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[BZ 1190] Make EOF sticky in stdio.

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C99 specifies that the EOF condition on a file is "sticky": once EOF
has been encountered, all subsequent reads should continue to return
EOF until the file is closed or something clears the "end-of-file
indicator" (e.g. fseek, clearerr).  This is arguably a change from
C89, where the wording was ambiguous; the BSDs always had sticky EOF,
but the System V lineage would attempt to read from the underlying fd
again.  GNU libc has followed System V for as long as we've been
using libio, but nowadays C99 conformance and BSD compatibility are
more important than System V compatibility.

You might wonder if changing the _underflow impls is sufficient to
apply the C99 semantics to all of the many stdio functions that
perform input.  It should be enough to cover all paths to _IO_SYSREAD,
and the only other functions that call _IO_SYSREAD are the _seekoff
impls, which is OK because seeking clears EOF, and the _xsgetn impls,
which, as far as I can tell, are unused within glibc.

The test programs in this patch use a pseudoterminal to set up the
necessary conditions.  To facilitate this I added a new test-support
function that sets up a pair of pty file descriptors for you; it's
almost the same as BSD openpty, the only differences are that it
allocates the optionally-returned tty pathname with malloc, and that
it crashes if anything goes wrong.

	[BZ #1190]
        [BZ #19476]
	* libio/fileops.c (_IO_new_file_underflow): Return EOF immediately
	if the _IO_EOF_SEEN bit is already set; update commentary.
	* libio/oldfileops.c (_IO_old_file_underflow): Likewise.
	* libio/wfileops.c (_IO_wfile_underflow): Likewise.

	* support/support_openpty.c, support/tty.h: New files.
	* support/Makefile (libsupport-routines): Add support_openpty.

	* libio/tst-fgetc-after-eof.c, wcsmbs/test-fgetwc-after-eof.c:
	New test cases.
	* libio/Makefile (tests): Add tst-fgetc-after-eof.
	* wcsmbs/Makefile (tests): Add tst-fgetwc-after-eof.
This commit is contained in:
Zack Weinberg 2018-02-21 19:12:51 -05:00
parent 778f197486
commit 2cc7bad0ae
12 changed files with 416 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
ChangeLog
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@ -1,3 +1,20 @@
2018-03-12 Zack Weinberg <zackw@panix.com>
[BZ #1190]
[BZ #19476]
* libio/fileops.c (_IO_new_file_underflow): Return EOF immediately
if the _IO_EOF_SEEN bit is already set; update commentary.
* libio/oldfileops.c (_IO_old_file_underflow): Likewise.
* libio/wfileops.c (_IO_wfile_underflow): Likewise.
* support/support_openpty.c, support/tty.h: New files.
* support/Makefile (libsupport-routines): Add support_openpty.
* libio/tst-fgetc-after-eof.c, wcsmbs/test-fgetwc-after-eof.c:
New test cases.
* libio/Makefile (tests): Add tst-fgetc-after-eof.
* wcsmbs/Makefile (tests): Add tst-fgetwc-after-eof.
2018-03-12 Dmitry V. Levin <ldv@altlinux.org>
* po/pt_BR.po: Update translations.

8
NEWS
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@ -28,6 +28,14 @@ Deprecated and removed features, and other changes affecting compatibility:
investigate using (f)getc_unlocked and (f)putc_unlocked, and, if
necessary, flockfile and funlockfile.
* All stdio functions now treat end-of-file as a sticky condition. If you
read from a file until EOF, and then the file is enlarged by another
process, you must call clearerr or another function with the same effect
(e.g. fseek, rewind) before you can read the additional data. This
corrects a longstanding C99 conformance bug. It is most likely to affect
programs that use stdio to read interactive input from a terminal.
(Bug #1190.)
* The macros 'major', 'minor', and 'makedev' are now only available from
the header <sys/sysmacros.h>; not from <sys/types.h> or various other
headers that happen to include <sys/types.h>. These macros are rarely

3
libio/Makefile
View File

@ -64,7 +64,8 @@ tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc \
bug-memstream1 bug-wmemstream1 \
tst-setvbuf1 tst-popen1 tst-fgetwc bug-wsetpos tst-fseek \
tst-fwrite-error tst-ftell-partial-wide tst-ftell-active-handler \
tst-ftell-append tst-fputws tst-bz22415
tst-ftell-append tst-fputws tst-bz22415 tst-fgetc-after-eof
ifeq (yes,$(build-shared))
# Add test-fopenloc only if shared library is enabled since it depends on
# shared localedata objects.

