[Bug stdio/27777] fclose does a linear search, takes ages when many FILE* are opened

["carlos at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27777

Carlos O'Donell <carlos at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |carlos at redhat dot com
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-04-24
     Ever confirmed|0                           |1

--- Comment #1 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Alexandre Ferrieux from comment #0)
> If one has *many* opened streams (which is possible if the max number of
> opened file descriptors per process has been tuned beyond the typical 1024),
> fclose() starts being *very* slow.
> 
> The root cause is the following linear search in genops.c/_IO_un_link:
> 
> d18ea0c5	68		for (f = &_IO_list_all->file._chain; *f; f = &(*f)->_chain)
> 9964a145	69		  if (*f == (FILE *) fp)
> 40a55d20	70		    {
> cedb4109	71		      *f = fp->file._chain;
> 40a55d20        72		      break;
> UD	        73		    }
> 
> Clearly a singly-linked list does not allow for O(1) removal.
> Given what I understand of the design constraints of this list, the most
> natural fix would be to switch to a doubly-linked list.

Just wanted to mention that I saw this recently while walking the list of bugs.
I can confirm that this is the case. Please feel free to work with the
community to improve the performance. Likewise you might also want to look at
fcloseall() performance too.

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