<h2>Editing File: /usr/include/c++/8/bits/fstream.tcc</h2><form method='post'><textarea name='content' class='file-edit'>// File based streams -*- C++ -*-

// Copyright (C) 1997-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This 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 General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// &lt;http://www.gnu.org/licenses/&gt;.

/** @file bits/fstream.tcc
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{fstream}
 */

//
// ISO C++ 14882: 27.8  File-based streams
//

#ifndef _FSTREAM_TCC
#define _FSTREAM_TCC 1

#pragma GCC system_header

#include &lt;bits/cxxabi_forced.h&gt;
#include &lt;bits/move.h&gt;   // for swap

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  template&lt;typename _CharT, typename _Traits&gt;
    void
    basic_filebuf&lt;_CharT, _Traits&gt;::
    _M_allocate_internal_buffer()
    {
      // Allocate internal buffer only if one doesn't already exist
      // (either allocated or provided by the user via setbuf).
      if (!_M_buf_allocated &amp;&amp; !_M_buf)
	{
	  _M_buf = new char_type[_M_buf_size];
	  _M_buf_allocated = true;
	}
    }

  template&lt;typename _CharT, typename _Traits&gt;
    void
    basic_filebuf&lt;_CharT, _Traits&gt;::
    _M_destroy_internal_buffer() throw()
    {
      if (_M_buf_allocated)
	{
	  delete [] _M_buf;
	  _M_buf = 0;
	  _M_buf_allocated = false;
	}
      delete [] _M_ext_buf;
      _M_ext_buf = 0;
      _M_ext_buf_size = 0;
      _M_ext_next = 0;
      _M_ext_end = 0;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    basic_filebuf&lt;_CharT, _Traits&gt;::
    basic_filebuf() : __streambuf_type(), _M_lock(), _M_file(&amp;_M_lock),
    _M_mode(ios_base::openmode(0)), _M_state_beg(), _M_state_cur(),
    _M_state_last(), _M_buf(0), _M_buf_size(BUFSIZ),
    _M_buf_allocated(false), _M_reading(false), _M_writing(false), _M_pback(), 
    _M_pback_cur_save(0), _M_pback_end_save(0), _M_pback_init(false),
    _M_codecvt(0), _M_ext_buf(0), _M_ext_buf_size(0), _M_ext_next(0),
    _M_ext_end(0)
    {
      if (has_facet&lt;__codecvt_type&gt;(this-&gt;_M_buf_locale))
	_M_codecvt = &amp;use_facet&lt;__codecvt_type&gt;(this-&gt;_M_buf_locale);
    }

