src/tac.c

   1 /* tac - concatenate and print files in reverse
   2    Copyright (C) 1988-2024 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation, either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 /* Written by Jay Lepreau (lepreau@cs.utah.edu).
  18    GNU enhancements by David MacKenzie (djm@gnu.ai.mit.edu). */
  19
  20 /* Copy each FILE, or the standard input if none are given or when a
  21    FILE name of "-" is encountered, to the standard output with the
  22    order of the records reversed.  The records are separated by
  23    instances of a string, or a newline if none is given.  By default, the
  24    separator string is attached to the end of the record that it
  25    follows in the file.
  26
  27    Options:
  28    -b, --before                 The separator is attached to the beginning
  29                                 of the record that it precedes in the file.
  30    -r, --regex                  The separator is a regular expression.
  31    -s, --separator=separator    Use SEPARATOR as the record separator.
  32
  33    To reverse a file byte by byte, use (in bash, ksh, or sh):
  34 tac -r -s '.\|
  35 ' file */
  36
  37 #include <config.h>
  38
  39 #include <stdio.h>
  40 #include <getopt.h>
  41 #include <sys/types.h>
  42 #include "system.h"
  43
  44 #include <regex.h>
  45
  46 #include "filenamecat.h"
  47 #include "full-read.h"
  48 #include "safe-read.h"
  49 #include "temp-stream.h"
  50 #include "xbinary-io.h"
  51
  52 /* The official name of this program (e.g., no 'g' prefix).  */
  53 #define PROGRAM_NAME "tac"
  54
  55 #define AUTHORS \
  56   proper_name ("Jay Lepreau"), \
  57   proper_name ("David MacKenzie")
  58
  59
  60 /* The number of bytes per atomic read. */
  61 #define INITIAL_READSIZE 8192
  62
  63 /* The number of bytes per atomic write. */
  64 #define WRITESIZE 8192
  65
  66 /* The string that separates the records of the file. */
  67 static char const *separator;
  68
  69 /* True if we have ever read standard input.  */
  70 static bool have_read_stdin = false;
  71
  72 /* If true, print 'separator' along with the record preceding it
  73    in the file; otherwise with the record following it. */
  74 static bool separator_ends_record;
  75
  76 /* 0 if 'separator' is to be matched as a regular expression;
  77    otherwise, the length of 'separator', used as a sentinel to
  78    stop the search. */
  79 static size_t sentinel_length;
  80
  81 /* The length of a match with 'separator'.  If 'sentinel_length' is 0,
  82    'match_length' is computed every time a match succeeds;
  83    otherwise, it is simply the length of 'separator'. */
  84 static size_t match_length;
  85
  86 /* The input buffer. */
  87 static char *G_buffer;
  88
  89 /* The number of bytes to read at once into 'buffer'. */
  90 static size_t read_size;
  91
  92 /* The size of 'buffer'.  This is read_size * 2 + sentinel_length + 2.
  93    The extra 2 bytes allow 'past_end' to have a value beyond the
  94    end of 'G_buffer' and 'match_start' to run off the front of 'G_buffer'. */
  95 static size_t G_buffer_size;
  96
  97 /* The compiled regular expression representing 'separator'. */
  98 static struct re_pattern_buffer compiled_separator;
  99 static char compiled_separator_fastmap[UCHAR_MAX + 1];
 100 static struct re_registers regs;
 101
 102 static struct option const longopts[] =
 103 {
 104   {"before", no_argument, nullptr, 'b'},
 105   {"regex", no_argument, nullptr, 'r'},
 106   {"separator", required_argument, nullptr, 's'},
 107   {GETOPT_HELP_OPTION_DECL},
 108   {GETOPT_VERSION_OPTION_DECL},
 109   {nullptr, 0, nullptr, 0}
 110 };
 111
 112 void
 113 usage (int status)
 114 {
 115   if (status != EXIT_SUCCESS)
 116     emit_try_help ();
 117   else
 118     {
 119       printf (_("\
 120 Usage: %s [OPTION]... [FILE]...\n\
 121 "),
 122               program_name);
 123       fputs (_("\
 124 Write each FILE to standard output, last line first.\n\
 125 "), stdout);
 126
 127       emit_stdin_note ();
 128       emit_mandatory_arg_note ();
 129
 130       fputs (_("\
 131   -b, --before             attach the separator before instead of after\n\
 132   -r, --regex              interpret the separator as a regular expression\n\
 133   -s, --separator=STRING   use STRING as the separator instead of newline\n\
 134 "), stdout);
 135       fputs (HELP_OPTION_DESCRIPTION, stdout);
 136       fputs (VERSION_OPTION_DESCRIPTION, stdout);
 137       emit_ancillary_info (PROGRAM_NAME);
 138     }
 139   exit (status);
 140 }
 141
 142 /* Print the characters from START to PAST_END - 1.
