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commit e8b9e065a1ae9d7d8b888909ae761cbd5cf7be32
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Author: Siddhesh Poyarekar <siddhesh@redhat.com>
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Date:   Wed Mar 19 00:42:30 2014 +0530
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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
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    is kept in sync with the external file offset.  This however is not
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    true when O_APPEND is in effect since switching to write mode ought to
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    send the external file offset to the end of file without making the
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    necessary adjustment to _IO_read_end.
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    Hence in append mode, offset computation when writing should only
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    consider the effect of unflushed writes, i.e. from _IO_write_base to
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    _IO_write_ptr.
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diff --git a/libio/Makefile b/libio/Makefile
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index 69c25c0..4bedfad 100644
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--- a/libio/Makefile
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+++ b/libio/Makefile
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@@ -60,7 +60,8 @@ tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc   \
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 	tst-wmemstream1 tst-wmemstream2 \
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 	bug-memstream1 bug-wmemstream1 \
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 	tst-setvbuf1 tst-popen1 tst-fgetwc bug-wsetpos tst-fseek \
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-	tst-fwrite-error tst-ftell-active-handler
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+	tst-fwrite-error tst-ftell-active-handler \
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+	tst-ftell-append
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 ifeq (yes,$(build-shared))
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 # Add test-fopenloc only if shared library is enabled since it depends on
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 # shared localedata objects.
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diff --git a/libio/fileops.c b/libio/fileops.c
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index cf68dbf..204cfea 100644
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--- a/libio/fileops.c
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+++ b/libio/fileops.c
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@@ -91,7 +91,9 @@ extern struct __gconv_trans_data __libio_translit attribute_hidden;
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    The position in the buffer that corresponds to the position
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    in external file system is normally _IO_read_end, except in putback
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-   mode, when it is _IO_save_end.
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+   mode, when it is _IO_save_end and also when the file is in append mode,
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+   since switching from read to write mode automatically sends the position in
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+   the external file system to the end of file.
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    If the field _fb._offset is >= 0, it gives the offset in
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    the file as a whole corresponding to eGptr(). (?)
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@@ -966,6 +968,14 @@ do_ftell (_IO_FILE *fp)
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       /* Adjust for unflushed data.  */
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       if (!was_writing)
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 	offset -= fp->_IO_read_end - fp->_IO_read_ptr;
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+      /* We don't trust _IO_read_end to represent the current file offset when
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+	 writing in append mode because the value would have to be shifted to
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+	 the end of the file during a flush.  Use the write base instead, along
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+	 with the new offset we got above when we did a seek to the end of the
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+	 file.  */
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+      else if (append_mode)
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+	offset += fp->_IO_write_ptr - fp->_IO_write_base;
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+      /* For all other modes, _IO_read_end represents the file offset.  */
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       else
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 	offset += fp->_IO_write_ptr - fp->_IO_read_end;
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     }
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diff --git a/libio/tst-ftell-append.c b/libio/tst-ftell-append.c
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new file mode 100644
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index 0000000..604dc03
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--- /dev/null
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+++ b/libio/tst-ftell-append.c
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@@ -0,0 +1,169 @@
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+/* Verify that ftell returns the correct value after a read and a write on a
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+   file opened in a+ mode.
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+   Copyright (C) 2014 Free Software Foundation, Inc.
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+   This file is part of the GNU C Library.
