Dogcows Code - chaz/tar/blob - src/misc.c

   1 /* Miscellaneous functions, not really specific to GNU tar.
   2
   3    Copyright (C) 1988, 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001,
   4    2003, 2004, 2005, 2006, 2007, 2009, 2010 Free Software Foundation, Inc.
   5
   6    This program is free software; you can redistribute it and/or modify it
   7    under the terms of the GNU General Public License as published by the
   8    Free Software Foundation; either version 3, or (at your option) any later
   9    version.
  10
  11    This program is distributed in the hope that it will be useful, but
  12    WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
  14    Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License along
  17    with this program; if not, write to the Free Software Foundation, Inc.,
  18    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
  19
  20 #include <system.h>
  21 #include <rmt.h>
  22 #include "common.h"
  23 #include <quotearg.h>
  24 #include <xgetcwd.h>
  25 #include <unlinkdir.h>
  26 #include <utimens.h>
  27
  28 #ifndef DOUBLE_SLASH_IS_DISTINCT_ROOT
  29 # define DOUBLE_SLASH_IS_DISTINCT_ROOT 0
  30 #endif
  31
  32 \f
  33 /* Handling strings.  */
  34
  35 /* Assign STRING to a copy of VALUE if not zero, or to zero.  If
  36    STRING was nonzero, it is freed first.  */
  37 void
  38 assign_string (char **string, const char *value)
  39 {
  40   free (*string);
  41   *string = value ? xstrdup (value) : 0;
  42 }
  43
  44 #if 0
  45 /* This function is currently unused; perhaps it should be removed?  */
  46
  47 /* Allocate a copy of the string quoted as in C, and returns that.  If
  48    the string does not have to be quoted, it returns a null pointer.
  49    The allocated copy should normally be freed with free() after the
  50    caller is done with it.
  51
  52    This is used in one context only: generating the directory file in
  53    incremental dumps.  The quoted string is not intended for human
  54    consumption; it is intended only for unquote_string.  The quoting
  55    is locale-independent, so that users needn't worry about locale
  56    when reading directory files.  This means that we can't use
  57    quotearg, as quotearg is locale-dependent and is meant for human
  58    consumption.  */
  59 static char *
  60 quote_copy_string (const char *string)
  61 {
  62   const char *source = string;
  63   char *destination = 0;
  64   char *buffer = 0;
  65   int copying = 0;
  66
  67   while (*source)
  68     {
  69       int character = *source++;
  70
  71       switch (character)
  72         {
  73         case '\n': case '\\':
  74           if (!copying)
  75             {
  76               size_t length = (source - string) - 1;
  77
  78               copying = 1;
  79               buffer = xmalloc (length + 2 + 2 * strlen (source) + 1);
  80               memcpy (buffer, string, length);
  81               destination = buffer + length;
  82             }
  83           *destination++ = '\\';
  84           *destination++ = character == '\\' ? '\\' : 'n';
  85           break;
  86
  87         default:
  88           if (copying)
  89             *destination++ = character;
  90           break;
  91         }
  92     }
  93   if (copying)
  94     {
  95       *destination = '\0';
  96       return buffer;
  97     }
  98   return 0;
  99 }
 100 #endif
 101
 102 /* Takes a quoted C string (like those produced by quote_copy_string)
 103    and turns it back into the un-quoted original.  This is done in
 104    place.  Returns 0 only if the string was not properly quoted, but
 105    completes the unquoting anyway.
 106
 107    This is used for reading the saved directory file in incremental
 108    dumps.  It is used for decoding old `N' records (demangling names).
