ln.c

linux开发技术详解一书的源码

/* `ln' program to create links between files.
   Copyright (C) 86, 89, 90, 91, 1995-2002 Free Software Foundation, Inc.

   This program 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 2, or (at your option)
   any later version.

   This program 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
   along with this program; if not, write to the Free Software Foundation,
   Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */
 

/* Written by Mike Parker and David MacKenzie. */

#ifdef _AIX
 #pragma alloca
#endif

#include <config.h>
#include <stdio.h>
#include <sys/types.h>
#include <getopt.h>

#include "system.h"
#include "same.h"
#include "backupfile.h"
#include "dirname.h"
#include "error.h"
#include "quote.h"

/* The official name of this program (e.g., no `g' prefix).  */
#define PROGRAM_NAME "ln"

#define AUTHORS N_ ("Mike Parker and David MacKenzie")

/* For long options that have no equivalent short option, use a
   non-character as a pseudo short option, starting with CHAR_MAX + 1.  */
enum
{
  TARGET_DIRECTORY_OPTION = CHAR_MAX + 1
};

int link ();			/* Some systems don't declare this anywhere. */

#ifdef S_ISLNK
int symlink ();
#endif

/* In being careful not even to try to make hard links to directories,
   we have to know whether link(2) follows symlinks.  If it does, then
   we have to *stat* the `source' to see if the resulting link would be
   to a directory.  Otherwise, we have to use *lstat* so that we allow
   users to make hard links to symlinks-that-point-to-directories.  */

#if LINK_FOLLOWS_SYMLINKS
# define STAT_LIKE_LINK(File, Stat_buf) \
  stat (File, Stat_buf)
#else
# define STAT_LIKE_LINK(File, Stat_buf) \
  lstat (File, Stat_buf)
#endif

/* Construct a string NEW_DEST by concatenating DEST, a slash, and
   basename(SOURCE) in alloca'd memory.  Don't modify DEST or SOURCE.  */

#define PATH_BASENAME_CONCAT(new_dest, dest, source)			\
    do									\
      {									\
	const char *source_base;					\
	char *tmp_source;						\
									\
	tmp_source = (char *) alloca (strlen ((source)) + 1);		\
	strcpy (tmp_source, (source));					\
	strip_trailing_slashes (tmp_source);				\
	source_base = base_name (tmp_source);				\
									\
	(new_dest) = (char *) alloca (strlen ((dest)) + 1		\
				      + strlen (source_base) + 1);	\
	stpcpy (stpcpy (stpcpy ((new_dest), (dest)), "/"), source_base);\
      }									\
    while (0)

int isdir ();
int yesno ();

/* The name by which the program was run, for error messages.  */
char *program_name;

/* FIXME: document */
enum backup_type backup_type;

/* A pointer to the function used to make links.  This will point to either
   `link' or `symlink'. */
static int (*linkfunc) ();

/* If nonzero, make symbolic links; otherwise, make hard links.  */
static int symbolic_link;

/* If nonzero, ask the user before removing existing files.  */
static int interactive;

/* If nonzero, remove existing files unconditionally.  */
static int remove_existing_files;

/* If nonzero, list each file as it is moved. */
static int verbose;

/* If nonzero, allow the superuser to make hard links to directories. */
static int hard_dir_link;

/* If nonzero, and the specified destination is a symbolic link to a
   directory, treat it just as if it were a directory.  Otherwise, the
   command `ln --force --no-dereference file symlink-to-dir' deletes
   symlink-to-dir before creating the new link.  */
static int dereference_dest_dir_symlinks = 1;

static struct option const long_options[] =
{
  {"backup", optional_argument, NULL, 'b'},
  {"directory", no_argument, NULL, 'F'},
  {"no-dereference", no_argument, NULL, 'n'},
  {"force", no_argument, NULL, 'f'},
  {"interactive", no_argument, NULL, 'i'},
  {"suffix", required_argument, NULL, 'S'},
  {"target-directory", required_argument, NULL, TARGET_DIRECTORY_OPTION},
  {"symbolic", no_argument, NULL, 's'},
  {"verbose", no_argument, NULL, 'v'},
  {"version-control", required_argument, NULL, 'V'}, /* Deprecated. FIXME. */
  {GETOPT_HELP_OPTION_DECL},
  {GETOPT_VERSION_OPTION_DECL},
  {NULL, 0, NULL, 0}
};

/* Make a link DEST to the (usually) existing file SOURCE.
   Symbolic links to nonexistent files are allowed.
   If DEST is a directory, put the link to SOURCE in that directory.
   Return 1 if there is an error, otherwise 0.  */

static int
do_link (const char *source, const char *dest)
{
  struct stat source_stats;
  struct stat dest_stats;
  char *dest_backup = NULL;
  int lstat_status;
  int backup_succeeded = 0;

