fileio.c 175 KB
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/* File IO for GNU Emacs.
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Copyright (C) 1985-1988, 1993-2011  Free Software Foundation, Inc.
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This file is part of GNU Emacs.

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GNU Emacs is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
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GNU Emacs 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.

You should have received a copy of the GNU General Public License
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along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
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#include <config.h>
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#include <limits.h>
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#include <fcntl.h>
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#include <stdio.h>
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#include <sys/types.h>
#include <sys/stat.h>
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#include <setjmp.h>
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#include <unistd.h>

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#ifdef HAVE_PWD_H
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#include <pwd.h>
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#endif

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#include <ctype.h>
#include <errno.h>

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#ifdef HAVE_LIBSELINUX
#include <selinux/selinux.h>
#include <selinux/context.h>
#endif

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#include "lisp.h"
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#include "intervals.h"
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#include "buffer.h"
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#include "character.h"
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#include "coding.h"
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#include "window.h"
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#include "blockinput.h"
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#include "frame.h"
#include "dispextern.h"
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#ifdef WINDOWSNT
#define NOMINMAX 1
#include <windows.h>
#include <fcntl.h>
#endif /* not WINDOWSNT */

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#ifdef MSDOS
#include "msdos.h"
#include <sys/param.h>
#include <fcntl.h>
#endif

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#ifdef DOS_NT
/* On Windows, drive letters must be alphabetic - on DOS, the Netware
   redirector allows the six letters between 'Z' and 'a' as well. */
#ifdef MSDOS
#define IS_DRIVE(x) ((x) >= 'A' && (x) <= 'z')
#endif
#ifdef WINDOWSNT
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#define IS_DRIVE(x) isalpha ((unsigned char) (x))
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#endif
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/* Need to lower-case the drive letter, or else expanded
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   filenames will sometimes compare unequal, because
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   `expand-file-name' doesn't always down-case the drive letter.  */
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#define DRIVE_LETTER(x) (tolower ((unsigned char) (x)))
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#endif

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#include "systime.h"
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#ifdef HPUX
#include <netio.h>
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#endif
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#include "commands.h"

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#ifndef FILE_SYSTEM_CASE
#define FILE_SYSTEM_CASE(filename)  (filename)
#endif

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/* Nonzero during writing of auto-save files */
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static int auto_saving;
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/* Set by auto_save_1 to mode of original file so Fwrite_region will create
   a new file with the same mode as the original */
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static int auto_save_mode_bits;
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/* Set by auto_save_1 if an error occurred during the last auto-save. */
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static int auto_save_error_occurred;
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/* The symbol bound to coding-system-for-read when
   insert-file-contents is called for recovering a file.  This is not
   an actual coding system name, but just an indicator to tell
   insert-file-contents to use `emacs-mule' with a special flag for
   auto saving and recovering a file.  */
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static Lisp_Object Qauto_save_coding;
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/* Property name of a file name handler,
   which gives a list of operations it handles..  */
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static Lisp_Object Qoperations;
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/* Lisp functions for translating file formats */
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static Lisp_Object Qformat_decode, Qformat_annotate_function;
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/* Lisp function for setting buffer-file-coding-system and the
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   multibyteness of the current buffer after inserting a file.  */
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static Lisp_Object Qafter_insert_file_set_coding;
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static Lisp_Object Qwrite_region_annotate_functions;
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/* Each time an annotation function changes the buffer, the new buffer
   is added here.  */
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static Lisp_Object Vwrite_region_annotation_buffers;
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#ifdef HAVE_FSYNC
#endif

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static Lisp_Object Qdelete_by_moving_to_trash;
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/* Lisp function for moving files to trash.  */
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static Lisp_Object Qmove_file_to_trash;
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/* Lisp function for recursively copying directories.  */
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static Lisp_Object Qcopy_directory;
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/* Lisp function for recursively deleting directories.  */
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static Lisp_Object Qdelete_directory;
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#ifdef WINDOWSNT
#endif

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Lisp_Object Qfile_error;
static Lisp_Object Qfile_already_exists, Qfile_date_error;
static Lisp_Object Qexcl;
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Lisp_Object Qfile_name_history;