 109    But also, it is used for decoding file arguments, would they come
 110    from the shell or a -T file, and for decoding the --exclude
 111    argument.  */
 112 int
 113 unquote_string (char *string)
 114 {
 115   int result = 1;
 116   char *source = string;
 117   char *destination = string;
 118
 119   /* Escape sequences other than \\ and \n are no longer generated by
 120      quote_copy_string, but accept them for backwards compatibility,
 121      and also because unquote_string is used for purposes other than
 122      parsing the output of quote_copy_string.  */
 123
 124   while (*source)
 125     if (*source == '\\')
 126       switch (*++source)
 127         {
 128         case '\\':
 129           *destination++ = '\\';
 130           source++;
 131           break;
 132
 133         case 'a':
 134           *destination++ = '\a';
 135           source++;
 136           break;
 137
 138         case 'b':
 139           *destination++ = '\b';
 140           source++;
 141           break;
 142
 143         case 'f':
 144           *destination++ = '\f';
 145           source++;
 146           break;
 147
 148         case 'n':
 149           *destination++ = '\n';
 150           source++;
 151           break;
 152
 153         case 'r':
 154           *destination++ = '\r';
 155           source++;
 156           break;
 157
 158         case 't':
 159           *destination++ = '\t';
 160           source++;
 161           break;
 162
 163         case 'v':
 164           *destination++ = '\v';
 165           source++;
 166           break;
 167
 168         case '?':
 169           *destination++ = 0177;
 170           source++;
 171           break;
 172
 173         case '0':
 174         case '1':
 175         case '2':
 176         case '3':
 177         case '4':
 178         case '5':
 179         case '6':
 180         case '7':
 181           {
 182             int value = *source++ - '0';
 183
 184             if (*source < '0' || *source > '7')
 185               {
 186                 *destination++ = value;
 187                 break;
 188               }
 189             value = value * 8 + *source++ - '0';
 190             if (*source < '0' || *source > '7')
 191               {
 192                 *destination++ = value;
 193                 break;
 194               }
 195             value = value * 8 + *source++ - '0';
 196             *destination++ = value;
 197             break;
 198           }
 199
 200         default:
 201           result = 0;
 202           *destination++ = '\\';
 203           if (*source)
 204             *destination++ = *source++;
 205           break;
 206         }
 207     else if (source != destination)
 208       *destination++ = *source++;
 209     else
 210       source++, destination++;
 211
 212   if (source != destination)
 213     *destination = '\0';
 214   return result;
 215 }
 216
 217 /* Zap trailing slashes.  */
 218 char *
 219 zap_slashes (char *name)
 220 {
 221   char *q;
 222
 223   if (!name || *name == 0)
 224     return name;
 225   q = name + strlen (name) - 1;
 226   while (q > name && ISSLASH (*q))
 227     *q-- = '\0';
 228   return name;
 229 }
 230
 231 /* Normalize FILE_NAME by removing redundant slashes and "."
 232    components, including redundant trailing slashes.  Leave ".."
 233    alone, as it may be significant in the presence of symlinks and on
 234    platforms where "/.." != "/".  Destructive version: modifies its
 235    argument. */
 236 static void
 237 normalize_filename_x (char *file_name)
 238 {
 239   char *name = file_name + FILE_SYSTEM_PREFIX_LEN (file_name);
 240   char *p;
 241   char const *q;
 242   char c;
 243
 244   /* Don't squeeze leading "//" to "/", on hosts where they're distinct.  */
 245   name += (DOUBLE_SLASH_IS_DISTINCT_ROOT
 246            && ISSLASH (*name) && ISSLASH (name[1]) && ! ISSLASH (name[2]));
 247
 248   /* Omit redundant leading "." components.  */
 249   for (q = p = name; (*p = *q) == '.' && ISSLASH (q[1]); p += !*q)
 250     for (q += 2; ISSLASH (*q); q++)
 251       continue;
 252
 253   /* Copy components from Q to P, omitting redundant slashes and
 254      internal "."  components.  */
 255   while ((*p++ = c = *q++) != '\0')
 256     if (ISSLASH (c))
 257       while (ISSLASH (q[*q == '.']))
 258         q += (*q == '.') + 1;
 259
 260   /* Omit redundant trailing "." component and slash.  */
 261   if (2 < p - name)
 262     {
 263       p -= p[-2] == '.' && ISSLASH (p[-3]);
 264       p -= 2 < p - name && ISSLASH (p[-2]);
 265       p[-1] = '\0';
 266     }
 267 }
 268
 269 /* Normalize NAME by removing redundant slashes and "." components,
 270    including redundant trailing slashes.  Return a normalized
 271    newly-allocated copy.  */
 272
 273 char *
 274 normalize_filename (const char *name)
 275 {
 276   char *copy = NULL;
 277
 278   if (IS_RELATIVE_FILE_NAME (name))
 279     {
 280       /* Set COPY to the absolute file name if possible.
