libio: Multiple fixes for open_{w}memstram (BZ#18241 and BZ#20181)

This patches fixes multiples issues on open_{w}memstream reported on both
BZ#18241 and BZ#20181:

  - failed fseek does not set errno.
  - negative offset in fseek fails even when resulting position is
    a valid one.
  - a flush after write if the current write position is not at the
    end of the stream currupt data.

The main fix is on seek operation for memstream (_IO_{w}str_seekoff), where
both _IO_read_ptr and _IO_read_end pointer are updated if a write operation
has occured (similar to default file operations).  Also, to calculate the
offset on both read and write pointers, a temporary value is instead of
updating the argument supplied value.  Negative offset are valid if resulting
internal pointer is within the range of _IO_{read,write}_base and
_IO_{read,write}_end.

Also POSIX states that a null or wide null shall be appended to the current
buffer iff a write moves the position to a value larger than the current
lenght.  Current implementation appends a null or wide null regardless
of this condition.  This patch fixes it by removing the 'else' condition
on _IO_{w}mem_sync.

Checked on x86_64.

	[BZ #18241]
	[BZ #20181]
	* libio/Makefile (test): Add tst-memstream3 and tst-wmemstream3.
	* libio/memstream.c (_IO_mem_sync): Only append a null byte if
	write position is at the end the buffer.
	* libio/wmemstream.c (_IO_wmem_sync): Likewise.
	* libio/strops.c (_IO_str_switch_to_get_mode): New function.
	(_IO_str_seekoff): Set correct offset from negative displacement and
	set EINVAL for invalid ones.
	* libio/wstrops.c (enlarge_userbuf): Use correct function to calculate
	buffer length.
	(_IO_wstr_switch_to_get_mode): New function.
	(_IO_wstr_seekoff): Set correct offset from negative displacement and
	set EINVAL for invalid ones.
	* libio/tst-memstream3.c: New file.
	* libio/tst-wmemstream3.c: Likewise.
	* manual/examples/memstrm.c: Remove warning when priting size_t.

1 files changed, 44 insertions, 0 deletions

diff --git a/libio/tst-wmemstream3.c b/libio/tst-wmemstream3.c
new file mode 100644
index 0000000..878db2d
--- /dev/null
+++ b/libio/tst-wmemstream3.c
@@ -0,0 +1,44 @@
+/* Test for open_memstream implementation.
+   Copyright (C) 2016 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 <wchar.h>
+
+/* Straighforward implementation so tst-memstream3 could use check
+   fwrite on open_memstream.  */
+static size_t
+fwwrite (const void *ptr, size_t size, size_t nmemb, FILE *arq)
+{
+  const wchar_t *wcs = (const wchar_t*) (ptr);
+  for (size_t s = 0; s < size; s++)
+    {
+      for (size_t n = 0; n < nmemb; n++)
+        if (fputwc (wcs[n], arq) == WEOF)
+          return n;
+    }
+  return size * nmemb;
+}
+
+#define CHAR_T wchar_t
+#define W(o) L##o
+#define OPEN_MEMSTREAM open_wmemstream
+#define PRINTF wprintf
+#define FWRITE fwwrite
+#define FPUTC  fputwc
+#define STRCMP wcscmp
+
+#include "tst-memstream3.c"