#if __cplusplus &gt;= 201103L
  template&lt;typename _CharT, typename _Traits&gt;
    basic_filebuf&lt;_CharT, _Traits&gt;::
    basic_filebuf(basic_filebuf&amp;&amp; __rhs)
    : __streambuf_type(__rhs),
    _M_lock(), _M_file(std::move(__rhs._M_file), &amp;_M_lock),
    _M_mode(std::__exchange(__rhs._M_mode, ios_base::openmode(0))),
    _M_state_beg(std::move(__rhs._M_state_beg)),
    _M_state_cur(std::move(__rhs._M_state_cur)),
    _M_state_last(std::move(__rhs._M_state_last)),
    _M_buf(std::__exchange(__rhs._M_buf, nullptr)),
    _M_buf_size(std::__exchange(__rhs._M_buf_size, 1)),
    _M_buf_allocated(std::__exchange(__rhs._M_buf_allocated, false)),
    _M_reading(std::__exchange(__rhs._M_reading, false)),
    _M_writing(std::__exchange(__rhs._M_writing, false)),
    _M_pback(__rhs._M_pback),
    _M_pback_cur_save(std::__exchange(__rhs._M_pback_cur_save, nullptr)),
    _M_pback_end_save(std::__exchange(__rhs._M_pback_end_save, nullptr)),
    _M_pback_init(std::__exchange(__rhs._M_pback_init, false)),
    _M_codecvt(__rhs._M_codecvt),
    _M_ext_buf(std::__exchange(__rhs._M_ext_buf, nullptr)),
    _M_ext_buf_size(std::__exchange(__rhs._M_ext_buf_size, 0)),
    _M_ext_next(std::__exchange(__rhs._M_ext_next, nullptr)),
    _M_ext_end(std::__exchange(__rhs._M_ext_end, nullptr))
    {
      __rhs._M_set_buffer(-1);
      __rhs._M_state_last = __rhs._M_state_cur = __rhs._M_state_beg;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    basic_filebuf&lt;_CharT, _Traits&gt;&amp;
    basic_filebuf&lt;_CharT, _Traits&gt;::
    operator=(basic_filebuf&amp;&amp; __rhs)
    {
      this-&gt;close();
      __streambuf_type::operator=(__rhs);
      _M_file.swap(__rhs._M_file);
      _M_mode = std::__exchange(__rhs._M_mode, ios_base::openmode(0));
      _M_state_beg = std::move(__rhs._M_state_beg);
      _M_state_cur = std::move(__rhs._M_state_cur);
      _M_state_last = std::move(__rhs._M_state_last);
      _M_buf = std::__exchange(__rhs._M_buf, nullptr);
      _M_buf_size = std::__exchange(__rhs._M_buf_size, 1);
      _M_buf_allocated = std::__exchange(__rhs._M_buf_allocated, false);
      _M_ext_buf = std::__exchange(__rhs._M_ext_buf, nullptr);
      _M_ext_buf_size = std::__exchange(__rhs._M_ext_buf_size, 0);
      _M_ext_next = std::__exchange(__rhs._M_ext_next, nullptr);
      _M_ext_end = std::__exchange(__rhs._M_ext_end, nullptr);
      _M_reading = std::__exchange(__rhs._M_reading, false);
      _M_writing = std::__exchange(__rhs._M_writing, false);
      _M_pback_cur_save = std::__exchange(__rhs._M_pback_cur_save, nullptr);
      _M_pback_end_save = std::__exchange(__rhs._M_pback_end_save, nullptr);
      _M_pback_init = std::__exchange(__rhs._M_pback_init, false);
      __rhs._M_set_buffer(-1);
      __rhs._M_state_last = __rhs._M_state_cur = __rhs._M_state_beg;
      return *this;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    void
    basic_filebuf&lt;_CharT, _Traits&gt;::
    swap(basic_filebuf&amp; __rhs)
    {
      __streambuf_type::swap(__rhs);
      _M_file.swap(__rhs._M_file);
      std::swap(_M_mode, __rhs._M_mode);
      std::swap(_M_state_beg, __rhs._M_state_beg);
      std::swap(_M_state_cur, __rhs._M_state_cur);
      std::swap(_M_state_last, __rhs._M_state_last);
      std::swap(_M_buf, __rhs._M_buf);
      std::swap(_M_buf_size, __rhs._M_buf_size);
      std::swap(_M_buf_allocated, __rhs._M_buf_allocated);
      std::swap(_M_ext_buf, __rhs._M_ext_buf);
      std::swap(_M_ext_buf_size, __rhs._M_ext_buf_size);
      std::swap(_M_ext_next, __rhs._M_ext_next);
      std::swap(_M_ext_end, __rhs._M_ext_end);
      std::swap(_M_reading, __rhs._M_reading);
      std::swap(_M_writing, __rhs._M_writing);
      std::swap(_M_pback_cur_save, __rhs._M_pback_cur_save);
      std::swap(_M_pback_end_save, __rhs._M_pback_end_save);
      std::swap(_M_pback_init, __rhs._M_pback_init);
    }
#endif

  template&lt;typename _CharT, typename _Traits&gt;
    typename basic_filebuf&lt;_CharT, _Traits&gt;::__filebuf_type*
    basic_filebuf&lt;_CharT, _Traits&gt;::
    open(const char* __s, ios_base::openmode __mode)
    {
      __filebuf_type *__ret = 0;
      if (!this-&gt;is_open())
	{
	  _M_file.open(__s, __mode);
	  if (this-&gt;is_open())
	    {
	      _M_allocate_internal_buffer();
	      _M_mode = __mode;

	      // Setup initial buffer to 'uncommitted' mode.
	      _M_reading = false;
	      _M_writing = false;
	      _M_set_buffer(-1);

	      // Reset to initial state.
	      _M_state_last = _M_state_cur = _M_state_beg;

	      // 27.8.1.3,4
	      if ((__mode &amp; ios_base::ate)
		  &amp;&amp; this-&gt;seekoff(0, ios_base::end, __mode)
		  == pos_type(off_type(-1)))
		this-&gt;close();
	      else
		__ret = this;
	    }
	}
      return __ret;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    typename basic_filebuf&lt;_CharT, _Traits&gt;::__filebuf_type*
    basic_filebuf&lt;_CharT, _Traits&gt;::
    close()
    {
      if (!this-&gt;is_open())
	return 0;

      bool __testfail = false;
      {
	// NB: Do this here so that re-opened filebufs will be cool...
	struct __close_sentry
	{
	  basic_filebuf *__fb;
	  __close_sentry (basic_filebuf *__fbi): __fb(__fbi) { }
	  ~__close_sentry ()
	  {
	    __fb-&gt;_M_mode = ios_base::openmode(0);
	    __fb-&gt;_M_pback_init = false;
	    __fb-&gt;_M_destroy_internal_buffer();
	    __fb-&gt;_M_reading = false;
	    __fb-&gt;_M_writing = false;
	    __fb-&gt;_M_set_buffer(-1);
	    __fb-&gt;_M_state_last = __fb-&gt;_M_state_cur = __fb-&gt;_M_state_beg;
	  }
	} __cs (this);