 281
 282          FIXME: There should be no need to get the absolute file name.
 283          getcwd is slow, it might fail, and it does not necessarily
 284          return a canonical name even when it succeeds.  Perhaps we
 285          can use dev+ino pairs instead of names?  */
 286       copy = xgetcwd ();
 287       if (copy)
 288         {
 289           size_t copylen = strlen (copy);
 290           bool need_separator = ! (DOUBLE_SLASH_IS_DISTINCT_ROOT
 291                                    && copylen == 2 && ISSLASH (copy[1]));
 292           copy = xrealloc (copy, copylen + need_separator + strlen (name) + 1);
 293           copy[copylen] = DIRECTORY_SEPARATOR;
 294           strcpy (copy + copylen + need_separator, name);
 295         }
 296       else
 297         WARN ((0, errno, _("Cannot get working directory")));
 298     }
 299
 300   if (! copy)
 301     copy = xstrdup (name);
 302   normalize_filename_x (copy);
 303   return copy;
 304 }
 305
 306 \f
 307 void
 308 replace_prefix (char **pname, const char *samp, size_t slen,
 309                 const char *repl, size_t rlen)
 310 {
 311   char *name = *pname;
 312   size_t nlen = strlen (name);
 313   if (nlen > slen && memcmp (name, samp, slen) == 0 && ISSLASH (name[slen]))
 314     {
 315       if (rlen > slen)
 316         {
 317           name = xrealloc (name, nlen - slen + rlen + 1);
 318           *pname = name;
 319         }
 320       memmove (name + rlen, name + slen, nlen - slen + 1);
 321       memcpy (name, repl, rlen);
 322     }
 323 }
 324
 325 \f
 326 /* Handling numbers.  */
 327
 328 /* Output fraction and trailing digits appropriate for a nanoseconds
 329    count equal to NS, but don't output unnecessary '.' or trailing
 330    zeros.  */
 331
 332 void
 333 code_ns_fraction (int ns, char *p)
 334 {
 335   if (ns == 0)
 336     *p = '\0';
 337   else
 338     {
 339       int i = 9;
 340       *p++ = '.';
 341
 342       while (ns % 10 == 0)
 343         {
 344           ns /= 10;
 345           i--;
 346         }
 347
 348       p[i] = '\0';
 349
 350       for (;;)
 351         {
 352           p[--i] = '0' + ns % 10;
 353           if (i == 0)
 354             break;
 355           ns /= 10;
 356         }
 357     }
 358 }
 359
 360 char const *
 361 code_timespec (struct timespec t, char sbuf[TIMESPEC_STRSIZE_BOUND])
 362 {
 363   time_t s = t.tv_sec;
 364   int ns = t.tv_nsec;
 365   char *np;
 366   bool negative = s < 0;
 367
 368   /* ignore invalid values of ns */
 369   if (BILLION <= ns || ns < 0)
 370     ns = 0;
 371
 372   if (negative && ns != 0)
 373     {
 374       s++;
 375       ns = BILLION - ns;
 376     }
 377
 378   np = umaxtostr (negative ? - (uintmax_t) s : (uintmax_t) s, sbuf + 1);
 379   if (negative)
 380     *--np = '-';
 381   code_ns_fraction (ns, sbuf + UINTMAX_STRSIZE_BOUND);
 382   return np;
 383 }
 384 \f
 385 /* File handling.  */
 386
 387 /* Saved names in case backup needs to be undone.  */
 388 static char *before_backup_name;
 389 static char *after_backup_name;
 390
 391 /* Return 1 if FILE_NAME is obviously "." or "/".  */
 392 bool
 393 must_be_dot_or_slash (char const *file_name)
 394 {
 395   file_name += FILE_SYSTEM_PREFIX_LEN (file_name);
 396
 397   if (ISSLASH (file_name[0]))
 398     {
 399       for (;;)
 400         if (ISSLASH (file_name[1]))
 401           file_name++;
 402         else if (file_name[1] == '.'