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static Lisp_Object Qcar_less_than_car;
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static Lisp_Object Fmake_symbolic_link (Lisp_Object, Lisp_Object, Lisp_Object);
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static int a_write (int, Lisp_Object, EMACS_INT, EMACS_INT,
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                    Lisp_Object *, struct coding_system *);
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static int e_write (int, Lisp_Object, EMACS_INT, EMACS_INT,
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		    struct coding_system *);
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153

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void
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report_file_error (const char *string, Lisp_Object data)
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{
  Lisp_Object errstring;
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  int errorno = errno;
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  char *str;
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  synchronize_system_messages_locale ();
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  str = strerror (errorno);
  errstring = code_convert_string_norecord (make_unibyte_string (str,
								 strlen (str)),
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					    Vlocale_coding_system, 0);

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  while (1)
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    switch (errorno)
      {
      case EEXIST:
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	xsignal (Qfile_already_exists, Fcons (errstring, data));
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	break;
      default:
	/* System error messages are capitalized.  Downcase the initial
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	   unless it is followed by a slash.  (The slash case caters to
	   error messages that begin with "I/O" or, in German, "E/A".)  */
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	if (STRING_MULTIBYTE (errstring)
	    && ! EQ (Faref (errstring, make_number (1)), make_number ('/')))
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	  {
	    int c;

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	    str = SSDATA (errstring);
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	    c = STRING_CHAR ((unsigned char *) str);
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	    Faset (errstring, make_number (0), make_number (downcase (c)));
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	  }
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	xsignal (Qfile_error,
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		 Fcons (build_string (string), Fcons (errstring, data)));
      }
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}
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Lisp_Object
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close_file_unwind (Lisp_Object fd)
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{
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  emacs_close (XFASTINT (fd));
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  return Qnil;
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}
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/* Restore point, having saved it as a marker.  */

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Lisp_Object
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restore_point_unwind (Lisp_Object location)
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{
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  Fgoto_char (location);
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  Fset_marker (location, Qnil, Qnil);
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  return Qnil;
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}
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static Lisp_Object Qexpand_file_name;
static Lisp_Object Qsubstitute_in_file_name;
static Lisp_Object Qdirectory_file_name;
static Lisp_Object Qfile_name_directory;
static Lisp_Object Qfile_name_nondirectory;
static Lisp_Object Qunhandled_file_name_directory;
static Lisp_Object Qfile_name_as_directory;
static Lisp_Object Qcopy_file;
static Lisp_Object Qmake_directory_internal;
static Lisp_Object Qmake_directory;
static Lisp_Object Qdelete_directory_internal;
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Lisp_Object Qdelete_file;
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static Lisp_Object Qrename_file;
static Lisp_Object Qadd_name_to_file;
static Lisp_Object Qmake_symbolic_link;
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Lisp_Object Qfile_exists_p;
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static Lisp_Object Qfile_executable_p;
static Lisp_Object Qfile_readable_p;
static Lisp_Object Qfile_writable_p;
static Lisp_Object Qfile_symlink_p;
static Lisp_Object Qaccess_file;
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Lisp_Object Qfile_directory_p;
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static Lisp_Object Qfile_regular_p;
static Lisp_Object Qfile_accessible_directory_p;
static Lisp_Object Qfile_modes;
static Lisp_Object Qset_file_modes;
static Lisp_Object Qset_file_times;
static Lisp_Object Qfile_selinux_context;
static Lisp_Object Qset_file_selinux_context;
static Lisp_Object Qfile_newer_than_file_p;
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Lisp_Object Qinsert_file_contents;
Lisp_Object Qwrite_region;
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static Lisp_Object Qverify_visited_file_modtime;
static Lisp_Object Qset_visited_file_modtime;
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DEFUN ("find-file-name-handler", Ffind_file_name_handler,
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       Sfind_file_name_handler, 2, 2, 0,
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       doc: /* Return FILENAME's handler function for OPERATION, if it has one.
Otherwise, return nil.
A file name is handled if one of the regular expressions in
`file-name-handler-alist' matches it.

