[TUHS] #defines and enums

[Larry McVoy]

FYI.

----- Forwarded message from Linus Torvalds <torvalds at linux-foundation.org> -----

Date: Wed, 13 Nov 2019 12:37:50 -0800
From: Linus Torvalds <torvalds at linux-foundation.org>
To: Larry McVoy <lm at mcvoy.com>
Subject: Re: enum style?

On Wed, Nov 13, 2019 at 10:28 AM Larry McVoy <lm at mcvoy.com> wrote:
>
> and asked what was the point of the #defines.  I couldn't answer, the only
> thing I can think of is so you can say
>
>         int     flags = MS_RDONLY;
>
> Which is cool, but why bother with the enum?

For the kernel we actually have this special "type-safe enum" checker
thing, which warns about assigning one enum type to another.

It's not really C, but it's close. It's the same tool we use for some
other kernel-specific type checking (user pointers vs kernel pointers
etc): 'sparse'.

  http://man7.org/linux/man-pages/man1/sparse.1.html

and in particular the "-Wenum-mismatch" flag to enable that warning
when you assign an enum to another enum.

It's quite useful for verifying that you pass the right kind of enum
to functions etc - which is a really easy mistake to make in C, since
they all just devolve into 'int' when they are used.

However, we don't use it for the MS_xyz flag: those are just plain
#define's in the kernel. But maybe somebody at some point wanted to do
something similar for the ones you point at?

The only other reason I can think of is that somebody really wanted to
use an enum for namespace reasons, and then noticed that other people
had used a #define and used "#ifdef XYZ" to check whether it was
available, and then instead of changing the enums to #defines, they
just added the self-defines.

In the kernel we do that "use #define for discoberability" quite a lot
particularly with architecture-specific helper functions. So you migth
have

   static inline some_arch_fn(..) ...
   #define some_arch_fn some_arch_fn

in an architecture file, and then in generic code we have

   #ifndef some_arch_fn
   static inline some_arch_fn(.,,) /* generic implemenbtation goes here */
   #endif

as a way to avoid extra configuration variables for the "do I have a
special version X?"

There's no way to ask "is the C symbol X available in this scope", so
using the pre-processor for that is as close as you can get.

               Linus

----- End forwarded message -----

-- 
---
Larry McVoy            	     lm at mcvoy.com             http://www.mcvoy.com/lm