 403                  && ISSLASH (file_name[2 + (file_name[2] == '.')]))
 404           file_name += 2 + (file_name[2] == '.');
 405         else
 406           return ! file_name[1];
 407     }
 408   else
 409     {
 410       while (file_name[0] == '.' && ISSLASH (file_name[1]))
 411         {
 412           file_name += 2;
 413           while (ISSLASH (*file_name))
 414             file_name++;
 415         }
 416
 417       return ! file_name[0] || (file_name[0] == '.' && ! file_name[1]);
 418     }
 419 }
 420
 421 /* Some implementations of rmdir let you remove '.' or '/'.
 422    Report an error with errno set to zero for obvious cases of this;
 423    otherwise call rmdir.  */
 424 static int
 425 safer_rmdir (const char *file_name)
 426 {
 427   if (must_be_dot_or_slash (file_name))
 428     {
 429       errno = 0;
 430       return -1;
 431     }
 432
 433   return unlinkat (chdir_fd, file_name, AT_REMOVEDIR);
 434 }
 435
 436 /* Remove FILE_NAME, returning 1 on success.  If FILE_NAME is a directory,
 437    then if OPTION is RECURSIVE_REMOVE_OPTION is set remove FILE_NAME
 438    recursively; otherwise, remove it only if it is empty.  If FILE_NAME is
 439    a directory that cannot be removed (e.g., because it is nonempty)
 440    and if OPTION is WANT_DIRECTORY_REMOVE_OPTION, then return -1.
 441    Return 0 on error, with errno set; if FILE_NAME is obviously the working
 442    directory return zero with errno set to zero.  */
 443 int
 444 remove_any_file (const char *file_name, enum remove_option option)
 445 {
 446   /* Try unlink first if we cannot unlink directories, as this saves
 447      us a system call in the common case where we're removing a
 448      non-directory.  */
 449   bool try_unlink_first = cannot_unlink_dir ();
 450
 451   if (try_unlink_first)
 452     {
 453       if (unlinkat (chdir_fd, file_name, 0) == 0)
 454         return 1;
 455
 456       /* POSIX 1003.1-2001 requires EPERM when attempting to unlink a
 457          directory without appropriate privileges, but many Linux
 458          kernels return the more-sensible EISDIR.  */
 459       if (errno != EPERM && errno != EISDIR)
 460         return 0;
 461     }
 462
 463   if (safer_rmdir (file_name) == 0)
 464     return 1;
 465
 466   switch (errno)
 467     {
 468     case ENOTDIR:
 469       return !try_unlink_first && unlinkat (chdir_fd, file_name, 0) == 0;
 470
 471     case 0:
 472     case EEXIST:
 473 #if defined ENOTEMPTY && ENOTEMPTY != EEXIST
 474     case ENOTEMPTY:
 475 #endif
 476       switch (option)
 477         {
 478         case ORDINARY_REMOVE_OPTION:
 479           break;
 480
 481         case WANT_DIRECTORY_REMOVE_OPTION:
 482           return -1;
 483
 484         case RECURSIVE_REMOVE_OPTION:
 485           {
 486             char *directory = savedir (file_name);
 487             char const *entry;
 488             size_t entrylen;
 489
 490             if (! directory)
 491               return 0;
 492
 493             for (entry = directory;
 494                  (entrylen = strlen (entry)) != 0;
 495                  entry += entrylen + 1)
 496               {
 497                 char *file_name_buffer = new_name (file_name, entry);
 498                 int r = remove_any_file (file_name_buffer,
 499                                          RECURSIVE_REMOVE_OPTION);
 500                 int e = errno;
 501                 free (file_name_buffer);
 502
 503                 if (! r)
 504                   {
 505                     free (directory);
 506                     errno = e;
 507                     return 0;
 508                   }
 509               }
 510
 511             free (directory);
 512             return safer_rmdir (file_name) == 0;
 513           }
 514         }
 515       break;
 516     }
 517
 518   return 0;
 519 }
 520
 521 /* Check if FILE_NAME already exists and make a backup of it right now.
