Re: list of open descriptors

[CRPence <CRPbottle@xxxxxxxxx>]

On 06 Jun 2013 13:22, Vernon Hamberg wrote:
> On 06 Jun 2013 05:24, Vernon Hamberg wrote:
> > Did a little looking and wonder if the fcntl() function will
> > do the trick. You'd need to do a loop until it returns an error
> > - that'd be the first available descriptor. <<SNIP>>
> <<SNIP>> I even have a doubt about my suggestion to use fcntl() - I
> don't remember or know if there can be gaps in the used file
> descriptors - each does refer to an open file, and if one closes one
> with a lower FD while one with a higher FD is open, is the lower one
> reused? <<SNIP>>

  I was going to compose a response to the earlier message, but the 
more recent of the above quoted messages effectively gives the lead-in I 
would have had to compose.

  There could always be gaps irrespective of the reuse algorithm; i.e. 
the integer descriptor is not transient, so a close of a lower FD does 
not renumber the others.

  Ignoring the possibility of a zero-value descriptor, ignoring any 
special processing that might occur for zero to three, and ignoring any 
multi-threading...

  If the identifiers one through seven are all in-use, and number five 
is closed, then there is a gap of one.  Whether the number five is 
reused immediately for the next open [or if instead the number eight is 
used] is moot; i.e. there remains a gap.  Of course the gap could grow 
too.  Close numbers four and six, there is now a gap of three; four 
remain open [1,2,3,7].

  Given the lowest number is always reused...

  Knowing that there were only four remaining open is of no direct 
assist to know how many to process sequentially.  Having that number 
only allows knowing when to _stop looking_ for those that are open.  By 
processing the descriptors sequentially from one, until an incremented 
verified-as-open-count matches a known total open-descriptor-count, the 
"when" to _stop looking_ for more open descriptors is obvious.  In the 
just-posited example with only four open, all seven would need to be 
checked to find that each of one to three and seven remained open.  If 
instead numbers four, six, and seven had been closed, again four remain 
open [1,2,3,5], yet only five, instead of all seven, would need to be 
interrogated for their open status.