[Bug libc/27747] New: Inconsistent file pointer fgets with multiprocess

[Bug libc/27747] New: Inconsistent file pointer fgets with multiprocess

[gigacms at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=27747

            Bug ID: 27747
           Summary: Inconsistent file pointer fgets with multiprocess
           Product: glibc
           Version: 2.31
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: gigacms at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Created attachment 13373
  --> https://sourceware.org/bugzilla/attachment.cgi?id=13373&action=edit
source code, compiled binary, stdoutput of testcase

Hi!
In multiprocess environment, file pointer express wrong and inconsistent seek
position after using fgets.

test case is in attachment and have 3 point sequence:
    1. file open
    2. fork
    3. fgets & printf (each process)


I tried to see memory using GDB, there are empty memory(set 0) such as
_IO_read_base.
It seems about file lock. But I can't assure it.

My input was the string follow: "one\ntwo\nthree\nfour"
So, expected result is 2nd of executed output below.


[ stdout of testcase ]
-----------------------------------------------

liberty@ubuntu:~/glibc/test$ ./fgets_fork_inner 
13252   0
0       0
0       4
13252   4
0       (null)
13252   one

liberty@ubuntu:~/glibc/test$ ./fgets_fork_inner 
13254   0
13254   4
13254   one

0       4
0       8
0       two

liberty@ubuntu:~/glibc/test$ ./fgets_fork_inner 
13256   0
0       0
13256   4
13256   one

0       8
0       two

liberty@ubuntu:~/glibc/test$ ./fgets_fork_inner 
13258   0
13258   4
13258   one

0       0
0       8
0       two

-----------------------------------------------


Libc Path:/usr/lib/x86_64-linux-gnu/libc-2.31.so
System Info: "Linux ubuntu 5.8.0-50-generic #56~20.04.1-Ubuntu SMP Mon Apr 12
21:46:35 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux"

Thank you.

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[Bug libc/27747] Inconsistent file pointer fgets with multiprocess

[schwab@linux-m68k.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27747

Andreas Schwab <schwab@linux-m68k.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |NOTABUG

--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> ---
You need to follow the processes described in this section:

https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_05_01

Accessing the same stream in both processes after fork is undefined otherwise.

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[Bug libc/27747] Inconsistent file pointer fgets with multiprocess

[gigacms at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=27747

--- Comment #2 from Minseong <gigacms at gmail dot com> ---
(In reply to Andreas Schwab from comment #1)
> You need to follow the processes described in this section:
> 
> https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.
> html#tag_15_05_01
> 
> Accessing the same stream in both processes after fork is undefined
> otherwise.

Oh I see. Thanks a lot!

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