If OPERATION equals `inhibit-file-name-operation', then we ignore
any handlers that are members of `inhibit-file-name-handlers',
but we still do run any other handlers.  This lets handlers
use the standard functions without calling themselves recursively.  */)
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  (Lisp_Object filename, Lisp_Object operation)
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{
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  /* This function must not munge the match data.  */
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  Lisp_Object chain, inhibited_handlers, result;
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  int pos = -1;
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  result = Qnil;
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  CHECK_STRING (filename);
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  if (EQ (operation, Vinhibit_file_name_operation))
    inhibited_handlers = Vinhibit_file_name_handlers;
  else
    inhibited_handlers = Qnil;
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  for (chain = Vfile_name_handler_alist; CONSP (chain);
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       chain = XCDR (chain))
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    {
      Lisp_Object elt;
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      elt = XCAR (chain);
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      if (CONSP (elt))
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	{
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	  Lisp_Object string = XCAR (elt);
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	  EMACS_INT match_pos;
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	  Lisp_Object handler = XCDR (elt);
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	  Lisp_Object operations = Qnil;

	  if (SYMBOLP (handler))
	    operations = Fget (handler, Qoperations);
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	  if (STRINGP (string)
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	      && (match_pos = fast_string_match (string, filename)) > pos
	      && (NILP (operations) || ! NILP (Fmemq (operation, operations))))
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	    {
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	      Lisp_Object tem;
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	      handler = XCDR (elt);
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	      tem = Fmemq (handler, inhibited_handlers);
	      if (NILP (tem))
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		{
		  result = handler;
		  pos = match_pos;
		}
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	    }
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	}
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      QUIT;
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    }
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  return result;
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}

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DEFUN ("file-name-directory", Ffile_name_directory, Sfile_name_directory,
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       1, 1, 0,
       doc: /* Return the directory component in file name FILENAME.
Return nil if FILENAME does not include a directory.
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Otherwise return a directory name.
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Given a Unix syntax file name, returns a string ending in slash.  */)
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  (Lisp_Object filename)
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{
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#ifndef DOS_NT
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  register const char *beg;
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#else
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  register char *beg;
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#endif
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  register const char *p;
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  Lisp_Object handler;
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  CHECK_STRING (filename);
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  /* If the file name has special constructs in it,
     call the corresponding file handler.  */
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  handler = Ffind_file_name_handler (filename, Qfile_name_directory);
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  if (!NILP (handler))
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    return call2 (handler, Qfile_name_directory, filename);
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  filename = FILE_SYSTEM_CASE (filename);
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#ifdef DOS_NT
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  beg = (char *) alloca (SBYTES (filename) + 1);
  memcpy (beg, SSDATA (filename), SBYTES (filename) + 1);
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#else
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  beg = SSDATA (filename);
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#endif
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  p = beg + SBYTES (filename);
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  while (p != beg && !IS_DIRECTORY_SEP (p[-1])
#ifdef DOS_NT
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	 /* only recognize drive specifier at the beginning */
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	 && !(p[-1] == ':'
	      /* handle the "/:d:foo" and "/:foo" cases correctly  */
	      && ((p == beg + 2 && !IS_DIRECTORY_SEP (*beg))
		  || (p == beg + 4 && IS_DIRECTORY_SEP (*beg))))
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#endif
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	 ) p--;

  if (p == beg)
    return Qnil;
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#ifdef DOS_NT
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  /* Expansion of "c:" to drive and default directory.  */
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  if (p[-1] == ':')
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    {
      /* MAXPATHLEN+1 is guaranteed to be enough space for getdefdir.  */
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      char *res = alloca (MAXPATHLEN + 1);
      char *r = res;
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      if (p == beg + 4 && IS_DIRECTORY_SEP (*beg) && beg[1] == ':')
	{
	  strncpy (res, beg, 2);
	  beg += 2;
	  r += 2;
	}