  /* Use stat here instead of lstat.
     On SVR4, link does not follow symlinks, so this check disallows
     making hard links to symlinks that point to directories.  Big deal.
     On other systems, link follows symlinks, so this check is right.  */
  if (!symbolic_link)
    {
      if (STAT_LIKE_LINK (source, &source_stats) != 0)
	{
	  error (0, errno, _("accessing %s"), quote (source));
	  return 1;
	}

      if (S_ISLNK (source_stats.st_mode))
	{
	  error (0, 0, _("%s: warning: making a hard link to a symbolic link\
 is not portable"),
		 quote (source));
	}

      if (!hard_dir_link && S_ISDIR (source_stats.st_mode))
	{
	  error (0, 0, _("%s: hard link not allowed for directory"),
		 quote (source));
	  return 1;
	}
    }

  lstat_status = lstat (dest, &dest_stats);
  if (lstat_status != 0 && errno != ENOENT)
    {
      error (0, errno, _("accessing %s"), quote (dest));
      return 1;
    }

  /* If the destination is a directory or (it is a symlink to a directory
     and the user has not specified --no-dereference), then form the
     actual destination name by appending base_name (source) to the
     specified destination directory.  */
  if ((lstat_status == 0
       && S_ISDIR (dest_stats.st_mode))
#ifdef S_ISLNK
      || (dereference_dest_dir_symlinks
	  && (lstat_status == 0
	      && S_ISLNK (dest_stats.st_mode)
	      && isdir (dest)))
#endif
     )
    {
      /* Target is a directory; build the full filename. */
      char *new_dest;
      PATH_BASENAME_CONCAT (new_dest, dest, source);
      dest = new_dest;

      /* Get stats for new DEST.  */
      lstat_status = lstat (dest, &dest_stats);
      if (lstat_status != 0 && errno != ENOENT)
	{
	  error (0, errno, _("accessing %s"), quote (dest));
	  return 1;
	}
    }

  /* If --force (-f) has been specified without --backup, then before
     making a link ln must remove the destination file if it exists.
     (with --backup, it just renames any existing destination file)
     But if the source and destination are the same, don't remove
     anything and fail right here.  */
  if (remove_existing_files
      && lstat_status == 0
      /* Allow `ln -sf --backup k k' to succeed in creating the
	 self-referential symlink, but don't allow the hard-linking
	 equivalent: `ln -f k k' (with or without --backup) to get
	 beyond this point, because the error message you'd get is
	 misleading.  */
      && (backup_type == none || !symbolic_link)
      && (!symbolic_link || stat (source, &source_stats) == 0)
      && source_stats.st_dev == dest_stats.st_dev
      && source_stats.st_ino == dest_stats.st_ino
      /* The following detects whether removing DEST will also remove
 	 SOURCE.  If the file has only one link then both are surely
 	 the same link.  Otherwise check whether they point to the same
	 name in the same directory.  */
      && (source_stats.st_nlink == 1 || same_name (source, dest)))
    {
      error (0, 0, _("%s and %s are the same file"),
	     quote_n (0, source), quote_n (1, dest));
      return 1;
    }

  if (lstat_status == 0 || lstat (dest, &dest_stats) == 0)
    {
      if (S_ISDIR (dest_stats.st_mode))
	{
	  error (0, 0, _("%s: cannot overwrite directory"), quote (dest));
	  return 1;
	}
      if (interactive)
	{
	  fprintf (stderr, _("%s: replace %s? "), program_name, quote (dest));
	  if (!yesno ())
	    return 0;
	}
      else if (!remove_existing_files && backup_type == none)
	{
	  error (0, 0, _("%s: File exists"), quote (dest));
	  return 1;
	}

      if (backup_type != none)
	{
	  char *tmp_backup = find_backup_file_name (dest, backup_type);
	  if (tmp_backup == NULL)
	    xalloc_die ();
	  dest_backup = (char *) alloca (strlen (tmp_backup) + 1);
	  strcpy (dest_backup, tmp_backup);
	  free (tmp_backup);
	  if (rename (dest, dest_backup))
	    {
	      if (errno != ENOENT)
		{
		  error (0, errno, _("cannot backup %s"), quote (dest));
		  return 1;
		}
	      else
		dest_backup = NULL;
	    }
	  else
	    {
	      backup_succeeded = 1;
	    }
	}

      /* Try to unlink DEST even if we may have renamed it.  In some unusual
	 cases (when DEST and DEST_BACKUP are hard-links that refer to the
	 same file), rename succeeds and DEST remains.  If we didn't remove
	 DEST in that case, the subsequent LINKFUNC call would fail.  */