linux-kernel - Re: [PATCH 21/21] posix_types: Remove fd

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Date:	Tue, 14 Feb 2012 11:44:30 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	David Howells <dhowells@...hat.com>
CC:	Tony Luck <tony.luck@...el.com>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 21/21] posix_types: Remove fd_set macros

On 02/14/2012 11:18 AM, David Howells wrote:
> Tony Luck <tony.luck@...el.com> wrote:
> 
>> Adding a #include <linux/string.h> to include/linux/time.h fixes them for me.
> 
> Which is one of the reasons I'd rather not see these things cropping up in
> linux/time.h.  Either put them in linux/select.h (mirroring userspace) or
> stick them in linux/poll.h with the rest of the polling paraphernalia.
> 
> __FD_SET() and co aren't exported to userspace anyway, as I understand it, so
> it shouldn't hurt to bung them in linux/poll.h, and if they are exported to
> userspace, they should stay in posix_types.h, shouldn't they?
> 
> To support this, at least three #includes will have to be added to
> linux/time.h: linux/posix_types.h (which is fair enough), linux/bitops.h and
> linux/string.h.

We can do that, but we'd almost certainly have to #include
<linux/select.h> in <linux/time.h>.  The following macros are defined in
<linux/time.h> and *exported to userspace*:

#define NFDBITS                 __NFDBITS

#define FD_SETSIZE              __FD_SETSIZE
#define FD_SET(fd,fdsetp)       __FD_SET(fd,fdsetp)
#define FD_CLR(fd,fdsetp)       __FD_CLR(fd,fdsetp)
#define FD_ISSET(fd,fdsetp)     __FD_ISSET(fd,fdsetp)
#define FD_ZERO(fdsetp)         __FD_ZERO(fdsetp)

Now, this largely mimics what has happened in userspace over time; to
quote POSIX:

The <sys/time.h> header shall define the fd_set type as described in
<sys/select.h>.

The <sys/time.h> header shall define the following as described in
<sys/select.h>:
FD_CLR( )
FD_ISSET( )
FD_SET( )
FD_ZERO( )
FD_SETSIZE

[...]

Inclusion of the <sys/time.h> header may make visible all symbols from
the <sys/select.h> header.

Cleaning up <linux/time.h> would indeed be a worthwhile goal -- it's not
clear to me that it can safely be used to build a POSIX-compliant
userspace.  In particular, it mixes the functions of <sys/time.h> and
<time.h>, but <time.h> symbols are not permitted to leak into
<sys/time.h> space as far as I read the specification.  However, this
feels like a slightly orthogonal task (in fact, it might belong more in
your uabi work.)

	-hpa
-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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