 143    If START is null, just flush the buffer. */
 144
 145 static void
 146 output (char const *start, char const *past_end)
 147 {
 148   static char buffer[WRITESIZE];
 149   static size_t bytes_in_buffer = 0;
 150   size_t bytes_to_add = past_end - start;
 151   size_t bytes_available = WRITESIZE - bytes_in_buffer;
 152
 153   if (start == 0)
 154     {
 155       fwrite (buffer, 1, bytes_in_buffer, stdout);
 156       bytes_in_buffer = 0;
 157       return;
 158     }
 159
 160   /* Write out as many full buffers as possible. */
 161   while (bytes_to_add >= bytes_available)
 162     {
 163       memcpy (buffer + bytes_in_buffer, start, bytes_available);
 164       bytes_to_add -= bytes_available;
 165       start += bytes_available;
 166       fwrite (buffer, 1, WRITESIZE, stdout);
 167       bytes_in_buffer = 0;
 168       bytes_available = WRITESIZE;
 169     }
 170
 171   memcpy (buffer + bytes_in_buffer, start, bytes_to_add);
 172   bytes_in_buffer += bytes_to_add;
 173 }
 174
 175 /* Print in reverse the file open on descriptor FD for reading FILE.
 176    The file is already positioned at FILE_POS, which should be near its end.
 177    Return true if successful.  */
 178
 179 static bool
 180 tac_seekable (int input_fd, char const *file, off_t file_pos)
 181 {
 182   /* Pointer to the location in 'G_buffer' where the search for
 183      the next separator will begin. */
 184   char *match_start;
 185
 186   /* Pointer to one past the rightmost character in 'G_buffer' that
 187      has not been printed yet. */
 188   char *past_end;
 189
 190   /* Length of the record growing in 'G_buffer'. */
 191   ptrdiff_t saved_record_size;
 192
 193   /* True if 'output' has not been called yet for any file.
 194      Only used when the separator is attached to the preceding record. */
 195   bool first_time = true;
 196   char first_char = *separator; /* Speed optimization, non-regexp. */
 197   char const *separator1 = separator + 1; /* Speed optimization, non-regexp. */
 198   size_t match_length1 = match_length - 1; /* Speed optimization, non-regexp. */
 199
 200   /* Arrange for the first read to lop off enough to leave the rest of the
 201      file a multiple of 'read_size'.  Since 'read_size' can change, this may
 202      not always hold during the program run, but since it usually will, leave
 203      it here for i/o efficiency (page/sector boundaries and all that).
 204      Note: the efficiency gain has not been verified. */
 205   size_t remainder = file_pos % read_size;
 206   if (remainder != 0)
 207     {
 208       file_pos -= remainder;
 209       if (lseek (input_fd, file_pos, SEEK_SET) < 0)
 210         error (0, errno, _("%s: seek failed"), quotef (file));
 211     }
 212
 213   /* Scan backward, looking for end of file.  This caters to proc-like
 214      file systems where the file size is just an estimate.  */
 215   while ((saved_record_size = safe_read (input_fd, G_buffer, read_size)) == 0
 216          && file_pos != 0)
 217     {
 218       off_t rsize = read_size;
 219       if (lseek (input_fd, -rsize, SEEK_CUR) < 0)
 220         error (0, errno, _("%s: seek failed"), quotef (file));
 221       file_pos -= read_size;
 222     }
 223
 224   /* Now scan forward, looking for end of file.  */
 225   while (saved_record_size == read_size)
 226     {
 227       ptrdiff_t nread = safe_read (input_fd, G_buffer, read_size);
 228       if (nread == 0)
 229         break;
 230       saved_record_size = nread;
 231       if (saved_record_size < 0)
 232         break;
 233       file_pos += nread;
 234     }
 235
 236   if (saved_record_size < 0)
 237     {
 238       error (0, errno, _("%s: read error"), quotef (file));
 239       return false;
 240     }
 241
 242   match_start = past_end = G_buffer + saved_record_size;
 243   /* For non-regexp search, move past impossible positions for a match. */
 244   if (sentinel_length)
 245     match_start -= match_length1;
 246
 247   while (true)
 248     {
 249       /* Search backward from 'match_start' - 1 to 'G_buffer' for a match
 250          with 'separator'; for speed, use strncmp if 'separator' contains no
 251          metacharacters.