 522    Return success (nonzero) only if the backup is either unneeded, or
 523    successful.  For now, directories are considered to never need
 524    backup.  If THIS_IS_THE_ARCHIVE is nonzero, this is the archive and
 525    so, we do not have to backup block or character devices, nor remote
 526    entities.  */
 527 bool
 528 maybe_backup_file (const char *file_name, bool this_is_the_archive)
 529 {
 530   struct stat file_stat;
 531
 532   assign_string (&before_backup_name, file_name);
 533
 534   /* A run situation may exist between Emacs or other GNU programs trying to
 535      make a backup for the same file simultaneously.  If theoretically
 536      possible, real problems are unlikely.  Doing any better would require a
 537      convention, GNU-wide, for all programs doing backups.  */
 538
 539   assign_string (&after_backup_name, 0);
 540
 541   /* Check if we really need to backup the file.  */
 542
 543   if (this_is_the_archive && _remdev (file_name))
 544     return true;
 545
 546   if (deref_stat (file_name, &file_stat) != 0)
 547     {
 548       if (errno == ENOENT)
 549         return true;
 550
 551       stat_error (file_name);
 552       return false;
 553     }
 554
 555   if (S_ISDIR (file_stat.st_mode))
 556     return true;
 557
 558   if (this_is_the_archive
 559       && (S_ISBLK (file_stat.st_mode) || S_ISCHR (file_stat.st_mode)))
 560     return true;
 561
 562   after_backup_name = find_backup_file_name (file_name, backup_type);
 563   if (! after_backup_name)
 564     xalloc_die ();
 565
 566   if (renameat (chdir_fd, before_backup_name, chdir_fd, after_backup_name)
 567       == 0)
 568     {
 569       if (verbose_option)
 570         fprintf (stdlis, _("Renaming %s to %s\n"),
 571                  quote_n (0, before_backup_name),
 572                  quote_n (1, after_backup_name));
 573       return true;
 574     }
 575   else
 576     {
 577       /* The backup operation failed.  */
 578       int e = errno;
 579       ERROR ((0, e, _("%s: Cannot rename to %s"),
 580               quotearg_colon (before_backup_name),
 581               quote_n (1, after_backup_name)));
 582       assign_string (&after_backup_name, 0);
 583       return false;
 584     }
 585 }
 586
 587 /* Try to restore the recently backed up file to its original name.
 588    This is usually only needed after a failed extraction.  */
 589 void
 590 undo_last_backup (void)
 591 {
 592   if (after_backup_name)
 593     {
 594       if (renameat (chdir_fd, after_backup_name, chdir_fd, before_backup_name)
 595           != 0)
 596         {
 597           int e = errno;
 598           ERROR ((0, e, _("%s: Cannot rename to %s"),
 599                   quotearg_colon (after_backup_name),
 600                   quote_n (1, before_backup_name)));
 601         }
 602       if (verbose_option)
 603         fprintf (stdlis, _("Renaming %s back to %s\n"),
 604                  quote_n (0, after_backup_name),
 605                  quote_n (1, before_backup_name));
 606       assign_string (&after_backup_name, 0);
 607     }
 608 }
 609
 610 /* Apply either stat or lstat to (NAME, BUF), depending on the
 611    presence of the --dereference option.  NAME is relative to the
 612    most-recent argument to chdir_do.  */
 613 int
 614 deref_stat (char const *name, struct stat *buf)
 615 {
 616   return fstatat (chdir_fd, name, buf, fstatat_flags);
 617 }
 618
 619 /* Read from FD into the buffer BUF with COUNT bytes.  Attempt to fill
 620    BUF.  Wait until input is available; this matters because files are
 621    opened O_NONBLOCK for security reasons, and on some file systems
 622    this can cause read to fail with errno == EAGAIN.  Return the
 623    actual number of bytes read, zero for EOF, or
 624    SAFE_READ_ERROR upon error.  */
 625 size_t
 626 blocking_read (int fd, void *buf, size_t count)
 627 {
 628   size_t bytes = safe_read (fd, buf, count);
 629
 630 #if defined F_SETFL && O_NONBLOCK
 631   if (bytes == SAFE_READ_ERROR && errno == EAGAIN)
 632     {
 633       int flags = fcntl (fd, F_GETFL);
 634       if (0 <= flags && flags & O_NONBLOCK
 635           && fcntl (fd, F_SETFL, flags & ~O_NONBLOCK) != -1)
 636         bytes = safe_read (fd, buf, count);
 637     }
 638 #endif
 639
 640   return bytes;
 641 }
 642
 643 /* Write to FD from the buffer BUF with COUNT bytes.  Do a full write.