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      if (getdefdir (toupper ((unsigned char) *beg) - 'A' + 1, r))
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	{
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	  if (!IS_DIRECTORY_SEP (res[strlen (res) - 1]))
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	    strcat (res, "/");
	  beg = res;
	  p = beg + strlen (beg);
	}
    }
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  dostounix_filename (beg);
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#endif /* DOS_NT */
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  return make_specified_string (beg, -1, p - beg, STRING_MULTIBYTE (filename));
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}

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DEFUN ("file-name-nondirectory", Ffile_name_nondirectory,
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       Sfile_name_nondirectory, 1, 1, 0,
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       doc: /* Return file name FILENAME sans its directory.
For example, in a Unix-syntax file name,
this is everything after the last slash,
or the entire name if it contains no slash.  */)
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  (Lisp_Object filename)
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{
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  register const char *beg, *p, *end;
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  Lisp_Object handler;
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  CHECK_STRING (filename);
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  /* If the file name has special constructs in it,
     call the corresponding file handler.  */
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  handler = Ffind_file_name_handler (filename, Qfile_name_nondirectory);
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  if (!NILP (handler))
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    return call2 (handler, Qfile_name_nondirectory, filename);
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  beg = SSDATA (filename);
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  end = p = beg + SBYTES (filename);
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  while (p != beg && !IS_DIRECTORY_SEP (p[-1])
#ifdef DOS_NT
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	 /* only recognize drive specifier at beginning */
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	 && !(p[-1] == ':'
	      /* handle the "/:d:foo" case correctly  */
	      && (p == beg + 2 || (p == beg + 4 && IS_DIRECTORY_SEP (*beg))))
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#endif
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	 )
    p--;
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  return make_specified_string (p, -1, end - p, STRING_MULTIBYTE (filename));
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}
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DEFUN ("unhandled-file-name-directory", Funhandled_file_name_directory,
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       Sunhandled_file_name_directory, 1, 1, 0,
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       doc: /* Return a directly usable directory name somehow associated with FILENAME.
A `directly usable' directory name is one that may be used without the
intervention of any file handler.
If FILENAME is a directly usable file itself, return
\(file-name-directory FILENAME).
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If FILENAME refers to a file which is not accessible from a local process,
then this should return nil.
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The `call-process' and `start-process' functions use this function to
get a current directory to run processes in.  */)
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  (Lisp_Object filename)
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{
  Lisp_Object handler;

  /* If the file name has special constructs in it,
     call the corresponding file handler.  */
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  handler = Ffind_file_name_handler (filename, Qunhandled_file_name_directory);
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  if (!NILP (handler))
    return call2 (handler, Qunhandled_file_name_directory, filename);

  return Ffile_name_directory (filename);
}

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439

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static char *
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file_name_as_directory (char *out, const char *in)
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{
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  ptrdiff_t len = strlen (in);
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  if (len == 0)
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    {
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      out[0] = '.';
      out[1] = '/';
      out[2] = 0;
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      return out;
    }

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  strcpy (out, in);

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  /* For Unix syntax, Append a slash if necessary */
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  if (!IS_DIRECTORY_SEP (out[len - 1]))
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    {
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      out[len] = DIRECTORY_SEP;
      out[len + 1] = '\0';
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    }
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#ifdef DOS_NT
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  dostounix_filename (out);
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#endif
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  return out;
}

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DEFUN ("file-name-as-directory", Ffile_name_as_directory,
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       Sfile_name_as_directory, 1, 1, 0,
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       doc: /* Return a string representing the file name FILE interpreted as a directory.
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This operation exists because a directory is also a file, but its name as
a directory is different from its name as a file.
The result can be used as the value of `default-directory'
or passed as second argument to `expand-file-name'.
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For a Unix-syntax file name, just appends a slash.  */)
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  (Lisp_Object file)
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{
  char *buf;
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  Lisp_Object handler;
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  CHECK_STRING (file);
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  if (NILP (file))
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    return Qnil;
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  /* If the file name has special constructs in it,
     call the corresponding file handler.  */
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  handler = Ffind_file_name_handler (file, Qfile_name_as_directory);
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  if (!NILP (handler))
    return call2 (handler, Qfile_name_as_directory, file);

