src/filelock.c

   1 /* Lock files for editing.
   2    Copyright (C) 1985-1987, 1993-1994, 1996, 1998-2012
   3                  Free Software Foundation, Inc.
   4
   5 This file is part of GNU Emacs.
   6
   7 GNU Emacs is free software: you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation, either version 3 of the License, or
  10 (at your option) any later version.
  11
  12 GNU Emacs is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20
  21 #include <config.h>
  22 #include <sys/types.h>
  23 #include <sys/stat.h>
  24 #include <signal.h>
  25 #include <stdio.h>
  26
  27 #ifdef HAVE_PWD_H
  28 #include <pwd.h>
  29 #endif
  30
  31 #include <sys/file.h>
  32 #include <fcntl.h>
  33 #include <unistd.h>
  34
  35 #ifdef __FreeBSD__
  36 #include <sys/sysctl.h>
  37 #endif /* __FreeBSD__ */
  38
  39 #include <errno.h>
  40
  41 #include "lisp.h"
  42 #include "character.h"
  43 #include "buffer.h"
  44 #include "coding.h"
  45 #include "systime.h"
  46
  47 #ifdef CLASH_DETECTION
  48
  49 #ifdef HAVE_UTMP_H
  50 #include <utmp.h>
  51 #endif
  52
  53 /* A file whose last-modified time is just after the most recent boot.
  54    Define this to be NULL to disable checking for this file.  */
  55 #ifndef BOOT_TIME_FILE
  56 #define BOOT_TIME_FILE "/var/run/random-seed"
  57 #endif
  58
  59 #ifndef WTMP_FILE
  60 #define WTMP_FILE "/var/log/wtmp"
  61 #endif
  62
  63 /* The strategy: to lock a file FN, create a symlink .#FN in FN's
  64    directory, with link data `user@host.pid'.  This avoids a single
  65    mount (== failure) point for lock files.
  66
  67    When the host in the lock data is the current host, we can check if
  68    the pid is valid with kill.
  69
  70    Otherwise, we could look at a separate file that maps hostnames to
  71    reboot times to see if the remote pid can possibly be valid, since we
  72    don't want Emacs to have to communicate via pipes or sockets or
  73    whatever to other processes, either locally or remotely; rms says
  74    that's too unreliable.  Hence the separate file, which could
  75    theoretically be updated by daemons running separately -- but this
  76    whole idea is unimplemented; in practice, at least in our
  77    environment, it seems such stale locks arise fairly infrequently, and
  78    Emacs' standard methods of dealing with clashes suffice.
  79
  80    We use symlinks instead of normal files because (1) they can be
  81    stored more efficiently on the filesystem, since the kernel knows
  82    they will be small, and (2) all the info about the lock can be read
  83    in a single system call (readlink).  Although we could use regular
  84    files to be useful on old systems lacking symlinks, nowadays
  85    virtually all such systems are probably single-user anyway, so it
  86    didn't seem worth the complication.
  87
  88    Similarly, we don't worry about a possible 14-character limit on
  89    file names, because those are all the same systems that don't have
  90    symlinks.
  91
  92    This is compatible with the locking scheme used by Interleaf (which
  93    has contributed this implementation for Emacs), and was designed by
  94    Ethan Jacobson, Kimbo Mundy, and others.
  95
  96    --karl@cs.umb.edu/karl@hq.ileaf.com.  */
  97
  98 \f
  99 /* Return the time of the last system boot.  */
 100
 101 static time_t boot_time;
 102 static bool boot_time_initialized;
 103
 104 #ifdef BOOT_TIME
 105 static void get_boot_time_1 (const char *, bool);
 106 #endif
 107
 108 static time_t
 109 get_boot_time (void)
 110 {
 111 #if defined (BOOT_TIME)
 112   int counter;
 113 #endif
 114
 115   if (boot_time_initialized)
 116     return boot_time;
 117   boot_time_initialized = 1;
 118
 119 #if defined (CTL_KERN) && defined (KERN_BOOTTIME)
 120   {
 121     int mib[2];
 122     size_t size;
 123     struct timeval boottime_val;
 124
 125     mib[0] = CTL_KERN;
 126     mib[1] = KERN_BOOTTIME;
 127     size = sizeof (boottime_val);
 128
 129     if (sysctl (mib, 2, &boottime_val, &size, NULL, 0) >= 0)
 130       {
 131         boot_time = boottime_val.tv_sec;
 132         return boot_time;
 133       }
 134   }
 135 #endif /* defined (CTL_KERN) && defined (KERN_BOOTTIME) */
 136
 137   if (BOOT_TIME_FILE)
 138     {
 139       struct stat st;
 140       if (stat (BOOT_TIME_FILE, &st) == 0)
 141         {
 142           boot_time = st.st_mtime;
 143           return boot_time;
 144         }
 145     }
 146
 147 #if defined (BOOT_TIME)
 148 #ifndef CANNOT_DUMP
 149   /* The utmp routines maintain static state.