7
libio/fileops.c
View File

@ -468,11 +468,10 @@ int
_IO_new_file_underflow (FILE *fp)
{
ssize_t count;
#if 0
/* SysV does not make this test; take it out for compatibility */
/* C99 requires EOF to be "sticky". */
if (fp->_flags & _IO_EOF_SEEN)
return (EOF);
#endif
return EOF;
if (fp->_flags & _IO_NO_READS)
{

7
libio/oldfileops.c
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@ -294,11 +294,10 @@ attribute_compat_text_section
_IO_old_file_underflow (FILE *fp)
{
ssize_t count;
#if 0
/* SysV does not make this test; take it out for compatibility */
/* C99 requires EOF to be "sticky". */
if (fp->_flags & _IO_EOF_SEEN)
return (EOF);
#endif
return EOF;
if (fp->_flags & _IO_NO_READS)
{

109
libio/tst-fgetc-after-eof.c Normal file
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@ -0,0 +1,109 @@
/* Bug 1190: EOF conditions are supposed to be sticky.
Copyright (C) 2018 Free Software Foundation.
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved. This file is offered as-is,
without any warranty. */
/* ISO C1999 specification of fgetc:
#include <stdio.h>
int fgetc (FILE *stream);
Description
If the end-of-file indicator for the input stream pointed to by
stream is not set and a next character is present, the fgetc
function obtains that character as an unsigned char converted to
an int and advances the associated file position indicator for
the stream (if defined).
Returns
If the end-of-file indicator for the stream is set, or if the
stream is at end-of-file, the end-of-file indicator for the
stream is set and the fgetc function returns EOF. Otherwise, the
fgetc function returns the next character from the input stream
pointed to by stream. If a read error occurs, the error indicator
for the stream is set and the fgetc function returns EOF.
The requirement to return EOF "if the end-of-file indicator for the
stream is set" was new in C99; the language in the 1989 edition of
the standard was ambiguous. Historically, BSD-derived Unix always
had the C99 behavior, whereas in System V fgetc would attempt to
call read() again before returning EOF again. Prior to version 2.28,
glibc followed the System V behavior even though this does not
comply with C99.
See
<https://sourceware.org/bugzilla/show_bug.cgi?id=1190>,
<https://sourceware.org/bugzilla/show_bug.cgi?id=19476>,
and the thread at
<https://sourceware.org/ml/libc-alpha/2012-09/msg00343.html>
for more detail. */
#include <support/tty.h>
#include <support/check.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define XWRITE(fd, s, msg) do { \
if (write (fd, s, sizeof s - 1) != sizeof s - 1) \
{ \
perror ("write " msg); \
return 1; \
} \
} while (0)
int
do_test (void)
{
/* The easiest way to set up the conditions under which you can
notice whether the end-of-file indicator is sticky, is with a
pseudo-tty. This is also the case which applications are most
likely to care about. And it avoids any question of whether and
how it is legitimate to access the same physical file with two
independent FILE objects. */
int outer_fd, inner_fd;
FILE *fp;
support_openpty (&outer_fd, &inner_fd, 0, 0, 0);
fp = fdopen (inner_fd, "r+");
if (!fp)
{
perror ("fdopen");
return 1;
}
XWRITE (outer_fd, "abc\n\004", "first line + EOF");
TEST_COMPARE (fgetc (fp), 'a');
TEST_COMPARE (fgetc (fp), 'b');
TEST_COMPARE (fgetc (fp), 'c');
TEST_COMPARE (fgetc (fp), '\n');
TEST_COMPARE (fgetc (fp), EOF);
TEST_VERIFY_EXIT (feof (fp));
TEST_VERIFY_EXIT (!ferror (fp));
XWRITE (outer_fd, "d\n", "second line");
/* At this point, there is a new full line of input waiting in the
kernelside input buffer, but we should still observe EOF from
stdio, because the end-of-file indicator has not been cleared. */
TEST_COMPARE (fgetc (fp), EOF);
/* Clearing EOF should reveal the next line of input. */
clearerr (fp);
TEST_COMPARE (fgetc (fp), 'd');
TEST_COMPARE (fgetc (fp), '\n');
fclose (fp);
close (outer_fd);
return 0;
}
#include <support/test-driver.c>

