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Fix offset computation for append+ mode on switching from read (BZ #16724)

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The offset computation in write mode uses the fact that _IO_read_end
is kept in sync with the external file offset.  This however is not
true when O_APPEND is in effect since switching to write mode ought to
send the external file offset to the end of file without making the
necessary adjustment to _IO_read_end.

Hence in append mode, offset computation when writing should only
consider the effect of unflushed writes, i.e. from _IO_write_base to
_IO_write_ptr.

The wiki has a detailed document that describes the rationale for
offsets returned by ftell in various conditions:

https://sourceware.org/glibc/wiki/File%20offsets%20in%20a%20stdio%20stream%20and%20ftell
This commit is contained in:
Siddhesh Poyarekar 2014-05-27 13:54:19 +05:30
parent bab900166e
commit 2482ae433a
6 changed files with 207 additions and 10 deletions
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9
ChangeLog
View file

@ -1,3 +1,12 @@
2014-05-27 Siddhesh Poyarekar <siddhesh@redhat.com>
[BZ #16724]
* libio/tst-ftell-append.c: New test case.
* libio/Makefile (tests): Add test case.
* libio/fileops.c (do_ftell): Don't trust _IO_read_end when in
append mode.
* libio/wfileops.c (do_ftell_wide): Likewise.
2014-05-26 Adhemerval Zanella <azanella@linux.vnet.ibm.com>
* sysdeps/powerpc/fpu/libm-test-ulps: Update.

10
NEWS
View file

@ -14,11 +14,11 @@ Version 2.20
16516, 16532, 16545, 16564, 16574, 16599, 16600, 16609, 16610, 16611,
16613, 16619, 16623, 16629, 16632, 16634, 16639, 16642, 16648, 16649,
16670, 16674, 16677, 16680, 16683, 16689, 16695, 16701, 16706, 16707,
16712, 16713, 16714, 16731, 16739, 16740, 16743, 16754, 16758, 16759,
16760, 16770, 16786, 16789, 16791, 16796, 16799, 16800, 16815, 16823,
16824, 16831, 16838, 16849, 16854, 16876, 16877, 16878, 16885, 16888,
16890, 16912, 16915, 16916, 16917, 16922, 16927, 16928, 16932, 16943,
16958, 16966, 16967, 16965, 16977, 16978, 16984.
16712, 16713, 16714, 16724, 16731, 16739, 16740, 16743, 16754, 16758,
16759, 16760, 16770, 16786, 16789, 16791, 16796, 16799, 16800, 16815,
16823, 16824, 16831, 16838, 16849, 16854, 16876, 16877, 16878, 16885,
16888, 16890, 16912, 16915, 16916, 16917, 16922, 16927, 16928, 16932,
16943, 16958, 16966, 16967, 16965, 16977, 16978, 16984.
* The minimum Linux kernel version that this version of the GNU C Library
can be used with is 2.6.32.

4
libio/Makefile
View file

@ -60,7 +60,8 @@ tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc \
tst-wmemstream1 tst-wmemstream2 \
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-fwrite-error tst-ftell-partial-wide tst-ftell-active-handler \
tst-ftell-append
ifeq (yes,$(build-shared))
# Add test-fopenloc only if shared library is enabled since it depends on
# shared localedata objects.
@ -160,6 +161,7 @@ tst-fgetwc-ENV = LOCPATH=$(common-objpfx)localedata
tst-fseek-ENV = LOCPATH=$(common-objpfx)localedata
tst-ftell-partial-wide-ENV = LOCPATH=$(common-objpfx)localedata
tst-ftell-active-handler-ENV = LOCPATH=$(common-objpfx)localedata
tst-ftell-append-ENV = LOCPATH=$(common-objpfx)localedata
generated += tst-fopenloc.mtrace tst-fopenloc.check

12
libio/fileops.c
View file

@ -91,7 +91,9 @@ extern struct __gconv_trans_data __libio_translit attribute_hidden;
The position in the buffer that corresponds to the position
in external file system is normally _IO_read_end, except in putback
mode, when it is _IO_save_end.
mode, when it is _IO_save_end and also when the file is in append mode,
since switching from read to write mode automatically sends the position in
the external file system to the end of file.
If the field _fb._offset is >= 0, it gives the offset in
the file as a whole corresponding to eGptr(). (?)
@ -966,6 +968,14 @@ do_ftell (_IO_FILE *fp)
/* Adjust for unflushed data. */
if (!was_writing)
offset -= fp->_IO_read_end - fp->_IO_read_ptr;
/* We don't trust _IO_read_end to represent the current file offset when
writing in append mode because the value would have to be shifted to
the end of the file during a flush. Use the write base instead, along
with the new offset we got above when we did a seek to the end of the
file. */
else if (append_mode)
offset += fp->_IO_write_ptr - fp->_IO_write_base;
/* For all other modes, _IO_read_end represents the file offset. */
else
offset += fp->_IO_write_ptr - fp->_IO_read_end;
}