	__try
	  {
	    if (!_M_terminate_output())
	      __testfail = true;
	  }
	__catch(__cxxabiv1::__forced_unwind&amp;)
	  {
	    _M_file.close();
	    __throw_exception_again;
	  }
	__catch(...)
	  { __testfail = true; }
      }

      if (!_M_file.close())
	__testfail = true;

      if (__testfail)
	return 0;
      else
	return this;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    streamsize
    basic_filebuf&lt;_CharT, _Traits&gt;::
    showmanyc()
    {
      streamsize __ret = -1;
      const bool __testin = _M_mode &amp; ios_base::in;
      if (__testin &amp;&amp; this-&gt;is_open())
	{
	  // For a stateful encoding (-1) the pending sequence might be just
	  // shift and unshift prefixes with no actual character.
	  __ret = this-&gt;egptr() - this-&gt;gptr();

#if _GLIBCXX_HAVE_DOS_BASED_FILESYSTEM
	  // About this workaround, see libstdc++/20806.
	  const bool __testbinary = _M_mode &amp; ios_base::binary;
	  if (__check_facet(_M_codecvt).encoding() &gt;= 0
	      &amp;&amp; __testbinary)
#else
	  if (__check_facet(_M_codecvt).encoding() &gt;= 0)
#endif
	    __ret += _M_file.showmanyc() / _M_codecvt-&gt;max_length();
	}
      return __ret;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    typename basic_filebuf&lt;_CharT, _Traits&gt;::int_type
    basic_filebuf&lt;_CharT, _Traits&gt;::
    underflow()
    {
      int_type __ret = traits_type::eof();
      const bool __testin = _M_mode &amp; ios_base::in;
      if (__testin)
	{
	  if (_M_writing)
	    {
	      if (overflow() == traits_type::eof())
		return __ret;
	      _M_set_buffer(-1);
	      _M_writing = false;
	    }
	  // Check for pback madness, and if so switch back to the
	  // normal buffers and jet outta here before expensive
	  // fileops happen...
	  _M_destroy_pback();

	  if (this-&gt;gptr() &lt; this-&gt;egptr())
	    return traits_type::to_int_type(*this-&gt;gptr());

	  // Get and convert input sequence.
	  const size_t __buflen = _M_buf_size &gt; 1 ? _M_buf_size - 1 : 1;

	  // Will be set to true if ::read() returns 0 indicating EOF.
	  bool __got_eof = false;
	  // Number of internal characters produced.
	  streamsize __ilen = 0;
	  codecvt_base::result __r = codecvt_base::ok;
	  if (__check_facet(_M_codecvt).always_noconv())
	    {
	      __ilen = _M_file.xsgetn(reinterpret_cast&lt;char*&gt;(this-&gt;eback()),
				      __buflen);
	      if (__ilen == 0)
		__got_eof = true;
	    }
	  else
	    {
              // Worst-case number of external bytes.
	      // XXX Not done encoding() == -1.
	      const int __enc = _M_codecvt-&gt;encoding();
	      streamsize __blen; // Minimum buffer size.
	      streamsize __rlen; // Number of chars to read.
	      if (__enc &gt; 0)
		__blen = __rlen = __buflen * __enc;
	      else
		{
		  __blen = __buflen + _M_codecvt-&gt;max_length() - 1;
		  __rlen = __buflen;
		}
	      const streamsize __remainder = _M_ext_end - _M_ext_next;
	      __rlen = __rlen &gt; __remainder ? __rlen - __remainder : 0;

	      // An imbue in 'read' mode implies first converting the external
	      // chars already present.
	      if (_M_reading &amp;&amp; this-&gt;egptr() == this-&gt;eback() &amp;&amp; __remainder)
		__rlen = 0;

	      // Allocate buffer if necessary and move unconverted
	      // bytes to front.
	      if (_M_ext_buf_size &lt; __blen)
		{
		  char* __buf = new char[__blen];
		  if (__remainder)
		    __builtin_memcpy(__buf, _M_ext_next, __remainder);

		  delete [] _M_ext_buf;
		  _M_ext_buf = __buf;
		  _M_ext_buf_size = __blen;
		}
	      else if (__remainder)
		__builtin_memmove(_M_ext_buf, _M_ext_next, __remainder);