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  buf = (char *) alloca (SBYTES (file) + 10);
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  file_name_as_directory (buf, SSDATA (file));
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  return make_specified_string (buf, -1, strlen (buf),
				STRING_MULTIBYTE (file));
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}

/*
 * Convert from directory name to filename.
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 * On UNIX, it's simple: just make sure there isn't a terminating /
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 * Value is nonzero if the string output is different from the input.
 */

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static int
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directory_file_name (char *src, char *dst)
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{
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  ptrdiff_t slen;
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  slen = strlen (src);
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  /* Process as Unix format: just remove any final slash.
     But leave "/" unchanged; do not change it to "".  */
  strcpy (dst, src);
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  if (slen > 1
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      && IS_DIRECTORY_SEP (dst[slen - 1])
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#ifdef DOS_NT
      && !IS_ANY_SEP (dst[slen - 2])
#endif
      )
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    dst[slen - 1] = 0;
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#ifdef DOS_NT
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  dostounix_filename (dst);
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#endif
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  return 1;
}

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DEFUN ("directory-file-name", Fdirectory_file_name, Sdirectory_file_name,
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       1, 1, 0,
       doc: /* Returns the file name of the directory named DIRECTORY.
This is the name of the file that holds the data for the directory DIRECTORY.
This operation exists because a directory is also a file, but its name as
a directory is different from its name as a file.
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In Unix-syntax, this function just removes the final slash.  */)
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  (Lisp_Object directory)
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{
  char *buf;
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  Lisp_Object handler;
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  CHECK_STRING (directory);
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  if (NILP (directory))
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    return Qnil;
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  /* If the file name has special constructs in it,
     call the corresponding file handler.  */
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  handler = Ffind_file_name_handler (directory, Qdirectory_file_name);
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  if (!NILP (handler))
    return call2 (handler, Qdirectory_file_name, directory);

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  buf = (char *) alloca (SBYTES (directory) + 20);
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  directory_file_name (SSDATA (directory), buf);
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  return make_specified_string (buf, -1, strlen (buf),
				STRING_MULTIBYTE (directory));
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}

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static const char make_temp_name_tbl[64] =
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{
  'A','B','C','D','E','F','G','H',
  'I','J','K','L','M','N','O','P',
  'Q','R','S','T','U','V','W','X',
  'Y','Z','a','b','c','d','e','f',
  'g','h','i','j','k','l','m','n',
  'o','p','q','r','s','t','u','v',
  'w','x','y','z','0','1','2','3',
  '4','5','6','7','8','9','-','_'
};
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static unsigned make_temp_name_count, make_temp_name_count_initialized_p;

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/* Value is a temporary file name starting with PREFIX, a string.
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   The Emacs process number forms part of the result, so there is
   no danger of generating a name being used by another process.
   In addition, this function makes an attempt to choose a name
   which has no existing file.  To make this work, PREFIX should be
   an absolute file name.
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   BASE64_P non-zero means add the pid as 3 characters in base64
   encoding.  In this case, 6 characters will be added to PREFIX to
   form the file name.  Otherwise, if Emacs is running on a system
   with long file names, add the pid as a decimal number.

   This function signals an error if no unique file name could be
   generated.  */

Lisp_Object
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make_temp_name (Lisp_Object prefix, int base64_p)
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{
  Lisp_Object val;
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  int len, clen;
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  printmax_t pid;
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  char *p, *data;
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  char pidbuf[INT_BUFSIZE_BOUND (printmax_t)];
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  int pidlen;
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  CHECK_STRING (prefix);
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  /* VAL is created by adding 6 characters to PREFIX.  The first
     three are the PID of this process, in base 64, and the second
     three are incremented if the file already exists.  This ensures
     262144 unique file names per PID per PREFIX.  */