 644    Wait until an output buffer is available; this matters because
 645    files are opened O_NONBLOCK for security reasons, and on some file
 646    systems this can cause write to fail with errno == EAGAIN.  Return
 647    the actual number of bytes written, setting errno if that is less
 648    than COUNT.  */
 649 size_t
 650 blocking_write (int fd, void const *buf, size_t count)
 651 {
 652   size_t bytes = full_write (fd, buf, count);
 653
 654 #if defined F_SETFL && O_NONBLOCK
 655   if (bytes < count && errno == EAGAIN)
 656     {
 657       int flags = fcntl (fd, F_GETFL);
 658       if (0 <= flags && flags & O_NONBLOCK
 659           && fcntl (fd, F_SETFL, flags & ~O_NONBLOCK) != -1)
 660         {
 661           char const *buffer = buf;
 662           bytes += full_write (fd, buffer + bytes, count - bytes);
 663         }
 664     }
 665 #endif
 666
 667   return bytes;
 668 }
 669
 670 /* Set FD's (i.e., assuming the working directory is PARENTFD, FILE's)
 671    access time to ATIME.  */
 672 int
 673 set_file_atime (int fd, int parentfd, char const *file, struct timespec atime)
 674 {
 675   struct timespec ts[2];
 676   ts[0] = atime;
 677   ts[1].tv_nsec = UTIME_OMIT;
 678   return fdutimensat (fd, parentfd, file, ts, fstatat_flags);
 679 }
 680
 681 /* A description of a working directory.  */
 682 struct wd
 683 {
 684   /* The directory's name.  */
 685   char const *name;
 686
 687   /* If nonzero, the file descriptor of the directory, or AT_FDCWD if
 688      the working directory.  If zero, the directory needs to be opened
 689      to be used.  */
 690   int fd;
 691 };
 692
 693 /* A vector of chdir targets.  wd[0] is the initial working directory.  */
 694 static struct wd *wd;
 695
 696 /* The number of working directories in the vector.  */
 697 static size_t wd_count;
 698
 699 /* The allocated size of the vector.  */
 700 static size_t wd_alloc;
 701
 702 /* The maximum number of chdir targets with open directories.
 703    Don't make it too large, as many operating systems have a small
 704    limit on the number of open file descriptors.  Also, the current
 705    implementation does not scale well.  */
 706 enum { CHDIR_CACHE_SIZE = 16 };
 707
 708 /* Indexes into WD of chdir targets with open file descriptors, sorted
 709    most-recently used first.  Zero indexes are unused.  */
 710 static int wdcache[CHDIR_CACHE_SIZE];
 711
 712 /* Number of nonzero entries in WDCACHE.  */
 713 static size_t wdcache_count;
 714
 715 int
 716 chdir_count ()
 717 {
 718   if (wd_count == 0)
 719     return wd_count;
 720   return wd_count - 1;
 721 }
 722
 723 /* DIR is the operand of a -C option; add it to vector of chdir targets,
 724    and return the index of its location.  */
 725 int
 726 chdir_arg (char const *dir)
 727 {
 728   if (wd_count == wd_alloc)
 729     {
 730       if (wd_alloc == 0)
 731         {
 732           wd_alloc = 2;
 733           wd = xmalloc (sizeof *wd * wd_alloc);
 734         }
 735       else
 736         wd = x2nrealloc (wd, &wd_alloc, sizeof *wd);
 737
 738       if (! wd_count)
 739         {
 740           wd[wd_count].name = ".";
 741           wd[wd_count].fd = AT_FDCWD;
 742           wd_count++;
 743         }
 744     }
 745
 746   /* Optimize the common special case of the working directory,
 747      or the working directory as a prefix.  */
 748   if (dir[0])
 749     {
 750       while (dir[0] == '.' && ISSLASH (dir[1]))
 751         for (dir += 2;  ISSLASH (*dir);  dir++)
 752           continue;
 753       if (! dir[dir[0] == '.'])