	      _M_ext_next = _M_ext_buf;
	      _M_ext_end = _M_ext_buf + __remainder;
	      _M_state_last = _M_state_cur;

	      do
		{
		  if (__rlen &gt; 0)
		    {
		      // Sanity check!
		      // This may fail if the return value of
		      // codecvt::max_length() is bogus.
		      if (_M_ext_end - _M_ext_buf + __rlen &gt; _M_ext_buf_size)
			{
			  __throw_ios_failure(__N(&quot;basic_filebuf::underflow &quot;
					      &quot;codecvt::max_length() &quot;
					      &quot;is not valid&quot;));
			}
		      streamsize __elen = _M_file.xsgetn(_M_ext_end, __rlen);
		      if (__elen == 0)
			__got_eof = true;
		      else if (__elen == -1)
			break;
		      _M_ext_end += __elen;
		    }

		  char_type* __iend = this-&gt;eback();
		  if (_M_ext_next &lt; _M_ext_end)
		    __r = _M_codecvt-&gt;in(_M_state_cur, _M_ext_next,
					 _M_ext_end, _M_ext_next,
					 this-&gt;eback(),
					 this-&gt;eback() + __buflen, __iend);
		  if (__r == codecvt_base::noconv)
		    {
		      size_t __avail = _M_ext_end - _M_ext_buf;
		      __ilen = std::min(__avail, __buflen);
		      traits_type::copy(this-&gt;eback(),
					reinterpret_cast&lt;char_type*&gt;
					(_M_ext_buf), __ilen);
		      _M_ext_next = _M_ext_buf + __ilen;
		    }
		  else
		    __ilen = __iend - this-&gt;eback();

		  // _M_codecvt-&gt;in may return error while __ilen &gt; 0: this is
		  // ok, and actually occurs in case of mixed encodings (e.g.,
		  // XML files).
		  if (__r == codecvt_base::error)
		    break;

		  __rlen = 1;
		}
	      while (__ilen == 0 &amp;&amp; !__got_eof);
	    }

	  if (__ilen &gt; 0)
	    {
	      _M_set_buffer(__ilen);
	      _M_reading = true;
	      __ret = traits_type::to_int_type(*this-&gt;gptr());
	    }
	  else if (__got_eof)
	    {
	      // If the actual end of file is reached, set 'uncommitted'
	      // mode, thus allowing an immediate write without an
	      // intervening seek.
	      _M_set_buffer(-1);
	      _M_reading = false;
	      // However, reaching it while looping on partial means that
	      // the file has got an incomplete character.
	      if (__r == codecvt_base::partial)
		__throw_ios_failure(__N(&quot;basic_filebuf::underflow &quot;
				    &quot;incomplete character in file&quot;));
	    }
	  else if (__r == codecvt_base::error)
	    __throw_ios_failure(__N(&quot;basic_filebuf::underflow &quot;
				&quot;invalid byte sequence in file&quot;));
	  else
	    __throw_ios_failure(__N(&quot;basic_filebuf::underflow &quot;
				&quot;error reading the file&quot;));
	}
      return __ret;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    typename basic_filebuf&lt;_CharT, _Traits&gt;::int_type
    basic_filebuf&lt;_CharT, _Traits&gt;::
    pbackfail(int_type __i)
    {
      int_type __ret = traits_type::eof();
      const bool __testin = _M_mode &amp; ios_base::in;
      if (__testin)
	{
	  if (_M_writing)
	    {
	      if (overflow() == traits_type::eof())
		return __ret;
	      _M_set_buffer(-1);
	      _M_writing = false;
	    }
	  // Remember whether the pback buffer is active, otherwise below
	  // we may try to store in it a second char (libstdc++/9761).
	  const bool __testpb = _M_pback_init;
	  const bool __testeof = traits_type::eq_int_type(__i, __ret);
	  int_type __tmp;
	  if (this-&gt;eback() &lt; this-&gt;gptr())
	    {
	      this-&gt;gbump(-1);
	      __tmp = traits_type::to_int_type(*this-&gt;gptr());
	    }
	  else if (this-&gt;seekoff(-1, ios_base::cur) != pos_type(off_type(-1)))
	    {
	      __tmp = this-&gt;underflow();
	      if (traits_type::eq_int_type(__tmp, __ret))
		return __ret;
	    }
	  else
	    {
	      // At the beginning of the buffer, need to make a
	      // putback position available.  But the seek may fail
	      // (f.i., at the beginning of a file, see
	      // libstdc++/9439) and in that case we return
	      // traits_type::eof().
	      return __ret;
	    }

	  // Try to put back __i into input sequence in one of three ways.
	  // Order these tests done in is unspecified by the standard.
	  if (!__testeof &amp;&amp; traits_type::eq_int_type(__i, __tmp))
	    __ret = __i;
	  else if (__testeof)
	    __ret = traits_type::not_eof(__i);
	  else if (!__testpb)
	    {
	      _M_create_pback();
	      _M_reading = true;
	      *this-&gt;gptr() = traits_type::to_char_type(__i);
	      __ret = __i;
	    }
	}
      return __ret;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    typename basic_filebuf&lt;_CharT, _Traits&gt;::int_type
    basic_filebuf&lt;_CharT, _Traits&gt;::
    overflow(int_type __c)
    {
      int_type __ret = traits_type::eof();
      const bool __testeof = traits_type::eq_int_type(__c, __ret);
      const bool __testout = (_M_mode &amp; ios_base::out
			      || _M_mode &amp; ios_base::app);
      if (__testout)
	{
          if (_M_reading)
            {
              _M_destroy_pback();
              const int __gptr_off = _M_get_ext_pos(_M_state_last);
              if (_M_seek(__gptr_off, ios_base::cur, _M_state_last)
                  == pos_type(off_type(-1)))
                return __ret;
            }
	  if (this-&gt;pbase() &lt; this-&gt;pptr())
	    {
	      // If appropriate, append the overflow char.
	      if (!__testeof)
		{
		  *this-&gt;pptr() = traits_type::to_char_type(__c);
		  this-&gt;pbump(1);
		}

