new file mode 100644
--- /dev/null
+++ b/lib/unistdio/u-vsprintf.h
@@ -0,0 +1,65 @@
+/* Formatted output to strings.
+   Copyright (C) 1999, 2002, 2006-2007 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify it
+   under the terms of the GNU Library General Public License as published
+   by the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program 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
+   Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public
+   License along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
+   USA.  */
+
+/* Some systems, like OSF/1 4.0 and Woe32, don't have EOVERFLOW.  */
+#ifndef EOVERFLOW
+# define EOVERFLOW E2BIG
+#endif
+
+#ifndef SIZE_MAX
+# define SIZE_MAX ((size_t) -1)
+#endif
+
+int
+VSPRINTF (DCHAR_T *buf, const FCHAR_T *format, va_list args)
+{
+  /* Pass an infinite length.  But note that *vasnprintf may fail if the buffer
+     argument is larger than INT_MAX (if that fits into a 'size_t' at all).
+     Also note that glibc's iconv fails with E2BIG when we pass a length that
+     is so large that buf + length wraps around, i.e.
+     (uintptr_t) (buf + length) < (uintptr_t) buf.  */
+  size_t length;
+  DCHAR_T *result;
+
+  /* Set length = min (SIZE_MAX, INT_MAX, - (uintptr_t) buf - 1).  */
+  length = (SIZE_MAX < INT_MAX ? SIZE_MAX : INT_MAX);
+  if (length > (~ (uintptr_t) buf) / sizeof (DCHAR_T))
+    length = (~ (uintptr_t) buf) / sizeof (DCHAR_T);
+
+  result = VASNPRINTF (buf, &length, format, args);
+  if (result == NULL)
+    return -1;
+
+  /* The infinite buffer size guarantees that the result is not malloc()ed.  */
+  if (result != buf)
+    {
+      /* length is near SIZE_MAX.  */
+      free (result);
+      errno = EOVERFLOW;
+      return -1;
+    }
+
+  if (length > INT_MAX)
+    {
+      errno = EOVERFLOW;
+      return -1;
+    }
+
+  /* Return the number of resulting units, excluding the trailing NUL.  */
+  return length;
+}