 754         return wd_count - 1;
 755     }
 756
 757   wd[wd_count].name = dir;
 758   wd[wd_count].fd = 0;
 759   return wd_count++;
 760 }
 761
 762 /* Index of current directory.  */
 763 int chdir_current;
 764
 765 /* Value suitable for use as the first argument to openat, and in
 766    similar locations for fstatat, etc.  This is an open file
 767    descriptor, or AT_FDCWD if the working directory is current.  It is
 768    valid until the next invocation of chdir_do.  */
 769 int chdir_fd = AT_FDCWD;
 770
 771 /* Change to directory I, in a virtual way.  This does not actually
 772    invoke chdir; it merely sets chdir_fd to an int suitable as the
 773    first argument for openat, etc.  If I is 0, change to the initial
 774    working directory; otherwise, I must be a value returned by
 775    chdir_arg.  */
 776 void
 777 chdir_do (int i)
 778 {
 779   if (chdir_current != i)
 780     {
 781       struct wd *curr = &wd[i];
 782       int fd = curr->fd;
 783
 784       if (! fd)
 785         {
 786           if (! IS_ABSOLUTE_FILE_NAME (curr->name))
 787             chdir_do (i - 1);
 788           fd = openat (chdir_fd, curr->name,
 789                        open_searchdir_flags & ~ O_NOFOLLOW);
 790           if (fd < 0)
 791             open_fatal (curr->name);
 792
 793           curr->fd = fd;
 794
 795           /* Add I to the cache, tossing out the lowest-ranking entry if the
 796              cache is full.  */
 797           if (wdcache_count < CHDIR_CACHE_SIZE)
 798             wdcache[wdcache_count++] = i;
 799           else
 800             {
 801               struct wd *stale = &wd[wdcache[CHDIR_CACHE_SIZE - 1]];
 802               if (close (stale->fd) != 0)
 803                 close_diag (stale->name);
 804               stale->fd = 0;
 805               wdcache[CHDIR_CACHE_SIZE - 1] = i;
 806             }
 807         }
 808
 809       if (0 < fd)
 810         {
 811           /* Move the i value to the front of the cache.  This is
 812              O(CHDIR_CACHE_SIZE), but the cache is small.  */
 813           size_t ci;
 814           int prev = wdcache[0];
 815           for (ci = 1; prev != i; ci++)
 816             {
 817               int curr = wdcache[ci];
 818               wdcache[ci] = prev;
 819               if (curr == i)
 820                 break;
 821               prev = curr;
 822             }
 823           wdcache[0] = i;
 824         }
 825
 826       chdir_current = i;
 827       chdir_fd = fd;
 828     }
 829 }
 830 \f
 831 void
 832 close_diag (char const *name)
 833 {
 834   if (ignore_failed_read_option)
 835     close_warn (name);
 836   else
 837     close_error (name);
 838 }
 839
 840 void
 841 open_diag (char const *name)
 842 {
 843   if (ignore_failed_read_option)
 844     open_warn (name);
 845   else
 846     open_error (name);
 847 }
 848
 849 void
 850 read_diag_details (char const *name, off_t offset, size_t size)
 851 {
 852   if (ignore_failed_read_option)
 853     read_warn_details (name, offset, size);
 854   else
 855     read_error_details (name, offset, size);
 856 }
 857
 858 void
 859 readlink_diag (char const *name)
 860 {
 861   if (ignore_failed_read_option)
 862     readlink_warn (name);
 863   else
 864     readlink_error (name);
 865 }
 866
 867 void
 868 savedir_diag (char const *name)
 869 {
 870   if (ignore_failed_read_option)
 871     savedir_warn (name);
 872   else