 150      Don't touch that state unless we are initialized,
 151      since it might not survive dumping.  */
 152   if (! initialized)
 153     return boot_time;
 154 #endif /* not CANNOT_DUMP */
 155
 156   /* Try to get boot time from utmp before wtmp,
 157      since utmp is typically much smaller than wtmp.
 158      Passing a null pointer causes get_boot_time_1
 159      to inspect the default file, namely utmp.  */
 160   get_boot_time_1 ((char *) 0, 0);
 161   if (boot_time)
 162     return boot_time;
 163
 164   /* Try to get boot time from the current wtmp file.  */
 165   get_boot_time_1 (WTMP_FILE, 1);
 166
 167   /* If we did not find a boot time in wtmp, look at wtmp, and so on.  */
 168   for (counter = 0; counter < 20 && ! boot_time; counter++)
 169     {
 170       char cmd_string[sizeof WTMP_FILE ".19.gz"];
 171       Lisp_Object tempname, filename;
 172       bool delete_flag = 0;
 173
 174       filename = Qnil;
 175
 176       tempname = make_formatted_string
 177         (cmd_string, "%s.%d", WTMP_FILE, counter);
 178       if (! NILP (Ffile_exists_p (tempname)))
 179         filename = tempname;
 180       else
 181         {
 182           tempname = make_formatted_string (cmd_string, "%s.%d.gz",
 183                                             WTMP_FILE, counter);
 184           if (! NILP (Ffile_exists_p (tempname)))
 185             {
 186               Lisp_Object args[6];
 187
 188               /* The utmp functions on mescaline.gnu.org accept only
 189                  file names up to 8 characters long.  Choose a 2
 190                  character long prefix, and call make_temp_file with
 191                  second arg non-zero, so that it will add not more
 192                  than 6 characters to the prefix.  */
 193               filename = Fexpand_file_name (build_string ("wt"),
 194                                             Vtemporary_file_directory);
 195               filename = make_temp_name (filename, 1);
 196               args[0] = build_string ("gzip");
 197               args[1] = Qnil;
 198               args[2] = list2 (QCfile, filename);
 199               args[3] = Qnil;
 200               args[4] = build_string ("-cd");
 201               args[5] = tempname;
 202               Fcall_process (6, args);
 203               delete_flag = 1;
 204             }
 205         }
 206
 207       if (! NILP (filename))
 208         {
 209           get_boot_time_1 (SSDATA (filename), 1);
 210           if (delete_flag)
 211             unlink (SSDATA (filename));
 212         }
 213     }
 214
 215   return boot_time;
 216 #else
 217   return 0;
 218 #endif
 219 }
 220
 221 #ifdef BOOT_TIME
 222 /* Try to get the boot time from wtmp file FILENAME.
 223    This succeeds if that file contains a reboot record.
 224
 225    If FILENAME is zero, use the same file as before;
 226    if no FILENAME has ever been specified, this is the utmp file.
 227    Use the newest reboot record if NEWEST,
 228    the first reboot record otherwise.
