glibc/stdio/ftell.c

/* Copyright (C) 1991, 1992, 1994, 1996, 1997 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, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <errno.h>
#include <stdio.h>

/* Return the offset in bytes from the beginning
   of the file of the file position of STREAM.  */
long int
ftell (stream)
     FILE *stream;
{
  long int pos;

  if (!__validfp (stream))
    {
      __set_errno (EINVAL);
      return -1L;
    }

  if (__stdio_check_offset (stream) == EOF)
    return -1L;

  /* Start with the file position associated with the beginning
     of our buffer.  */
  pos = stream->__target;

  if (stream->__pushed_back)
    /* ungetc was just called, so our real buffer pointer is squirreled
       away in STREAM->__pushback_bufp, not in STREAM->__bufp as normal.
       Calling ungetc is supposed to decrement the file position.  ANSI
       says the file position is unspecified if you ungetc when the
       position is zero; -1 seems as good as anything to me.  */
    pos += stream->__pushback_bufp - stream->__buffer - 1;
  else
    pos += stream->__bufp - stream->__buffer;

  return pos;
}