169
libio/tst-ftell-append.c Normal file
View file

@ -0,0 +1,169 @@
/* Verify that ftell returns the correct value after a read and a write on a
file opened in a+ mode.
Copyright (C) 2014 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 <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <locale.h>
#include <wchar.h>
/* data points to either char_data or wide_data, depending on whether we're
testing regular file mode or wide mode respectively. Similarly,
fputs_func points to either fputs or fputws. data_len keeps track of the
length of the current data and file_len maintains the current file
length. */
#define BUF_LEN 4
static void *buf;
static char char_buf[BUF_LEN];
static wchar_t wide_buf[BUF_LEN];
static const void *data;
static const char *char_data = "abcdefghijklmnopqrstuvwxyz";
static const wchar_t *wide_data = L"abcdefghijklmnopqrstuvwxyz";
static size_t data_len;
static size_t file_len;
typedef int (*fputs_func_t) (const void *data, FILE *fp);
fputs_func_t fputs_func;
typedef void *(*fgets_func_t) (void *s, int size, FILE *stream);
fgets_func_t fgets_func;
static int do_test (void);
#define TEST_FUNCTION do_test ()
#include "../test-skeleton.c"
static FILE *
init_file (const char *filename)
{
FILE *fp = fopen (filename, "w");
if (fp == NULL)
{
printf ("fopen: %m\n");
return NULL;
}
int written = fputs_func (data, fp);
if (written == EOF)
{
printf ("fputs failed to write data\n");
fclose (fp);
return NULL;
}
file_len = data_len;
fclose (fp);
fp = fopen (filename, "a+");
if (fp == NULL)
{
printf ("fopen(a+): %m\n");
return NULL;
}
return fp;
}
static int
do_one_test (const char *filename)
{
FILE *fp = init_file (filename);
if (fp == NULL)
return 1;
void *ret = fgets_func (buf, BUF_LEN, fp);
if (ret == NULL)
{
printf ("read failed: %m\n");
fclose (fp);
return 1;
}
int written = fputs_func (data, fp);
if (written == EOF)
{
printf ("fputs failed to write data\n");
fclose (fp);
return 1;
}
file_len += data_len;
long off = ftell (fp);
if (off != file_len)
{
printf ("Incorrect offset %ld, expected %zu\n", off, file_len);
fclose (fp);
return 1;
}
else
printf ("Correct offset %ld after write.\n", off);
return 0;
}
/* Run the tests for regular files and wide mode files. */
static int
do_test (void)
{
int ret = 0;
char *filename;
int fd = create_temp_file ("tst-ftell-append-tmp.", &filename);
if (fd == -1)
{
printf ("create_temp_file: %m\n");
return 1;
}
close (fd);
/* Tests for regular files. */
puts ("Regular mode:");
fputs_func = (fputs_func_t) fputs;
fgets_func = (fgets_func_t) fgets;
data = char_data;
buf = char_buf;
data_len = strlen (char_data);
ret |= do_one_test (filename);
/* Tests for wide files. */
puts ("Wide mode:");
if (setlocale (LC_ALL, "en_US.UTF-8") == NULL)
{
printf ("Cannot set en_US.UTF-8 locale.\n");
return 1;
}
fputs_func = (fputs_func_t) fputws;
fgets_func = (fgets_func_t) fgetws;
data = wide_data;
buf = wide_buf;
data_len = wcslen (wide_data);
ret |= do_one_test (filename);
return ret;
}

13
libio/wfileops.c
View file

@ -713,9 +713,16 @@ do_ftell_wide (_IO_FILE *fp)
offset += outstop - out;
}
/* _IO_read_end coincides with fp._offset, so the actual file
position is fp._offset - (_IO_read_end - new_write_ptr). */
offset -= fp->_IO_read_end - fp->_IO_write_ptr;
/* We don't trust _IO_read_end to represent the current file offset
when writing in append mode because the value would have to be
shifted to the end of the file during a flush. Use the write base
instead, along with the new offset we got above when we did a seek
to the end of the file. */
if (append_mode)
offset += fp->_IO_write_ptr - fp->_IO_write_base;
/* For all other modes, _IO_read_end represents the file offset. */
else
offset += fp->_IO_write_ptr - fp->_IO_read_end;
}
}