lib/unistdio/u-vsprintf.h

/* Formatted output to strings.
   Copyright (C) 1999, 2002, 2006-2026 Free Software Foundation, Inc.

   This file is free software.
   It is dual-licensed under "the GNU LGPLv3+ or the GNU GPLv2+".
   You can redistribute it and/or modify it under either
     - the terms of the GNU Lesser General Public License as published
       by the Free Software Foundation, either version 3, or (at your
       option) any later version, or
     - the terms of the GNU General Public License as published by the
       Free Software Foundation; either version 2, or (at your option)
       any later version, or
     - the same dual license "the GNU LGPLv3+ or the GNU GPLv2+".

   This file 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 and the GNU General Public License
   for more details.

   You should have received a copy of the GNU Lesser General Public
   License and of the GNU General Public License along with this
   program.  If not, see <https://www.gnu.org/licenses/>.  */

#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.  */

  /* Set length = min (SIZE_MAX, INT_MAX, - (uintptr_t) buf - 1).  */
  size_t length = (SIZE_MAX < INT_MAX ? SIZE_MAX : INT_MAX);
  if (length > (~ (uintptr_t) buf) / sizeof (DCHAR_T))
    length = (~ (uintptr_t) buf) / sizeof (DCHAR_T);

  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;
}