man scanf:

​

       These  functions  return the number of input items successfully matched

   and assigned, which can be fewer than provided for, or even zero in the

   event of an early matching failure.

​

/* Copyright (C) 1991-2019 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 <stdarg.h>

#include <libio/strfile.h>

/* Read formatted input from S, according to the format string FORMAT.  */

int

__isoc99_sscanf (const char *s, const char *format, ...)

{

  va_list arg;

  int done;

  _IO_strfile sf;

  FILE *f = _IO_strfile_read (&sf, s);

  va_start (arg, format);

  done = __vfscanf_internal (f, format, arg, SCANF_ISOC99_A);

  va_end (arg);

  return done;

}

libc_hidden_def (__isoc99_sscanf)