add closefrom() call

[Robert Watson]

On Fri, 6 Jul 2007, Julian Elischer wrote:

> Ed Schouten wrote:
>> * LI Xin <delphij at delphij.net> wrote:
>>> Here is my implementation for FreeBSD.  Some difference between my and 
>>> DragonFly's implementation:
>>>
>>>  - closefrom(-1) would be no-op on DragonFly, my version would close all 
>>> open files (From my understanding of OpenSolaris's userland 
>>> implementation, this is Solaris's behavior).
>>>  - my version closefrom(very_big_fd) would result in EBADF.  I am not very 
>>> sure whether this is correct, but it does not hurt for applications that 
>>> thinks closefrom() would return void.
>> 
>> Wouldn't it be better to just implement it through fcntl() and implement 
>> closefrom() in libc?
>
> that's a possibility but I personally thing the huge difference in 
> efficiency makes it worth putting it in the kernel. Quite a few programs I 
> know of could really help their startup time with this as the first thing 
> they do is "close the first 2000 file descriptors.

The Solaris implementation appears to implement two strategies:

(1) If procfs is mounted, list the fd directory to get a list of open fds,
     then close those by number.

(2) If procfs is not mounted, query the number of open fds using the resource
     limit interface, then sequentially close until the right number close.

Hence my question as to whether there's actually a big benefit or not -- do we 
think closefrom() is a performance-critical function?

Robert N M Watson
Computer Laboratory
University of Cambridge