[TUHS] #defines and enums
lm at mcvoy.com
Thu Nov 14 11:35:21 AEST 2019
----- 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
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
static inline some_arch_fn(..) ...
#define some_arch_fn some_arch_fn
in an architecture file, and then in generic code we have
static inline some_arch_fn(.,,) /* generic implemenbtation goes here */
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.
----- End forwarded message -----
Larry McVoy lm at mcvoy.com http://www.mcvoy.com/lm
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