 229    Ignore all reboot records on or before BOOT_TIME.
 230    Success is indicated by setting BOOT_TIME to a larger value.  */
 231
 232 void
 233 get_boot_time_1 (const char *filename, bool newest)
 234 {
 235   struct utmp ut, *utp;
 236   int desc;
 237
 238   if (filename)
 239     {
 240       /* On some versions of IRIX, opening a nonexistent file name
 241          is likely to crash in the utmp routines.  */
 242       desc = emacs_open (filename, O_RDONLY, 0);
 243       if (desc < 0)
 244         return;
 245
 246       emacs_close (desc);
 247
 248       utmpname (filename);
 249     }
 250
 251   setutent ();
 252
 253   while (1)
 254     {
 255       /* Find the next reboot record.  */
 256       ut.ut_type = BOOT_TIME;
 257       utp = getutid (&ut);
 258       if (! utp)
 259         break;
 260       /* Compare reboot times and use the newest one.  */
 261       if (utp->ut_time > boot_time)
 262         {
 263           boot_time = utp->ut_time;
 264           if (! newest)
 265             break;
 266         }
 267       /* Advance on element in the file
 268          so that getutid won't repeat the same one.  */
 269       utp = getutent ();
 270       if (! utp)
 271         break;
 272     }
 273   endutent ();
 274 }
 275 #endif /* BOOT_TIME */
 276 \f
 277 /* Here is the structure that stores information about a lock.  */
 278
 279 typedef struct
 280 {
 281   char *user;
 282   char *host;
 283   pid_t pid;
 284   time_t boot_time;
 285 } lock_info_type;
 286
 287 /* Free the two dynamically-allocated pieces in PTR.  */
 288 #define FREE_LOCK_INFO(i) do { xfree ((i).user); xfree ((i).host); } while (0)
 289
 290
 291 /* Write the name of the lock file for FN into LFNAME.  Length will be
 292    that of FN plus two more for the leading `.#' plus 1 for the
 293    trailing period plus one for the digit after it plus one for the
 294    null.  */
 295 #define MAKE_LOCK_NAME(lock, file) \
 296   (lock = alloca (SBYTES (file) + 2 + 1 + 1 + 1), \
 297    fill_in_lock_file_name (lock, (file)))
 298
 299 static void
 300 fill_in_lock_file_name (register char *lockfile, register Lisp_Object fn)
 301 {
 302   ptrdiff_t length = SBYTES (fn);
 303   register char *p;
 304   struct stat st;
 305   int count = 0;
 306
 307   strcpy (lockfile, SSDATA (fn));
 308
 309   /* Shift the nondirectory part of the file name (including the null)
 310      right two characters.  Here is one of the places where we'd have to
 311      do something to support 14-character-max file names.  */
 312   for (p = lockfile + length; p != lockfile && *p != '/'; p--)
 313     p[2] = *p;
 314
 315   /* Insert the `.#'.  */
 316   p[1] = '.';
 317   p[2] = '#';
 318
 319   p = p + length + 2;
 320
 321   while (lstat (lockfile, &st) == 0 && !S_ISLNK (st.st_mode))
 322     {
 323       if (count > 9)
 324         {
 325           *p = '\0';
 326           return;
 327         }
 328       sprintf (p, ".%d", count++);
 329     }
 330 }
 331
 332 /* Lock the lock file named LFNAME.
 333    If FORCE, do so even if it is already locked.
 334    Return true if successful.  */
 335
 336 static bool
 337 lock_file_1 (char *lfname, bool force)
 338 {
 339   int err;
 340   int symlink_errno;
 341   USE_SAFE_ALLOCA;
 342
 343   /* Call this first because it can GC.  */
 344   printmax_t boot = get_boot_time ();
 345
 346   Lisp_Object luser_name = Fuser_login_name (Qnil);
 347   char const *user_name = STRINGP (luser_name) ? SSDATA (luser_name) : "";
 348   Lisp_Object lhost_name = Fsystem_name ();
 349   char const *host_name = STRINGP (lhost_name) ? SSDATA (lhost_name) : "";
 350   ptrdiff_t lock_info_size = (strlen (user_name) + strlen (host_name)
 351                               + 2 * INT_STRLEN_BOUND (printmax_t)
 352                               + sizeof "@.:");
 353   char *lock_info_str = SAFE_ALLOCA (lock_info_size);
 354   printmax_t pid = getpid ();
 355
 356   esprintf (lock_info_str, boot ? "%s@%s.%"pMd":%"pMd : "%s@%s.%"pMd,
 357             user_name, host_name, pid, boot);
 358