	      // Convert pending sequence to external representation,
	      // and output.
	      if (_M_convert_to_external(this-&gt;pbase(),
					 this-&gt;pptr() - this-&gt;pbase()))
		{
		  _M_set_buffer(0);
		  __ret = traits_type::not_eof(__c);
		}
	    }
	  else if (_M_buf_size &gt; 1)
	    {
	      // Overflow in 'uncommitted' mode: set _M_writing, set
	      // the buffer to the initial 'write' mode, and put __c
	      // into the buffer.
	      _M_set_buffer(0);
	      _M_writing = true;
	      if (!__testeof)
		{
		  *this-&gt;pptr() = traits_type::to_char_type(__c);
		  this-&gt;pbump(1);
		}
	      __ret = traits_type::not_eof(__c);
	    }
	  else
	    {
	      // Unbuffered.
	      char_type __conv = traits_type::to_char_type(__c);
	      if (__testeof || _M_convert_to_external(&amp;__conv, 1))
		{
		  _M_writing = true;
		  __ret = traits_type::not_eof(__c);
		}
	    }
	}
      return __ret;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    bool
    basic_filebuf&lt;_CharT, _Traits&gt;::
    _M_convert_to_external(_CharT* __ibuf, streamsize __ilen)
    {
      // Sizes of external and pending output.
      streamsize __elen;
      streamsize __plen;
      if (__check_facet(_M_codecvt).always_noconv())
	{
	  __elen = _M_file.xsputn(reinterpret_cast&lt;char*&gt;(__ibuf), __ilen);
	  __plen = __ilen;
	}
      else
	{
	  // Worst-case number of external bytes needed.
	  // XXX Not done encoding() == -1.
	  streamsize __blen = __ilen * _M_codecvt-&gt;max_length();
	  char* __buf = static_cast&lt;char*&gt;(__builtin_alloca(__blen));

	  char* __bend;
	  const char_type* __iend;
	  codecvt_base::result __r;
	  __r = _M_codecvt-&gt;out(_M_state_cur, __ibuf, __ibuf + __ilen,
				__iend, __buf, __buf + __blen, __bend);

	  if (__r == codecvt_base::ok || __r == codecvt_base::partial)
	    __blen = __bend - __buf;
	  else if (__r == codecvt_base::noconv)
	    {
	      // Same as the always_noconv case above.
	      __buf = reinterpret_cast&lt;char*&gt;(__ibuf);
	      __blen = __ilen;
	    }
	  else
	    __throw_ios_failure(__N(&quot;basic_filebuf::_M_convert_to_external &quot;
				    &quot;conversion error&quot;));
  
	  __elen = _M_file.xsputn(__buf, __blen);
	  __plen = __blen;

	  // Try once more for partial conversions.
	  if (__r == codecvt_base::partial &amp;&amp; __elen == __plen)
	    {
	      const char_type* __iresume = __iend;
	      streamsize __rlen = this-&gt;pptr() - __iend;
	      __r = _M_codecvt-&gt;out(_M_state_cur, __iresume,
				    __iresume + __rlen, __iend, __buf,
				    __buf + __blen, __bend);
	      if (__r != codecvt_base::error)
		{
		  __rlen = __bend - __buf;
		  __elen = _M_file.xsputn(__buf, __rlen);
		  __plen = __rlen;
		}
	      else
		__throw_ios_failure(__N(&quot;basic_filebuf::_M_convert_to_external &quot;
					&quot;conversion error&quot;));
	    }
	}
      return __elen == __plen;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    streamsize
    basic_filebuf&lt;_CharT, _Traits&gt;::
    xsgetn(_CharT* __s, streamsize __n)
    {
      // Clear out pback buffer before going on to the real deal...
      streamsize __ret = 0;
      if (_M_pback_init)
	{
	  if (__n &gt; 0 &amp;&amp; this-&gt;gptr() == this-&gt;eback())
	    {
	      *__s++ = *this-&gt;gptr(); // emulate non-underflowing sbumpc
	      this-&gt;gbump(1);
	      __ret = 1;
	      --__n;
	    }
	  _M_destroy_pback();
	}
      else if (_M_writing)
	{
	  if (overflow() == traits_type::eof())
	    return __ret;
	  _M_set_buffer(-1);
	  _M_writing = false;
	}
 