4
libio/wfileops.c
View File

@ -116,6 +116,10 @@ _IO_wfile_underflow (FILE *fp)
enum __codecvt_result status;
ssize_t count;
/* C99 requires EOF to be "sticky". */
if (fp->_flags & _IO_EOF_SEEN)
return WEOF;
if (__glibc_unlikely (fp->_flags & _IO_NO_READS))
{
fp->_flags |= _IO_ERR_SEEN;

1
support/Makefile
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@ -52,6 +52,7 @@ libsupport-routines = \
support_format_hostent \
support_format_netent \
support_isolate_in_subprocess \
support_openpty \
support_record_failure \
support_run_diff \
support_shared_allocate \

109
support/support_openpty.c Normal file
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@ -0,0 +1,109 @@
/* Open a pseudoterminal.
Copyright (C) 2018 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
<http://www.gnu.org/licenses/>. */
#include <support/tty.h>
#include <support/check.h>
#include <support/support.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <termios.h>
#include <sys/ioctl.h>
#include <unistd.h>
/* As ptsname, but allocates space for an appropriately-sized string
using malloc. */
static char *
xptsname (int fd)
{
int rv;
size_t buf_len = 128;
char *buf = xmalloc (buf_len);
for (;;)
{
rv = ptsname_r (fd, buf, buf_len);
if (rv)
FAIL_EXIT1 ("ptsname_r: %s", strerror (errno));
if (memchr (buf, '\0', buf_len))
return buf; /* ptsname succeeded and the buffer was not truncated */
buf_len *= 2;
buf = xrealloc (buf, buf_len);
}
}
void
support_openpty (int *a_outer, int *a_inner, char **a_name,
const struct termios *termp,
const struct winsize *winp)
{
int outer = -1, inner = -1;
char *namebuf = 0;
outer = posix_openpt (O_RDWR | O_NOCTTY);
if (outer == -1)
FAIL_EXIT1 ("posix_openpt: %s", strerror (errno));
if (grantpt (outer))
FAIL_EXIT1 ("grantpt: %s", strerror (errno));
if (unlockpt (outer))
FAIL_EXIT1 ("unlockpt: %s", strerror (errno));
#ifdef TIOCGPTPEER
inner = ioctl (outer, TIOCGPTPEER, O_RDWR | O_NOCTTY);
#endif
if (inner == -1)
{
/* The kernel might not support TIOCGPTPEER, fall back to open
by name. */
namebuf = xptsname (outer);
inner = open (namebuf, O_RDWR | O_NOCTTY);
if (inner == -1)
FAIL_EXIT1 ("%s: %s", namebuf, strerror (errno));
}
if (termp)
{
if (tcsetattr (inner, TCSAFLUSH, termp))
FAIL_EXIT1 ("tcsetattr: %s", strerror (errno));
}
#ifdef TIOCSWINSZ
if (winp)
{
if (ioctl (inner, TIOCSWINSZ, winp))
FAIL_EXIT1 ("TIOCSWINSZ: %s", strerror (errno));
}
#endif
if (a_name)
{
if (!namebuf)
namebuf = xptsname (outer);
*a_name = namebuf;
}
else
free (namebuf);
*a_outer = outer;
*a_inner = inner;
}

45
support/tty.h Normal file
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@ -0,0 +1,45 @@
/* Support functions related to (pseudo)terminals.
Copyright (C) 2018 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
<http://www.gnu.org/licenses/>. */
#ifndef _SUPPORT_TTY_H
#define _SUPPORT_TTY_H 1
struct termios;
struct winsize;
/** Open a pseudoterminal pair. The outer fd is written to the address
A_OUTER and the inner fd to A_INNER.
If A_NAME is not NULL, it will be set to point to a string naming
the /dev/pts/NNN device corresponding to the inner fd; space for
this string is allocated with malloc and should be freed by the
caller when no longer needed. (This is different from the libutil
function 'openpty'.)
If TERMP is not NULL, the terminal parameters will be initialized
according to the termios structure it points to.
If WINP is not NULL, the terminal window size will be set
accordingly.
Terminates the process on failure (like xmalloc). */
extern void support_openpty (int *a_outer, int *a_inner, char **a_name,
const struct termios *termp,
const struct winsize *winp);
#endif