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  pid = getpid ();
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  if (base64_p)
    {
      pidbuf[0] = make_temp_name_tbl[pid & 63], pid >>= 6;
      pidbuf[1] = make_temp_name_tbl[pid & 63], pid >>= 6;
      pidbuf[2] = make_temp_name_tbl[pid & 63], pid >>= 6;
      pidlen = 3;
    }
  else
    {
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#ifdef HAVE_LONG_FILE_NAMES
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      pidlen = sprintf (pidbuf, "%"pMd, pid);
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#else
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      pidbuf[0] = make_temp_name_tbl[pid & 63], pid >>= 6;
      pidbuf[1] = make_temp_name_tbl[pid & 63], pid >>= 6;
      pidbuf[2] = make_temp_name_tbl[pid & 63], pid >>= 6;
      pidlen = 3;
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#endif
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    }
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  len = SBYTES (prefix); clen = SCHARS (prefix);
  val = make_uninit_multibyte_string (clen + 3 + pidlen, len + 3 + pidlen);
  if (!STRING_MULTIBYTE (prefix))
    STRING_SET_UNIBYTE (val);
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  data = SSDATA (val);
  memcpy (data, SSDATA (prefix), len);
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  p = data + len;

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  memcpy (p, pidbuf, pidlen);
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  p += pidlen;

  /* Here we try to minimize useless stat'ing when this function is
     invoked many times successively with the same PREFIX.  We achieve
     this by initializing count to a random value, and incrementing it
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     afterwards.

     We don't want make-temp-name to be called while dumping,
     because then make_temp_name_count_initialized_p would get set
     and then make_temp_name_count would not be set when Emacs starts.  */

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  if (!make_temp_name_count_initialized_p)
    {
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      make_temp_name_count = time (NULL);
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      make_temp_name_count_initialized_p = 1;
    }

  while (1)
    {
      struct stat ignored;
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      unsigned num = make_temp_name_count;
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      p[0] = make_temp_name_tbl[num & 63], num >>= 6;
      p[1] = make_temp_name_tbl[num & 63], num >>= 6;
      p[2] = make_temp_name_tbl[num & 63], num >>= 6;

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      /* Poor man's congruential RN generator.  Replace with
         ++make_temp_name_count for debugging.  */
      make_temp_name_count += 25229;
      make_temp_name_count %= 225307;

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      if (stat (data, &ignored) < 0)
	{
	  /* We want to return only if errno is ENOENT.  */
	  if (errno == ENOENT)
	    return val;
	  else
	    /* The error here is dubious, but there is little else we
	       can do.  The alternatives are to return nil, which is
	       as bad as (and in many cases worse than) throwing the
	       error, or to ignore the error, which will likely result
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	       in looping through 225307 stat's, which is not only
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	       dog-slow, but also useless since eventually nil would
	       have to be returned anyway.  */
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	    report_file_error ("Cannot create temporary name for prefix",
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			       Fcons (prefix, Qnil));
	  /* not reached */
	}
    }
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}
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DEFUN ("make-temp-name", Fmake_temp_name, Smake_temp_name, 1, 1, 0,
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       doc: /* Generate temporary file name (string) starting with PREFIX (a string).
The Emacs process number forms part of the result,
so there is no danger of generating a name being used by another process.

In addition, this function makes an attempt to choose a name
which has no existing file.  To make this work,
PREFIX should be an absolute file name.

There is a race condition between calling `make-temp-name' and creating the
file which opens all kinds of security holes.  For that reason, you should
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probably use `make-temp-file' instead, except in three circumstances:

* If you are creating the file in the user's home directory.
* If you are creating a directory rather than an ordinary file.
* If you are taking special precautions as `make-temp-file' does.  */)
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  (Lisp_Object prefix)
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{
  return make_temp_name (prefix, 0);
}


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DEFUN ("expand-file-name", Fexpand_file_name, Sexpand_file_name, 1, 2, 0,
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       doc: /* Convert filename NAME to absolute, and canonicalize it.
Second arg DEFAULT-DIRECTORY is directory to start with if NAME is relative
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\(does not start with slash or tilde); if DEFAULT-DIRECTORY is nil or missing,
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the current buffer's value of `default-directory' is used.
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NAME should be a string that is a valid file name for the underlying
filesystem.
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File name components that are `.' are removed, and
so are file name components followed by `..', along with the `..' itself;
note that these simplifications are done without checking the resulting
file names in the file system.
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Multiple consecutive slashes are collapsed into a single slash,
except at the beginning of the file name when they are significant (e.g.,
UNC file names on MS-Windows.)
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An initial `~/' expands to your home directory.
An initial `~USER/' expands to USER's home directory.
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See also the function `substitute-in-file-name'.