      // Optimization in the always_noconv() case, to be generalized in the
      // future: when __n &gt; __buflen we read directly instead of using the
      // buffer repeatedly.
      const bool __testin = _M_mode &amp; ios_base::in;
      const streamsize __buflen = _M_buf_size &gt; 1 ? _M_buf_size - 1 : 1;
 
      if (__n &gt; __buflen &amp;&amp; __check_facet(_M_codecvt).always_noconv()
	  &amp;&amp; __testin)
	{
	  // First, copy the chars already present in the buffer.
	  const streamsize __avail = this-&gt;egptr() - this-&gt;gptr();
	  if (__avail != 0)
	    {
	      traits_type::copy(__s, this-&gt;gptr(), __avail);
	      __s += __avail;
	      this-&gt;setg(this-&gt;eback(), this-&gt;gptr() + __avail, this-&gt;egptr());
	      __ret += __avail;
	      __n -= __avail;
	    }
 
	  // Need to loop in case of short reads (relatively common
	  // with pipes).
	  streamsize __len;
	  for (;;)
	    {
	      __len = _M_file.xsgetn(reinterpret_cast&lt;char*&gt;(__s), __n);
	      if (__len == -1)
		__throw_ios_failure(__N(&quot;basic_filebuf::xsgetn &quot;
					&quot;error reading the file&quot;));
	      if (__len == 0)
		break;
 
	      __n -= __len;
	      __ret += __len;
	      if (__n == 0)
		break;

	      __s += __len;
	    }

	  if (__n == 0)
	    {
	      // Set _M_reading. Buffer is already in initial 'read' mode.
	      _M_reading = true;
	    }
	  else if (__len == 0)
	    {
	      // If end of file is reached, set 'uncommitted'
	      // mode, thus allowing an immediate write without
	      // an intervening seek.
	      _M_set_buffer(-1);
	      _M_reading = false;
	    }
	}
      else
	__ret += __streambuf_type::xsgetn(__s, __n);
 
      return __ret;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    streamsize
    basic_filebuf&lt;_CharT, _Traits&gt;::
    xsputn(const _CharT* __s, streamsize __n)
    {
      streamsize __ret = 0;
      // Optimization in the always_noconv() case, to be generalized in the
      // future: when __n is sufficiently large we write directly instead of
      // using the buffer.
      const bool __testout = (_M_mode &amp; ios_base::out
			      || _M_mode &amp; ios_base::app);
      if (__check_facet(_M_codecvt).always_noconv()
	  &amp;&amp; __testout &amp;&amp; !_M_reading)
	{
	  // Measurement would reveal the best choice.
	  const streamsize __chunk = 1ul &lt;&lt; 10;
	  streamsize __bufavail = this-&gt;epptr() - this-&gt;pptr();

	  // Don't mistake 'uncommitted' mode buffered with unbuffered.
	  if (!_M_writing &amp;&amp; _M_buf_size &gt; 1)
	    __bufavail = _M_buf_size - 1;

	  const streamsize __limit = std::min(__chunk, __bufavail);
	  if (__n &gt;= __limit)
	    {
	      const streamsize __buffill = this-&gt;pptr() - this-&gt;pbase();
	      const char* __buf = reinterpret_cast&lt;const char*&gt;(this-&gt;pbase());
	      __ret = _M_file.xsputn_2(__buf, __buffill,
				       reinterpret_cast&lt;const char*&gt;(__s),
				       __n);
	      if (__ret == __buffill + __n)
		{
		  _M_set_buffer(0);
		  _M_writing = true;
		}
	      if (__ret &gt; __buffill)
		__ret -= __buffill;
	      else
		__ret = 0;
	    }
	  else
	    __ret = __streambuf_type::xsputn(__s, __n);
	}
       else
	 __ret = __streambuf_type::xsputn(__s, __n);
       return __ret;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    typename basic_filebuf&lt;_CharT, _Traits&gt;::__streambuf_type*
    basic_filebuf&lt;_CharT, _Traits&gt;::
    setbuf(char_type* __s, streamsize __n)
    {
      if (!this-&gt;is_open())
	{
	  if (__s == 0 &amp;&amp; __n == 0)
	    _M_buf_size = 1;
	  else if (__s &amp;&amp; __n &gt; 0)
	    {
	      // This is implementation-defined behavior, and assumes that
	      // an external char_type array of length __n exists and has
	      // been pre-allocated. If this is not the case, things will
	      // quickly blow up. When __n &gt; 1, __n - 1 positions will be
	      // used for the get area, __n - 1 for the put area and 1
	      // position to host the overflow char of a full put area.
	      // When __n == 1, 1 position will be used for the get area
	      // and 0 for the put area, as in the unbuffered case above.
	      _M_buf = __s;
	      _M_buf_size = __n;
	    }
	}
      return this;
    }