2
wcsmbs/Makefile
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@ -50,7 +50,7 @@ strop-tests := wcscmp wcsncmp wmemcmp wcslen wcschr wcsrchr wcscpy wcsnlen \
tests := tst-wcstof wcsmbs-tst1 tst-wcsnlen tst-btowc tst-mbrtowc \
tst-wcrtomb tst-wcpncpy tst-mbsrtowcs tst-wchar-h tst-mbrtowc2 \
tst-c16c32-1 wcsatcliff tst-wcstol-locale tst-wcstod-nan-locale \
tst-wcstod-round test-char-types \
tst-wcstod-round test-char-types tst-fgetwc-after-eof \
$(addprefix test-,$(strop-tests))
include ../Rules

114
wcsmbs/tst-fgetwc-after-eof.c Normal file
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@ -0,0 +1,114 @@
/* Bug 1190: EOF conditions are supposed to be sticky.
Copyright (C) 2018 Free Software Foundation.
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved. This file is offered as-is,
without any warranty. */
/* ISO C1999 specification of fgetwc:
#include <stdio.h>
#include <wchar.h>
wint_t fgetwc (FILE *stream);
Description
If the end-of-file indicator for the input stream pointed to by
stream is not set and a next wide character is present, the
fgetwc function obtains that wide character as a wchar_t
converted to a wint_t and advances the associated file position
indicator for the stream (if defined).
Returns
If the end-of-file indicator for the stream is set, or if the
stream is at end-of-file, the end- of-file indicator for the
stream is set and the fgetwc function returns WEOF. Otherwise,
the fgetwc function returns the next wide character from the
input stream pointed to by stream. If a read error occurs, the
error indicator for the stream is set and the fgetwc function
returns WEOF. If an encoding error occurs (including too few
bytes), the value of the macro EILSEQ is stored in errno and the
fgetwc function returns WEOF.
The requirement to return WEOF "if the end-of-file indicator for the
stream is set" was new in C99; the language in the 1995 edition of
the standard was ambiguous. Historically, BSD-derived Unix always
had the C99 behavior, whereas in System V fgetwc would attempt to
call read() again before returning EOF again. Prior to version 2.28,
glibc followed the System V behavior even though this does not
comply with C99.
See
<https://sourceware.org/bugzilla/show_bug.cgi?id=1190>,
<https://sourceware.org/bugzilla/show_bug.cgi?id=19476>,
and the thread at
<https://sourceware.org/ml/libc-alpha/2012-09/msg00343.html>
for more detail. */
#include <support/tty.h>
#include <support/check.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <wchar.h>
#define XWRITE(fd, s, msg) do { \
if (write (fd, s, sizeof s - 1) != sizeof s - 1) \
{ \
perror ("write " msg); \
return 1; \
} \
} while (0)
int
do_test (void)
{
/* The easiest way to set up the conditions under which you can
notice whether the end-of-file indicator is sticky, is with a
pseudo-tty. This is also the case which applications are most
likely to care about. And it avoids any question of whether and
how it is legitimate to access the same physical file with two
independent FILE objects. */
int outer_fd, inner_fd;
FILE *fp;
support_openpty (&outer_fd, &inner_fd, 0, 0, 0);
fp = fdopen (inner_fd, "r+");
if (!fp)
{
perror ("fdopen");
return 1;
}
XWRITE (outer_fd, "abc\n\004", "first line + EOF");
TEST_COMPARE (fgetwc (fp), L'a');
TEST_COMPARE (fgetwc (fp), L'b');
TEST_COMPARE (fgetwc (fp), L'c');
TEST_COMPARE (fgetwc (fp), L'\n');
TEST_COMPARE (fgetwc (fp), WEOF);
TEST_VERIFY_EXIT (feof (fp));
TEST_VERIFY_EXIT (!ferror (fp));
XWRITE (outer_fd, "d\n", "second line");
/* At this point, there is a new full line of input waiting in the
kernelside input buffer, but we should still observe EOF from
stdio, because the end-of-file indicator has not been cleared. */
TEST_COMPARE (fgetwc (fp), WEOF);
/* Clearing EOF should reveal the next line of input. */
clearerr (fp);
TEST_COMPARE (fgetwc (fp), L'd');
TEST_COMPARE (fgetwc (fp), L'\n');
fclose (fp);
close (outer_fd);
return 0;
}
#include <support/test-driver.c>