

Answer 2

Another thing these #defines achieve, besides allowing old code to continue the old names should the enum constant names be changed, and checking whether they are defined, is to prevent other code to define these symbols.

#include <linux/rtnetlink.h>
// for some reason, the author thinks
#define RTM_BASE 17.3f
// is a good idea

would not compile.

Answer 3

One thing I could think of is that you can check for the very existence of the enum by means of the macro:

#ifdef RTM_BASE
int flag = RTMBASE;
#else
int flag = 0;
#endif

No idea if that's what's going on, though.