 359   err = symlink (lock_info_str, lfname);
 360   if (errno == EEXIST && force)
 361     {
 362       unlink (lfname);
 363       err = symlink (lock_info_str, lfname);
 364     }
 365
 366   symlink_errno = errno;
 367   SAFE_FREE ();
 368   errno = symlink_errno;
 369   return err == 0;
 370 }
 371
 372 /* Return true if times A and B are no more than one second apart.  */
 373
 374 static bool
 375 within_one_second (time_t a, time_t b)
 376 {
 377   return (a - b >= -1 && a - b <= 1);
 378 }
 379 \f
 380 /* Return 0 if nobody owns the lock file LFNAME or the lock is obsolete,
 381    1 if another process owns it (and set OWNER (if non-null) to info),
 382    2 if the current process owns it,
 383    or -1 if something is wrong with the locking mechanism.  */
 384
 385 static int
 386 current_lock_owner (lock_info_type *owner, char *lfname)
 387 {
 388   int ret;
 389   ptrdiff_t len;
 390   lock_info_type local_owner;
 391   intmax_t n;
 392   char *at, *dot, *colon;
 393   char readlink_buf[READLINK_BUFSIZE];
 394   char *lfinfo = emacs_readlink (lfname, readlink_buf);
 395
 396   /* If nonexistent lock file, all is well; otherwise, got strange error. */
 397   if (!lfinfo)
 398     return errno == ENOENT ? 0 : -1;
 399
 400   /* Even if the caller doesn't want the owner info, we still have to
 401      read it to determine return value.  */
 402   if (!owner)
 403     owner = &local_owner;
 404
 405   /* Parse USER@HOST.PID:BOOT_TIME.  If can't parse, return -1.  */
 406   /* The USER is everything before the last @.  */
 407   at = strrchr (lfinfo, '@');
 408   dot = strrchr (lfinfo, '.');
 409   if (!at || !dot)
 410     {
 411       if (lfinfo != readlink_buf)
 412         xfree (lfinfo);
 413       return -1;
 414     }
 415   len = at - lfinfo;
 416   owner->user = xmalloc (len + 1);
 417   memcpy (owner->user, lfinfo, len);
 418   owner->user[len] = 0;
 419
 420   /* The PID is everything from the last `.' to the `:'.  */
 421   errno = 0;
 422   n = strtoimax (dot + 1, NULL, 10);
 423   owner->pid =
 424     ((0 <= n && n <= TYPE_MAXIMUM (pid_t)
 425       && (TYPE_MAXIMUM (pid_t) < INTMAX_MAX || errno != ERANGE))
 426      ? n : 0);
 427
 428   colon = strchr (dot + 1, ':');
 429   /* After the `:', if there is one, comes the boot time.  */
 430   n = 0;
 431   if (colon)
 432     {
 433       errno = 0;
 434       n = strtoimax (colon + 1, NULL, 10);
 435     }
 436   owner->boot_time =
 437     ((0 <= n && n <= TYPE_MAXIMUM (time_t)
 438       && (TYPE_MAXIMUM (time_t) < INTMAX_MAX || errno != ERANGE))
 439      ? n : 0);
 440
 441   /* The host is everything in between.  */
 442   len = dot - at - 1;
 443   owner->host = xmalloc (len + 1);
 444   memcpy (owner->host, at + 1, len);
 445   owner->host[len] = 0;
 446
 447   /* We're done looking at the link info.  */
 448   if (lfinfo != readlink_buf)
 449     xfree (lfinfo);
 450
 451   /* On current host?  */
 452   if (STRINGP (Fsystem_name ())
 453       && strcmp (owner->host, SSDATA (Fsystem_name ())) == 0)
 454     {
 455       if (owner->pid == getpid ())
 456         ret = 2; /* We own it.  */
 457       else if (owner->pid > 0
 458                && (kill (owner->pid, 0) >= 0 || errno == EPERM)
 459                && (owner->boot_time == 0
 460                    || within_one_second (owner->boot_time, get_boot_time ())))
 461         ret = 1; /* An existing process on this machine owns it.  */
 462       /* The owner process is dead or has a strange pid (<=0), so try to
 463          zap the lockfile.  */
 464       else if (unlink (lfname) < 0)
 465         ret = -1;
 466       else
 467         ret = 0;
 468     }
 469   else
 470     { /* If we wanted to support the check for stale locks on remote machines,
 471          here's where we'd do it.  */
 472       ret = 1;
 473     }
 474
 475   /* Avoid garbage.  */
 476   if (owner == &local_owner || ret <= 0)
 477     {
 478       FREE_LOCK_INFO (*owner);
 479     }
 480   return ret;
 481 }
 482
 483 \f
 484 /* Lock the lock named LFNAME if possible.
