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From: "alexandre.ferrieux at orange dot com"
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To: glibc-bugs@sourceware.org
Subject: [Bug stdio/27777] New: fclose does a linear search, takes ages when
 FILE* are opened
Date: Sat, 24 Apr 2021 21:50:58 +0000
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X-Bugzilla-Product: glibc
X-Bugzilla-Component: stdio
X-Bugzilla-Version: 2.34
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D27777

            Bug ID: 27777
           Summary: fclose does a linear search, takes ages when FILE* are
                    opened
           Product: glibc
           Version: 2.34
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: alexandre.ferrieux at orange dot com
  Target Milestone: ---

If one has *many* opened streams (which is possible if the max number of op=
ened
file descriptors per process has been tuned beyond the typical 1024), fclos=
e()
starts being *very* slow.

The root cause is the following linear search in genops.c/_IO_un_link:

d18ea0c5        68              for (f =3D &_IO_list_all->file._chain; *f; =
f =3D
&(*f)->_chain)
9964a145        69                if (*f =3D=3D (FILE *) fp)
40a55d20        70                  {
cedb4109        71                    *f =3D 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.

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