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

[Linus Torvalds]

On Tue, Jul 24, 2012 at 12:03 PM, Josh Boyer <jwboyer@xxxxxxxxxx> 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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