 873     savedir_error (name);
 874 }
 875
 876 void
 877 seek_diag_details (char const *name, off_t offset)
 878 {
 879   if (ignore_failed_read_option)
 880     seek_warn_details (name, offset);
 881   else
 882     seek_error_details (name, offset);
 883 }
 884
 885 void
 886 stat_diag (char const *name)
 887 {
 888   if (ignore_failed_read_option)
 889     stat_warn (name);
 890   else
 891     stat_error (name);
 892 }
 893
 894 void
 895 file_removed_diag (const char *name, bool top_level,
 896                    void (*diagfn) (char const *name))
 897 {
 898   if (!top_level && errno == ENOENT)
 899     {
 900       WARNOPT (WARN_FILE_REMOVED,
 901                (0, 0, _("%s: File removed before we read it"),
 902                 quotearg_colon (name)));
 903       set_exit_status (TAREXIT_DIFFERS);
 904     }
 905   else
 906     diagfn (name);
 907 }
 908
 909 void
 910 write_fatal_details (char const *name, ssize_t status, size_t size)
 911 {
 912   write_error_details (name, status, size);
 913   fatal_exit ();
 914 }
 915
 916 /* Fork, aborting if unsuccessful.  */
 917 pid_t
 918 xfork (void)
 919 {
 920   pid_t p = fork ();
 921   if (p == (pid_t) -1)
 922     call_arg_fatal ("fork", _("child process"));
 923   return p;
 924 }
 925
 926 /* Create a pipe, aborting if unsuccessful.  */
 927 void
 928 xpipe (int fd[2])
 929 {
 930   if (pipe (fd) < 0)
 931     call_arg_fatal ("pipe", _("interprocess channel"));
 932 }
 933
 934 /* Return PTR, aligned upward to the next multiple of ALIGNMENT.
 935    ALIGNMENT must be nonzero.  The caller must arrange for ((char *)
 936    PTR) through ((char *) PTR + ALIGNMENT - 1) to be addressable
 937    locations.  */
 938
 939 static inline void *
 940 ptr_align (void *ptr, size_t alignment)
 941 {
 942   char *p0 = ptr;
 943   char *p1 = p0 + alignment - 1;
 944   return p1 - (size_t) p1 % alignment;
 945 }
 946
 947 /* Return the address of a page-aligned buffer of at least SIZE bytes.
 948    The caller should free *PTR when done with the buffer.  */
 949
 950 void *
 951 page_aligned_alloc (void **ptr, size_t size)
 952 {
 953   size_t alignment = getpagesize ();
 954   size_t size1 = size + alignment;
 955   if (size1 < size)
 956     xalloc_die ();
 957   *ptr = xmalloc (size1);
 958   return ptr_align (*ptr, alignment);
 959 }
 960
 961 \f
 962
 963 struct namebuf
 964 {
 965   char *buffer;         /* directory, `/', and directory member */
 966   size_t buffer_size;   /* allocated size of name_buffer */
 967   size_t dir_length;    /* length of directory part in buffer */
 968 };
 969
 970 namebuf_t
 971 namebuf_create (const char *dir)
 972 {
 973   namebuf_t buf = xmalloc (sizeof (*buf));
 974   buf->buffer_size = strlen (dir) + 2;
 975   buf->buffer = xmalloc (buf->buffer_size);
 976   strcpy (buf->buffer, dir);
 977   buf->dir_length = strlen (buf->buffer);
 978   if (!ISSLASH (buf->buffer[buf->dir_length - 1]))
 979     buf->buffer[buf->dir_length++] = DIRECTORY_SEPARATOR;
 980   return buf;
 981 }
 982
 983 void
 984 namebuf_free (namebuf_t buf)
 985 {
 986   free (buf->buffer);
 987   free (buf);
 988 }
 989
 990 char *
 991 namebuf_name (namebuf_t buf, const char *name)
 992 {
 993   size_t len = strlen (name);
 994   while (buf->dir_length + len + 1 >= buf->buffer_size)
 995     buf->buffer = x2realloc (buf->buffer, &buf->buffer_size);
 996   strcpy (buf->buffer + buf->dir_length, name);
 997   return buf->buffer;
 998 }