 485    Return 0 in that case.
 486    Return positive if some other process owns the lock, and info about
 487      that process in CLASHER.
 488    Return -1 if cannot lock for any other reason.  */
 489
 490 static int
 491 lock_if_free (lock_info_type *clasher, register char *lfname)
 492 {
 493   while (! lock_file_1 (lfname, 0))
 494     {
 495       int locker;
 496
 497       if (errno != EEXIST)
 498         return -1;
 499
 500       locker = current_lock_owner (clasher, lfname);
 501       if (locker == 2)
 502         {
 503           FREE_LOCK_INFO (*clasher);
 504           return 0;   /* We ourselves locked it.  */
 505         }
 506       else if (locker == 1)
 507         return 1;  /* Someone else has it.  */
 508       else if (locker == -1)
 509         return -1;   /* current_lock_owner returned strange error.  */
 510
 511       /* We deleted a stale lock; try again to lock the file.  */
 512     }
 513   return 0;
 514 }
 515
 516 /* lock_file locks file FN,
 517    meaning it serves notice on the world that you intend to edit that file.
 518    This should be done only when about to modify a file-visiting
 519    buffer previously unmodified.
 520    Do not (normally) call this for a buffer already modified,
 521    as either the file is already locked, or the user has already
 522    decided to go ahead without locking.
 523
 524    When this returns, either the lock is locked for us,
 525    or the user has said to go ahead without locking.
 526
 527    If the file is locked by someone else, this calls
 528    ask-user-about-lock (a Lisp function) with two arguments,
 529    the file name and info about the user who did the locking.
 530    This function can signal an error, or return t meaning
 531    take away the lock, or return nil meaning ignore the lock.  */
 532
 533 void
 534 lock_file (Lisp_Object fn)
 535 {
 536   register Lisp_Object attack, orig_fn, encoded_fn;
 537   register char *lfname, *locker;
 538   ptrdiff_t locker_size;
 539   lock_info_type lock_info;
 540   printmax_t pid;
 541   struct gcpro gcpro1;
 542   USE_SAFE_ALLOCA;
 543
 544   /* Don't do locking if the user has opted out.  */
 545   if (! create_lockfiles)
 546     return;
 547
 548   /* Don't do locking while dumping Emacs.
 549      Uncompressing wtmp files uses call-process, which does not work
 550      in an uninitialized Emacs.  */
 551   if (! NILP (Vpurify_flag))
 552     return;
 553
 554   orig_fn = fn;
 555   GCPRO1 (fn);
 556   fn = Fexpand_file_name (fn, Qnil);
 557   encoded_fn = ENCODE_FILE (fn);
 558
 559   /* Create the name of the lock-file for file fn */
 560   MAKE_LOCK_NAME (lfname, encoded_fn);
 561
 562   /* See if this file is visited and has changed on disk since it was
 563      visited.  */
 564   {
 565     register Lisp_Object subject_buf;
 566
 567     subject_buf = get_truename_buffer (orig_fn);
 568
 569     if (!NILP (subject_buf)
 570         && NILP (Fverify_visited_file_modtime (subject_buf))
 571         && !NILP (Ffile_exists_p (fn)))
 572       call1 (intern ("ask-user-about-supersession-threat"), fn);
 573
 574   }
 575   UNGCPRO;
 576
 577   /* Try to lock the lock. */
 578   if (lock_if_free (&lock_info, lfname) <= 0)
 579     /* Return now if we have locked it, or if lock creation failed */
 580     return;
 581
 582   /* Else consider breaking the lock */
 583   locker_size = (strlen (lock_info.user) + strlen (lock_info.host)
 584                  + INT_STRLEN_BOUND (printmax_t)
 585                  + sizeof "@ (pid )");
 586   locker = SAFE_ALLOCA (locker_size);
 587   pid = lock_info.pid;
 588   esprintf (locker, "%s@%s (pid %"pMd")",
 589             lock_info.user, lock_info.host, pid);
 590   FREE_LOCK_INFO (lock_info);
 591
 592   attack = call2 (intern ("ask-user-about-lock"), fn, build_string (locker));
 593   SAFE_FREE ();
 594   if (!NILP (attack))
 595     /* User says take the lock */
 596     {
 597       lock_file_1 (lfname, 1);
 598       return;
 599     }
 600   /* User says ignore the lock */
 601 }
 602
 603 void
 604 unlock_file (register Lisp_Object fn)
 605 {
 606   register char *lfname;
 607
 608   fn = Fexpand_file_name (fn, Qnil);
 609   fn = ENCODE_FILE (fn);
 610
 611   MAKE_LOCK_NAME (lfname, fn);
 612
 613   if (current_lock_owner (0, lfname) == 2)
 614     unlink (lfname);
 615 }
 616
 617 void
 618 unlock_all_files (void)
 619 {
 620   register Lisp_Object tail;
 621   register struct buffer *b;
 622
 623   for (tail = Vbuffer_alist; CONSP (tail); tail = XCDR (tail))
 624     {
 625       b = XBUFFER (XCDR (XCAR (tail)));
 626       if (STRINGP (BVAR (b, file_truename)) && BUF_SAVE_MODIFF (b) < BUF_MODIFF (b))
 627         {
 628           unlock_file (BVAR (b, file_truename));
 629         }
 630     }
 631 }
 632 \f
 633 DEFUN ("lock-buffer", Flock_buffer, Slock_buffer,
 634        0, 1, 0,
 635        doc: /* Lock FILE, if current buffer is modified.
