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Re: bash: Correct usage of F_SETFD


From: Matthew Wilcox
Subject: Re: bash: Correct usage of F_SETFD
Date: Tue, 23 Nov 2010 07:42:39 -0700
User-agent: Mutt/1.5.18 (2008-05-17)

On Mon, Nov 22, 2010 at 04:04:46PM -0800, Sukadev Bhattiprolu wrote:
> Eric Blake address@hidden wrote:
> | On 11/22/2010 03:16 PM, Chet Ramey wrote:
> | >> include/filecntl.h in bash-4.1 has following:
> | >>
> | >> #define SET_CLOSE_ON_EXEC(fd)  (fcntl ((fd), F_SETFD, FD_CLOEXEC))
> | >>
> | >> Is that really the correct/intended usage of F_SETFD ?
> | > 
> | >      F_SETFD            Set the close-on-exec flag associated with fildes 
> to
> | >                         the low order bit of arg (0 or 1 as above).
> 
> Is that the POSIX definition ? Following man page does not limit F_SETFD to
> FD_CLOEXEC:
> 
>       http://www.kernel.org/doc/man-pages/online/pages/man2/fcntl.2.html
> 
>       F_SETFD (long)
>                   Set the file descriptor flags to the value specified by arg.

The POSIX definition can be found here:
http://www.opengroup.org/onlinepubs/009695399/functions/fcntl.html

> | In practice, there aren't any such systems; but POSIX warns that current
> | practice is no indicator of future systems, and that read-modify-write
> | is the only way to use F_SETFD.
> 
> Yes, that seems to make more sense.

I think future flags will be created such that they default to off,
and bash would have to affirmitively set them in order to use them.

So if bash is the one creating its file descriptors, there's no need to
use R/M/W since it knows what the state of them are.

-- 
Matthew Wilcox                          Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."



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