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Fix fflush after ungetc on input file (bug 5994) 

As discussed in bug 5994 (plus duplicates), POSIX requires fflush
after ungetc to discard pushed-back characters but preserve the file
position indicator.  For this purpose, each ungetc decrements the file
position indicator by 1; it is unspecified after ungetc at the start
of the file, and after ungetwc, so no special handling is needed for
either of those cases.

This is fixed with appropriate logic in _IO_new_file_sync.  I haven't
made any attempt to test or change things in this area for the "old"
functions; the case of files using mmap is addressed in a subsequent
patch (and there seem to be no problems in this area with files opened
with fmemopen).

Tested for x86_64.
This commit is contained in:
Joseph Myers 2025-01-28 19:38:27 +00:00
parent 1515f74fd8
commit 377e9733b5
3 changed files with 70 additions and 0 deletions
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5
libio/fileops.c
View File

@ -800,6 +800,11 @@ _IO_new_file_sync (FILE *fp)
if (fp->_IO_write_ptr > fp->_IO_write_base)
if (_IO_do_flush(fp)) return EOF;
delta = fp->_IO_read_ptr - fp->_IO_read_end;
if (_IO_in_backup (fp))
{
_IO_switch_to_main_get_area (fp);
delta += fp->_IO_read_ptr - fp->_IO_read_end;
}
if (delta != 0)
{
off64_t new_pos = _IO_SYSSEEK (fp, delta, 1);

1
stdio-common/Makefile
View File

@ -305,6 +305,7 @@ tests := \
tst-swscanf \
tst-tmpnam \
tst-ungetc \
tst-ungetc-fflush \
tst-ungetc-leak \
tst-ungetc-nomem \
tst-unlockedio \

64
stdio-common/tst-ungetc-fflush.c Normal file
View File

@ -0,0 +1,64 @@
/* Test flushing input file after ungetc (bug 5994).
Copyright (C) 2025 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <stdio.h>
#include <support/check.h>
#include <support/temp_file.h>
#include <support/xstdio.h>
#include <support/xunistd.h>
int
do_test (void)
{
char *filename = NULL;
int fd = create_temp_file ("tst-ungetc-fflush", &filename);
TEST_VERIFY_EXIT (fd != -1);
xclose (fd);
/* Test as in bug 5994. */
FILE *fp = xfopen (filename, "w");
TEST_VERIFY_EXIT (fputs ("#include", fp) >= 0);
xfclose (fp);
fp = xfopen (filename, "r");
TEST_COMPARE (fgetc (fp), '#');
TEST_COMPARE (fgetc (fp), 'i');
TEST_COMPARE (ungetc ('@', fp), '@');
TEST_COMPARE (fflush (fp), 0);
TEST_COMPARE (lseek (fileno (fp), 0, SEEK_CUR), 1);
TEST_COMPARE (fgetc (fp), 'i');
TEST_COMPARE (fgetc (fp), 'n');
xfclose (fp);
/* Test as in bug 12799 (duplicate of 5994). */
fp = xfopen (filename, "w+");
TEST_VERIFY_EXIT (fputs ("hello world", fp) >= 0);
rewind (fp);
TEST_VERIFY (fileno (fp) >= 0);
char buffer[10];
TEST_COMPARE (fread (buffer, 1, 5, fp), 5);
TEST_COMPARE (fgetc (fp), ' ');
TEST_COMPARE (ungetc ('@', fp), '@');
TEST_COMPARE (fflush (fp), 0);
TEST_COMPARE (fgetc (fp), ' ');
xfclose (fp);
return 0;
}
#include <support/test-driver.c>