linux-kernel - Re: [PATCH v2] posix_types.h: make __NFDBITS compatible with glibc definition

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Message-ID: <CA+55aFyn-yMUMdcY8jp65eeoGm3RPzy9p+St35-HGE_xXiVJag@mail.gmail.com>
Date:	Tue, 24 Jul 2012 12:09:41 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Josh Boyer <jwboyer@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, law@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] posix_types.h: make __NFDBITS compatible with glibc definition

On Tue, Jul 24, 2012 at 12:03 PM, Josh Boyer <jwboyer@...hat.com> wrote:
>
> FWIW, the definitions of __FD_ELT/__FD_MASK in glibc are:
>
> #define __FD_ELT(d)     ((d) / __NFDBITS)
> #define __FD_MASK(d)    ((__fd_mask) 1 << ((d) % __NFDBITS))
>
> where __fd_mask is 'typdef long int'.

Yeah, that's not good.

If __NFDBITS is signed (and it is), and "d" is a signed type, that
division and modulus now create stupid extra code with conditionals
(assuming 'd' isn't constant, of course).

So changing the sign of __NFDBITS has these kinds of subtle side
effects that clearly the glibc people didn't actually think about.

What was the *advantage* of that stupidity?

Quite frankly, if you want to make NFDBITS be an "int", then it should
have been done at that

   #define NFDBITS ((int)__NFDBITS)

level, not at "__NFDBITS". Exactly because the unsigned type there matters.

Does anybody in the glibc camp care about efficient and small code AT ALL?

                  Linus
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