mbstowcs: Document, test, and fix null pointer dst semantics (Bug 25219)

The function mbstowcs, by an XSI extension to POSIX, accepts a null
pointer for the destination wchar_t array.  This API behaviour allows
you to use the function to compute the length of the required wchar_t
array i.e. does the conversion without storing it and returns the
number of wide characters required.

We remove the __write_only__ markup for the first argument because it
is not true since the destination may be a null pointer, and so the
length argument may not apply.  We remove the markup otherwise the new
test case cannot be compiled with -Werror=nonnull.

We add a new test case for mbstowcs which exercises the destination is
a null pointer behaviour which we have now explicitly documented.

The mbsrtowcs and mbsnrtowcs behave similarly, and mbsrtowcs is
documented as doing this in C11, even if the standard doesn't come out
and call out this specific use case.  We add one note to each of
mbsrtowcs and mbsnrtowcs to call out that they support a null pointer
for the destination.

The wcsrtombs function behaves similarly but in the other way around
and allows you to use a null destination pointer to compute how many
bytes you would need to convert the wide character input.  We document
this particular case also, but leave wcsnrtombs as a references to
wcsrtombs, so the reader must still read the details of the semantics
for wcsrtombs.

2 files changed, 51 insertions, 1 deletions

diff --git a/wcsmbs/Makefile b/wcsmbs/Makefile
index f02167fa58..e638e45522 100644
--- a/wcsmbs/Makefile
+++ b/wcsmbs/Makefile
@@ -52,7 +52,7 @@ tests := tst-wcstof wcsmbs-tst1 tst-wcsnlen tst-btowc tst-mbrtowc \
 	 tst-c16c32-1 wcsatcliff tst-wcstol-locale tst-wcstod-nan-locale \
 	 tst-wcstod-round test-char-types tst-fgetwc-after-eof \
 	 tst-wcstod-nan-sign tst-c16-surrogate tst-c32-state \
-	 $(addprefix test-,$(strop-tests))
+	 $(addprefix test-,$(strop-tests)) tst-mbstowcs
 
 include ../Rules
 
diff --git a/wcsmbs/tst-mbstowcs.c b/wcsmbs/tst-mbstowcs.c
new file mode 100644
index 0000000000..b48891553e
--- /dev/null
+++ b/wcsmbs/tst-mbstowcs.c
@@ -0,0 +1,50 @@
+/* Test basic mbstowcs including wstring == NULL (Bug 25219).
+   Copyright (C) 2020 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdlib.h>
+#include <string.h>
+#include <support/check.h>
+
+static int
+do_test (void)
+{
+  char string[] = { '1', '2', '3' , '4', '5', '\0' };
+  size_t len = strlen (string);
+  wchar_t wstring[] = { L'1', L'2', L'3', L'4', L'5', L'\0' };
+#define NUM_WCHAR 6
+  wchar_t wout[NUM_WCHAR];
+  size_t result;
+
+  /* The input ASCII string in the C/POSIX locale must convert
+     to the matching WSTRING.  */
+  result = mbstowcs (wout, string, NUM_WCHAR);
+  TEST_VERIFY (result == (NUM_WCHAR - 1));
+  TEST_COMPARE_BLOB (wstring, sizeof (wchar_t) * (NUM_WCHAR - 1),
+		     wout, sizeof (wchar_t) * result);
+
+  /* The input ASCII string in the C/POSIX locale must be the
+     same length when using mbstowcs to compute the length of
+     the string required in the conversion.  Using mbstowcs
+     in this way is an XSI extension to POSIX.  */
+  result = mbstowcs (NULL, string, len);
+  TEST_VERIFY (result == len);
+
+  return 0;
+}
+
+#include <support/test-driver.c>