 636 FILE defaults to current buffer's visited file,
 637 or else nothing is done if current buffer isn't visiting a file.  */)
 638   (Lisp_Object file)
 639 {
 640   if (NILP (file))
 641     file = BVAR (current_buffer, file_truename);
 642   else
 643     CHECK_STRING (file);
 644   if (SAVE_MODIFF < MODIFF
 645       && !NILP (file))
 646     lock_file (file);
 647   return Qnil;
 648 }
 649
 650 DEFUN ("unlock-buffer", Funlock_buffer, Sunlock_buffer,
 651        0, 0, 0,
 652        doc: /* Unlock the file visited in the current buffer.
 653 If the buffer is not modified, this does nothing because the file
 654 should not be locked in that case.  */)
 655   (void)
 656 {
 657   if (SAVE_MODIFF < MODIFF
 658       && STRINGP (BVAR (current_buffer, file_truename)))
 659     unlock_file (BVAR (current_buffer, file_truename));
 660   return Qnil;
 661 }
 662
 663 /* Unlock the file visited in buffer BUFFER.  */
 664
 665 void
 666 unlock_buffer (struct buffer *buffer)
 667 {
 668   if (BUF_SAVE_MODIFF (buffer) < BUF_MODIFF (buffer)
 669       && STRINGP (BVAR (buffer, file_truename)))
 670     unlock_file (BVAR (buffer, file_truename));
 671 }
 672
 673 DEFUN ("file-locked-p", Ffile_locked_p, Sfile_locked_p, 1, 1, 0,
 674        doc: /* Return a value indicating whether FILENAME is locked.
 675 The value is nil if the FILENAME is not locked,
 676 t if it is locked by you, else a string saying which user has locked it.  */)
 677   (Lisp_Object filename)
 678 {
 679   Lisp_Object ret;
 680   register char *lfname;
 681   int owner;
 682   lock_info_type locker;
 683
 684   filename = Fexpand_file_name (filename, Qnil);
 685
 686   MAKE_LOCK_NAME (lfname, filename);
 687
 688   owner = current_lock_owner (&locker, lfname);
 689   if (owner <= 0)
 690     ret = Qnil;
 691   else if (owner == 2)
 692     ret = Qt;
 693   else
 694     ret = build_string (locker.user);
 695
 696   if (owner > 0)
 697     FREE_LOCK_INFO (locker);
 698
 699   return ret;
 700 }
 701
 702 #endif /* CLASH_DETECTION */
 703
 704 void
 705 syms_of_filelock (void)
 706 {
 707   DEFVAR_LISP ("temporary-file-directory", Vtemporary_file_directory,
 708                doc: /* The directory for writing temporary files.  */);
 709   Vtemporary_file_directory = Qnil;
 710
 711   DEFVAR_BOOL ("create-lockfiles", create_lockfiles,
 712                doc: /* Non-nil means use lockfiles to avoid editing collisions.  */);
 713   create_lockfiles = 1;
 714
 715 #ifdef CLASH_DETECTION
 716   defsubr (&Sunlock_buffer);
 717   defsubr (&Slock_buffer);
 718   defsubr (&Sfile_locked_p);
 719 #endif
 720 }