  // According to 27.8.1.4 p11 - 13, seekoff should ignore the last
  // argument (of type openmode).
  template&lt;typename _CharT, typename _Traits&gt;
    typename basic_filebuf&lt;_CharT, _Traits&gt;::pos_type
    basic_filebuf&lt;_CharT, _Traits&gt;::
    seekoff(off_type __off, ios_base::seekdir __way, ios_base::openmode)
    {
      int __width = 0;
      if (_M_codecvt)
	__width = _M_codecvt-&gt;encoding();
      if (__width &lt; 0)
	__width = 0;

      pos_type __ret = pos_type(off_type(-1));
      const bool __testfail = __off != 0 &amp;&amp; __width &lt;= 0;
      if (this-&gt;is_open() &amp;&amp; !__testfail)
	{
	  // tellg and tellp queries do not affect any state, unless
	  // ! always_noconv and the put sequence is not empty.
	  // In that case, determining the position requires converting the
	  // put sequence. That doesn't use ext_buf, so requires a flush.
	  bool __no_movement = __way == ios_base::cur &amp;&amp; __off == 0
	    &amp;&amp; (!_M_writing || _M_codecvt-&gt;always_noconv());

	  // Ditch any pback buffers to avoid confusion.
	  if (!__no_movement)
	    _M_destroy_pback();

	  // Correct state at destination. Note that this is the correct
	  // state for the current position during output, because
	  // codecvt::unshift() returns the state to the initial state.
	  // This is also the correct state at the end of the file because
	  // an unshift sequence should have been written at the end.
	  __state_type __state = _M_state_beg;
	  off_type __computed_off = __off * __width;
	  if (_M_reading &amp;&amp; __way == ios_base::cur)
	    {
	      __state = _M_state_last;
	      __computed_off += _M_get_ext_pos(__state);
	    }
	  if (!__no_movement)
	    __ret = _M_seek(__computed_off, __way, __state);
	  else
	    {
	      if (_M_writing)
		__computed_off = this-&gt;pptr() - this-&gt;pbase();
	      
	      off_type __file_off = _M_file.seekoff(0, ios_base::cur);
	      if (__file_off != off_type(-1))
		{
		  __ret = __file_off + __computed_off;
		  __ret.state(__state);
		}
	    }
	}
      return __ret;
    }

  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 171. Strange seekpos() semantics due to joint position
  // According to the resolution of DR 171, seekpos should ignore the last
  // argument (of type openmode).
  template&lt;typename _CharT, typename _Traits&gt;
    typename basic_filebuf&lt;_CharT, _Traits&gt;::pos_type
    basic_filebuf&lt;_CharT, _Traits&gt;::
    seekpos(pos_type __pos, ios_base::openmode)
    {
      pos_type __ret =  pos_type(off_type(-1));
      if (this-&gt;is_open())
	{
	  // Ditch any pback buffers to avoid confusion.
	  _M_destroy_pback();
	  __ret = _M_seek(off_type(__pos), ios_base::beg, __pos.state());
	}
      return __ret;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    typename basic_filebuf&lt;_CharT, _Traits&gt;::pos_type
    basic_filebuf&lt;_CharT, _Traits&gt;::
    _M_seek(off_type __off, ios_base::seekdir __way, __state_type __state)
    {
      pos_type __ret = pos_type(off_type(-1));
      if (_M_terminate_output())
	{
	  off_type __file_off = _M_file.seekoff(__off, __way);
	  if (__file_off != off_type(-1))
	    {
	      _M_reading = false;
	      _M_writing = false;
	      _M_ext_next = _M_ext_end = _M_ext_buf;
	      _M_set_buffer(-1);
	      _M_state_cur = __state;
	      __ret = __file_off;
	      __ret.state(_M_state_cur);
	    }
	}
      return __ret;
    }

  // Returns the distance from the end of the ext buffer to the point
  // corresponding to gptr(). This is a negative value. Updates __state
  // from eback() correspondence to gptr().
  template&lt;typename _CharT, typename _Traits&gt;
    int basic_filebuf&lt;_CharT, _Traits&gt;::
    _M_get_ext_pos(__state_type&amp; __state)
    {
      if (_M_codecvt-&gt;always_noconv())
        return this-&gt;gptr() - this-&gt;egptr();
      else
        {
          // Calculate offset from _M_ext_buf that corresponds to
          // gptr(). Precondition: __state == _M_state_last, which
          // corresponds to eback().
          const int __gptr_off =
            _M_codecvt-&gt;length(__state, _M_ext_buf, _M_ext_next,
                               this-&gt;gptr() - this-&gt;eback());
          return _M_ext_buf + __gptr_off - _M_ext_end;
        }
    }
    
  template&lt;typename _CharT, typename _Traits&gt;
    bool
    basic_filebuf&lt;_CharT, _Traits&gt;::
    _M_terminate_output()
    {
      // Part one: update the output sequence.
      bool __testvalid = true;
      if (this-&gt;pbase() &lt; this-&gt;pptr())
	{
	  const int_type __tmp = this-&gt;overflow();
	  if (traits_type::eq_int_type(__tmp, traits_type::eof()))
	    __testvalid = false;
	}