For technical reasons, this function can return correct but
non-intuitive results for the root directory; for instance,
\(expand-file-name ".." "/") returns "/..".  For this reason, use
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\(directory-file-name (file-name-directory dirname)) to traverse a
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filesystem tree, not (expand-file-name ".."  dirname).  */)
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  (Lisp_Object name, Lisp_Object default_directory)
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{
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  /* These point to SDATA and need to be careful with string-relocation
     during GC (via DECODE_FILE).  */
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  char *nm;
  const char *newdir;
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  /* This should only point to alloca'd data.  */
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  char *target;
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  ptrdiff_t tlen;
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  struct passwd *pw;
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#ifdef DOS_NT
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  int drive = 0;
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  int collapse_newdir = 1;
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  int is_escaped = 0;
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#endif /* DOS_NT */
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  ptrdiff_t length;
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  Lisp_Object handler, result;
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  int multibyte;
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  Lisp_Object hdir;
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  CHECK_STRING (name);
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  /* If the file name has special constructs in it,
     call the corresponding file handler.  */
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  handler = Ffind_file_name_handler (name, Qexpand_file_name);
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  if (!NILP (handler))
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    return call3 (handler, Qexpand_file_name, name, default_directory);
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  /* Use the buffer's default-directory if DEFAULT_DIRECTORY is omitted.  */
  if (NILP (default_directory))
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    default_directory = BVAR (current_buffer, directory);
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  if (! STRINGP (default_directory))
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    {
#ifdef DOS_NT
      /* "/" is not considered a root directory on DOS_NT, so using "/"
	 here causes an infinite recursion in, e.g., the following:

            (let (default-directory)
	      (expand-file-name "a"))

	 To avoid this, we set default_directory to the root of the
	 current drive.  */
      default_directory = build_string (emacs_root_dir ());
#else
      default_directory = build_string ("/");
#endif
    }
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  if (!NILP (default_directory))
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    {
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      handler = Ffind_file_name_handler (default_directory, Qexpand_file_name);
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      if (!NILP (handler))
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	return call3 (handler, Qexpand_file_name, name, default_directory);
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    }
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  {
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    char *o = SSDATA (default_directory);
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    /* Make sure DEFAULT_DIRECTORY is properly expanded.
       It would be better to do this down below where we actually use
       default_directory.  Unfortunately, calling Fexpand_file_name recursively
       could invoke GC, and the strings might be relocated.  This would
       be annoying because we have pointers into strings lying around
       that would need adjusting, and people would add new pointers to
       the code and forget to adjust them, resulting in intermittent bugs.
       Putting this call here avoids all that crud.

       The EQ test avoids infinite recursion.  */
    if (! NILP (default_directory) && !EQ (default_directory, name)
	/* Save time in some common cases - as long as default_directory
	   is not relative, it can be canonicalized with name below (if it
	   is needed at all) without requiring it to be expanded now.  */
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#ifdef DOS_NT
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	/* Detect MSDOS file names with drive specifiers.  */
	&& ! (IS_DRIVE (o[0]) && IS_DEVICE_SEP (o[1])
	      && IS_DIRECTORY_SEP (o[2]))
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#ifdef WINDOWSNT
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	/* Detect Windows file names in UNC format.  */
	&& ! (IS_DIRECTORY_SEP (o[0]) && IS_DIRECTORY_SEP (o[1]))
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#endif
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#else /* not DOS_NT */
      /* Detect Unix absolute file names (/... alone is not absolute on
	 DOS or Windows).  */
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	&& ! (IS_DIRECTORY_SEP (o[0]))
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#endif /* not DOS_NT */
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	)
      {
	struct gcpro gcpro1;
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	GCPRO1 (name);
	default_directory = Fexpand_file_name (default_directory, Qnil);
	UNGCPRO;
      }
  }
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  name = FILE_SYSTEM_CASE (name);
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  multibyte = STRING_MULTIBYTE (name);
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