The Unix Heritage Society mailing list
 help / color / mirror / Atom feed
* Re: [TUHS] #defines and enums
@ 2019-11-14  1:35 Larry McVoy
  0 siblings, 0 replies; 3+ messages in thread
From: Larry McVoy @ 2019-11-14  1:35 UTC (permalink / raw)
  To: tuhs

FYI.

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

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

On Wed, Nov 13, 2019 at 10:28 AM Larry McVoy <lm@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 

^ permalink raw reply	[flat|nested] 3+ messages in thread
* Re: [TUHS] Happy birthday Morris worm
@ 2019-11-12 20:56 Norman Wilson
  2019-11-13  7:35 ` arnold
  0 siblings, 1 reply; 3+ messages in thread
From: Norman Wilson @ 2019-11-12 20:56 UTC (permalink / raw)
  To: tuhs

I think I recall an explicit statement somewhere from an
interview with Robert that the worm was inspired partly
by Shockwave Rider.

I confess my immediate reaction to the worm was uncontrollable
laughter.  I was out of town when it happened, so I first
heard it from a newspaper article (and wasn't caught up in
fighting it or I'd have laughed a lot less, of course); and
it seemed to me hilarious when I read that Robert was behind
it.  He had interned with 1127 for a few summers while I was
there, so I knew him as very bright but often a bit careless
about details; that seemed an exact match for the worm.

My longer-term reaction was to completely drop my sloppy
old habit (common in those days not just in my code but in
that of many others) of ignoring possible buffer overflows.
I find it mind-boggling that people still make that mistake;
it has been literal decades since the lesson was rubbed in
our community's collective noses.  I am very disappointed
that programming education seems not to care enough about
this sort of thing, even today.

Norman Wilson
Toronto ON

^ permalink raw reply	[flat|nested] 3+ messages in thread

end of thread, other threads:[~2019-11-14  1:36 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-11-14  1:35 [TUHS] #defines and enums Larry McVoy
  -- strict thread matches above, loose matches on Subject: below --
2019-11-12 20:56 [TUHS] Happy birthday Morris worm Norman Wilson
2019-11-13  7:35 ` arnold
2019-11-13 18:02   ` [TUHS] Happy birthday Morris worm [ really programming education ] Jon Steinhart
2019-11-13 19:15     ` [TUHS] #defines and enums ron
2019-11-13 21:11       ` Warner Losh

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).