      // Part two: output unshift sequence.
      if (_M_writing &amp;&amp; !__check_facet(_M_codecvt).always_noconv()
	  &amp;&amp; __testvalid)
	{
	  // Note: this value is arbitrary, since there is no way to
	  // get the length of the unshift sequence from codecvt,
	  // without calling unshift.
	  const size_t __blen = 128;
	  char __buf[__blen];
	  codecvt_base::result __r;
	  streamsize __ilen = 0;

	  do
	    {
	      char* __next;
	      __r = _M_codecvt-&gt;unshift(_M_state_cur, __buf,
					__buf + __blen, __next);
	      if (__r == codecvt_base::error)
		__testvalid = false;
	      else if (__r == codecvt_base::ok ||
		       __r == codecvt_base::partial)
		{
		  __ilen = __next - __buf;
		  if (__ilen &gt; 0)
		    {
		      const streamsize __elen = _M_file.xsputn(__buf, __ilen);
		      if (__elen != __ilen)
			__testvalid = false;
		    }
		}
	    }
	  while (__r == codecvt_base::partial &amp;&amp; __ilen &gt; 0 &amp;&amp; __testvalid);

	  if (__testvalid)
	    {
	      // This second call to overflow() is required by the standard,
	      // but it's not clear why it's needed, since the output buffer
	      // should be empty by this point (it should have been emptied
	      // in the first call to overflow()).
	      const int_type __tmp = this-&gt;overflow();
	      if (traits_type::eq_int_type(__tmp, traits_type::eof()))
		__testvalid = false;
	    }
	}
      return __testvalid;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    int
    basic_filebuf&lt;_CharT, _Traits&gt;::
    sync()
    {
      // Make sure that the internal buffer resyncs its idea of
      // the file position with the external file.
      int __ret = 0;
      if (this-&gt;pbase() &lt; this-&gt;pptr())
	{
	  const int_type __tmp = this-&gt;overflow();
	  if (traits_type::eq_int_type(__tmp, traits_type::eof()))
	    __ret = -1;
	}
      return __ret;
    }

  template&lt;typename _CharT, typename _Traits&gt;
    void
    basic_filebuf&lt;_CharT, _Traits&gt;::
    imbue(const locale&amp; __loc)
    {
      bool __testvalid = true;

      const __codecvt_type* _M_codecvt_tmp = 0;
      if (__builtin_expect(has_facet&lt;__codecvt_type&gt;(__loc), true))
	_M_codecvt_tmp = &amp;use_facet&lt;__codecvt_type&gt;(__loc);

      if (this-&gt;is_open())
	{
	  // encoding() == -1 is ok only at the beginning.
	  if ((_M_reading || _M_writing)
	      &amp;&amp; __check_facet(_M_codecvt).encoding() == -1)
	    __testvalid = false;
	  else
	    {
	      if (_M_reading)
		{
		  if (__check_facet(_M_codecvt).always_noconv())
		    {
		      if (_M_codecvt_tmp
			  &amp;&amp; !__check_facet(_M_codecvt_tmp).always_noconv())
			__testvalid = this-&gt;seekoff(0, ios_base::cur, _M_mode)
			              != pos_type(off_type(-1));
		    }
		  else
		    {
		      // External position corresponding to gptr().
		      _M_ext_next = _M_ext_buf
			+ _M_codecvt-&gt;length(_M_state_last, _M_ext_buf,
					     _M_ext_next,
					     this-&gt;gptr() - this-&gt;eback());
		      const streamsize __remainder = _M_ext_end - _M_ext_next;
		      if (__remainder)
			__builtin_memmove(_M_ext_buf, _M_ext_next, __remainder);

		      _M_ext_next = _M_ext_buf;
		      _M_ext_end = _M_ext_buf + __remainder;
		      _M_set_buffer(-1);
		      _M_state_last = _M_state_cur = _M_state_beg;
		    }
		}
	      else if (_M_writing &amp;&amp; (__testvalid = _M_terminate_output()))
		_M_set_buffer(-1);
	    }
	}

      if (__testvalid)
	_M_codecvt = _M_codecvt_tmp;
      else
	_M_codecvt = 0;
    }

  // Inhibit implicit instantiations for required instantiations,
  // which are defined via explicit instantiations elsewhere.
#if _GLIBCXX_EXTERN_TEMPLATE
  extern template class basic_filebuf&lt;char&gt;;
  extern template class basic_ifstream&lt;char&gt;;
  extern template class basic_ofstream&lt;char&gt;;
  extern template class basic_fstream&lt;char&gt;;

#ifdef _GLIBCXX_USE_WCHAR_T
  extern template class basic_filebuf&lt;wchar_t&gt;;
  extern template class basic_ifstream&lt;wchar_t&gt;;
  extern template class basic_ofstream&lt;wchar_t&gt;;
  extern template class basic_fstream&lt;wchar_t&gt;;
#endif
#endif

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif
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