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From: "fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug stdio/31183] New: Wide stream buffer size reduced MB_LEN_MAX
 bytes after bug 17522 fix
Date: Wed, 20 Dec 2023 15:43:37 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: glibc
X-Bugzilla-Component: stdio
X-Bugzilla-Version: 2.21
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: fweimer at redhat dot com
X-Bugzilla-Status: NEW
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X-Bugzilla-Priority: P2
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D31183

            Bug ID: 31183
           Summary: Wide stream buffer size reduced MB_LEN_MAX bytes after
                    bug 17522 fix
           Product: glibc
           Version: 2.21
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: fweimer at redhat dot com
          Reporter: fweimer at redhat dot com
  Target Milestone: ---

After the fix from bug 17522, we do not use the remaining buffer space if t=
he
already-written buffer space is smaller than MB_LEN_MAX (16 bytes).  As we
start out with no written bytes (zero), we never use the full buffer, great=
ly
reducing performance.

The fix probably explains it better:

-         if (fp->_IO_write_ptr - fp->_IO_write_base < sizeof (mb_buf))
+         if (fp->_IO_buf_end - fp->_IO_write_ptr < sizeof (mb_buf))
            {
              /* Make sure we have room for at least one multibyte
                 character.  */
              write_ptr =3D write_base =3D mb_buf;
              buf_end =3D mb_buf + sizeof (mb_buf);
            }

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