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cdf0f88f97
When ungetc is called on an unused stream, the backup buffer is allocated without the main get area being present. This results in every subsequent ungetc (as the stream remains in the backup area) checking uninitialized memory in the backup buffer when trying to put a character back into the stream. Avoid comparing the input character with buffer contents when in backup to avoid this uninitialized read. The uninitialized read is harmless in this context since the location is promptly overwritten with the input character, thus fulfilling ungetc functionality. Also adjust wording in the manual to drop the paragraph that says glibc cannot do multiple ungetc back to back since with this change, ungetc can actually do this. Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org> Reviewed-by: Carlos O'Donell <carlos@redhat.com>
75 lines
2.2 KiB
C
75 lines
2.2 KiB
C
/* Test for ungetc bugs.
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Copyright (C) 1996-2024 Free Software Foundation, Inc.
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Copyright The GNU Toolchain Authors.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <stdio.h>
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#include <stdlib.h>
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#include <support/check.h>
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#include <support/support.h>
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#include <support/temp_file.h>
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#include <support/xstdio.h>
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#include <support/xunistd.h>
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static int
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do_test (void)
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{
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char *name = NULL;
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FILE *fp = NULL;
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int c;
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char buffer[64];
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int fd = create_temp_file ("tst-ungetc.", &name);
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if (fd == -1)
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FAIL_EXIT1 ("cannot create temporary file: %m");
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xclose (fd);
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fp = xfopen (name, "w");
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fputs ("bla", fp);
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xfclose (fp);
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fp = xfopen (name, "r");
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TEST_VERIFY_EXIT (ungetc ('z', fp) == 'z');
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TEST_VERIFY_EXIT (getc (fp) == 'z');
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TEST_VERIFY_EXIT (getc (fp) == 'b');
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TEST_VERIFY_EXIT (getc (fp) == 'l');
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TEST_VERIFY_EXIT (ungetc ('m', fp) == 'm');
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TEST_VERIFY_EXIT (ungetc ('n', fp) == 'n');
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TEST_VERIFY_EXIT (getc (fp) == 'n');
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TEST_VERIFY_EXIT (getc (fp) == 'm');
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TEST_VERIFY_EXIT ((c = getc (fp)) == 'a');
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TEST_VERIFY_EXIT (getc (fp) == EOF);
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TEST_VERIFY_EXIT (ungetc (c, fp) == c);
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TEST_VERIFY_EXIT (feof (fp) == 0);
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TEST_VERIFY_EXIT (getc (fp) == c);
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TEST_VERIFY_EXIT (getc (fp) == EOF);
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xfclose (fp);
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fp = xfopen (name, "r");
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TEST_VERIFY_EXIT (getc (fp) == 'b');
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TEST_VERIFY_EXIT (getc (fp) == 'l');
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TEST_VERIFY_EXIT (ungetc ('b', fp) == 'b');
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TEST_VERIFY_EXIT (fread (buffer, 1, 64, fp) == 2);
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TEST_VERIFY_EXIT (buffer[0] == 'b');
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TEST_VERIFY_EXIT (buffer[1] == 'a');
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xfclose (fp);
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return 0;
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}
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#include <support/test-driver.c>
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