[Bug stdio/30731] New: freopen unnecessarily flushes read-only streams

[Bug stdio/30731] New: freopen unnecessarily flushes read-only streams

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30731

            Bug ID: 30731
           Summary: freopen unnecessarily flushes read-only streams
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---

After

commit 0b727ed4d605d9318cb0d323c88abb0d5a441a9b
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Thu Jul 13 16:43:24 2017 -0300

    libio: Flush stream at freopen (BZ#21037)

    As POSIX states [1] a freopen call should first flush the stream as if by a
    call fflush.  C99 (n1256) and C11 (n1570) only states the function should
    first close any file associated with the specific stream.  Although current
    implementation only follow C specification, current BSD and other libc
    implementation (musl) are in sync with POSIX and fflush the stream.

    This patch change freopen{64} to fflush the stream before actually
reopening
    it (or returning if the stream does not support reopen).  It also changes
the
    Linux implementation to avoid a dynamic allocation on 'fd_to_filename'.

freopen seeks readable in read mode right before closing them. The fflush
machinery has to do this because it's necessary to make a stream's underlying
file descriptor ready for use with the low-level interfaces. But as we close
the stream immediately after, we do not actually need this.

Reported first as a side effect here:

is this a bug in glibc or readpst?
<https://sourceware.org/pipermail/libc-help/2022-November/006335.html>
<https://inbox.sourceware.org/libc-help/2cefc4fa95dd439c2581f4f06d520c004cd33708.camel@bonedaddy.net/>

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug stdio/30731] freopen unnecessarily flushes read-only streams

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30731

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at sourceware dot org   |fweimer at redhat dot com
                 CC|                            |fweimer at redhat dot com

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug stdio/30731] freopen unnecessarily flushes read-only streams

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30731

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from Florian Weimer <fweimer at redhat dot com> ---
Andreas Schwab pointed out on-list that the current flush-on-read behavior is
required by POSIX. I'm not sure if I agree. But upon second thought, I think
the current behavior is more generally useful.

[PATCH 2/2] libio: Perform write-only fflush as part of freopen (bug 30731)
<https://inbox.sourceware.org/libc-alpha/87ttt9fxdi.fsf@oldenburg.str.redhat.com/T/#u>

-- 
You are receiving this mail because:
You are on the CC list for the bug.