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+
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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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+
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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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+
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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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+   <http://www.gnu.org/licenses/>.  */
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+
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+#include <stdio.h>
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+#include <stdlib.h>
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+#include <string.h>
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+#include <errno.h>
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+#include <unistd.h>
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+#include <locale.h>
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+#include <wchar.h>
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+
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+/* data points to either char_data or wide_data, depending on whether we're
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+   testing regular file mode or wide mode respectively.  Similarly,
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+   fputs_func points to either fputs or fputws.  data_len keeps track of the
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+   length of the current data and file_len maintains the current file
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+   length.  */
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+#define BUF_LEN 4
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+static void *buf;
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+static char char_buf[BUF_LEN];
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+static wchar_t wide_buf[BUF_LEN];
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+static const void *data;
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+static const char *char_data = "abcdefghijklmnopqrstuvwxyz";
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+static const wchar_t *wide_data = L"abcdefghijklmnopqrstuvwxyz";
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+static size_t data_len;
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+static size_t file_len;
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+
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+typedef int (*fputs_func_t) (const void *data, FILE *fp);
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+fputs_func_t fputs_func;
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+
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+typedef void *(*fgets_func_t) (void *s, int size, FILE *stream);
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+fgets_func_t fgets_func;
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+
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+static int do_test (void);
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+
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+#define TEST_FUNCTION do_test ()
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+#include "../test-skeleton.c"
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+
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+static FILE *
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+init_file (const char *filename)
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+{
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+  FILE *fp = fopen (filename, "w");
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+  if (fp == NULL)
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+    {
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+      printf ("fopen: %m\n");
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+      return NULL;
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+    }
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+
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+  int written = fputs_func (data, fp);
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+
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+  if (written == EOF)
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+    {
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+      printf ("fputs failed to write data\n");
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+      fclose (fp);
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+      return NULL;
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+    }
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+
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+  file_len = data_len;
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+
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+  fclose (fp);
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+
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+  fp = fopen (filename, "a+");
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+  if (fp == NULL)
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+    {
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+      printf ("fopen(a+): %m\n");
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+      return NULL;
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+    }
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+
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+  return fp;
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+}
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+
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+static int
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+do_one_test (const char *filename)
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+{
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+  FILE *fp = init_file (filename);
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+
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+  if (fp == NULL)
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+    return 1;
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+
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+  void *ret = fgets_func (buf, BUF_LEN, fp);
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+
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+  if (ret == NULL)
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+    {
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+      printf ("read failed: %m\n");
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+      fclose (fp);
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+      return 1;
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+    }
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+
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+  int written = fputs_func (data, fp);
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+
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+  if (written == EOF)
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+    {
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+      printf ("fputs failed to write data\n");
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+      fclose (fp);
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+      return 1;
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+    }
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+
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+  file_len += data_len;
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+
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+  long off = ftell (fp);
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+
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+  if (off != file_len)
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+    {
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+      printf ("Incorrect offset %ld, expected %zu\n", off, file_len);
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+      fclose (fp);
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+      return 1;
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+    }
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+  else
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+    printf ("Correct offset %ld after write.\n", off);
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+
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+  return 0;
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+}
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+
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+/* Run the tests for regular files and wide mode files.  */
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+static int
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+do_test (void)
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+{
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+  int ret = 0;
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+  char *filename;
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+  int fd = create_temp_file ("tst-ftell-append-tmp.", &filename);
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+
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+  if (fd == -1)
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+    {
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+      printf ("create_temp_file: %m\n");
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+      return 1;
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+    }
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+
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+  close (fd);
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+
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+  /* Tests for regular files.  */
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+  puts ("Regular mode:");
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+  fputs_func = (fputs_func_t) fputs;
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+  fgets_func = (fgets_func_t) fgets;
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+  data = char_data;
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+  buf = char_buf;
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+  data_len = strlen (char_data);
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+  ret |= do_one_test (filename);
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+
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+  /* Tests for wide files.  */
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+  puts ("Wide mode:");
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+  if (setlocale (LC_ALL, "en_US.UTF-8") == NULL)
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+    {
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+      printf ("Cannot set en_US.UTF-8 locale.\n");
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+      return 1;
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+    }
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+  fputs_func = (fputs_func_t) fputws;
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+  fgets_func = (fgets_func_t) fgetws;
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+  data = wide_data;
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+  buf = wide_buf;
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+  data_len = wcslen (wide_data);
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+  ret |= do_one_test (filename);
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+
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+  return ret;
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+}
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diff --git a/libio/wfileops.c b/libio/wfileops.c
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index 3199861..f123add 100644
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--- a/libio/wfileops.c
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+++ b/libio/wfileops.c
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@@ -713,9 +713,16 @@ do_ftell_wide (_IO_FILE *fp)
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 	      offset += outstop - out;
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 	    }
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-	  /* _IO_read_end coincides with fp._offset, so the actual file
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-	     position is fp._offset - (_IO_read_end - new_write_ptr).  */
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-	  offset -= fp->_IO_read_end - fp->_IO_write_ptr;
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+	  /* We don't trust _IO_read_end to represent the current file offset
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+	     when writing in append mode because the value would have to be
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+	     shifted to the end of the file during a flush.  Use the write base
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+	     instead, along with the new offset we got above when we did a seek
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+	     to the end of the file.  */
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+	  if (append_mode)
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+	    offset += fp->_IO_write_ptr - fp->_IO_write_base;
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+	  /* For all other modes, _IO_read_end represents the file offset.  */
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+	  else
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+	    offset += fp->_IO_write_ptr - fp->_IO_read_end;
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 	}
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     }
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