Re: PATCH: netdev: add a cast NLMSG_OK to avoid a GCC warning in users' code

[David Miller <davem@davemloft.net>]

From: "D. Hugh Redelmeier" <hugh@mimosa.com>
Date: Wed, 9 Sep 2015 16:24:07 -0400 (EDT)

> | From: David Miller <davem@davemloft.net>
> 
> | From: "D. Hugh Redelmeier" <hugh@mimosa.com>
> | Date: Tue, 8 Sep 2015 09:46:56 -0400 (EDT)
> | 
> | > Using netlink.h's NLMSG_OK correctly will cause GCC to issue a warning
> | > on systems with 32-bit userland.  The definition can easily be changed
> | > to avoid this.
> | 
> | Everyone either uses an unsigned type (such as "size_t") or adds an
> | explicit cast to an unsinged type for the second argument.
> 
> 1) netlink(3) says that the type of the second parameter is "int".
>    From the synopsis:
>        int NLMSG_OK(struct nlmsghdr *nlh, int len);
>    Surely then "int" should be appropriate.

Documentation can, and often is, wrong.  The code that has been there
for more than two decades determines what the interface and semantics
actually are.

Whatever is actually in the macro is what people have to accomodate
and cope with.