 252          If the match succeeds, set 'match_start' to point to the start of
 253          the match and 'match_length' to the length of the match.
 254          Otherwise, make 'match_start' < 'G_buffer'. */
 255       if (sentinel_length == 0)
 256         {
 257           size_t i = match_start - G_buffer;
 258           regoff_t ri = i;
 259           regoff_t range = 1 - ri;
 260           regoff_t ret;
 261
 262           if (1 < range)
 263             error (EXIT_FAILURE, 0, _("record too large"));
 264
 265           if (range == 1
 266               || ((ret = re_search (&compiled_separator, G_buffer,
 267                                     i, i - 1, range, &regs))
 268                   == -1))
 269             match_start = G_buffer - 1;
 270           else if (ret == -2)
 271             error (EXIT_FAILURE, 0,
 272                    _("error in regular expression search"));
 273           else
 274             {
 275               match_start = G_buffer + regs.start[0];
 276               match_length = regs.end[0] - regs.start[0];
 277             }
 278         }
 279       else
 280         {
 281           /* 'match_length' is constant for non-regexp boundaries. */
 282           while (*--match_start != first_char
 283                  || (match_length1 && !STREQ_LEN (match_start + 1, separator1,
 284                                                   match_length1)))
 285             /* Do nothing. */ ;
 286         }
 287
 288       /* Check whether we backed off the front of 'G_buffer' without finding
 289          a match for 'separator'. */
 290       if (match_start < G_buffer)
 291         {
 292           if (file_pos == 0)
 293             {
 294               /* Hit the beginning of the file; print the remaining record. */
 295               output (G_buffer, past_end);
 296               return true;
 297             }
 298
 299           saved_record_size = past_end - G_buffer;
 300           if (saved_record_size > read_size)
 301             {
 302               /* 'G_buffer_size' is about twice 'read_size', so since
 303                  we want to read in another 'read_size' bytes before
 304                  the data already in 'G_buffer', we need to increase
 305                  'G_buffer_size'. */
 306               char *newbuffer;
 307               size_t offset = sentinel_length ? sentinel_length : 1;
 308               size_t old_G_buffer_size = G_buffer_size;
 309
 310               read_size *= 2;
 311               G_buffer_size = read_size * 2 + sentinel_length + 2;
 312               if (G_buffer_size < old_G_buffer_size)
 313                 xalloc_die ();
 314               newbuffer = xrealloc (G_buffer - offset, G_buffer_size);
 315               newbuffer += offset;
 316               G_buffer = newbuffer;
 317             }
 318
 319           /* Back up to the start of the next bufferfull of the file.  */
 320           if (file_pos >= read_size)
 321             file_pos -= read_size;
 322           else
 323             {
 324               read_size = file_pos;
 325               file_pos = 0;
 326             }
 327           if (lseek (input_fd, file_pos, SEEK_SET) < 0)
 328             error (0, errno, _("%s: seek failed"), quotef (file));
 329
 330           /* Shift the pending record data right to make room for the new.
 331              The source and destination regions probably overlap.  */
 332           memmove (G_buffer + read_size, G_buffer, saved_record_size);
 333           past_end = G_buffer + read_size + saved_record_size;
 334           /* For non-regexp searches, avoid unnecessary scanning. */
 335           if (sentinel_length)
 336             match_start = G_buffer + read_size;
 337           else
 338             match_start = past_end;
 339
 340           if (full_read (input_fd, G_buffer, read_size) != read_size)
 341             {
 342               error (0, errno, _("%s: read error"), quotef (file));
 343               return false;
 344             }
 345         }
 346       else
 347         {
 348           /* Found a match of 'separator'. */
 349           if (separator_ends_record)
 350             {
 351               char *match_end = match_start + match_length;
 352
 353               /* If this match of 'separator' isn't at the end of the
 354                  file, print the record. */
 355               if (!first_time || match_end != past_end)
 356                 output (match_end, past_end);
 357               past_end = match_end;
 358               first_time = false;
 359             }
 360           else
 361             {
 362               output (match_start, past_end);
 363               past_end = match_start;
 364             }
 365
 366           /* For non-regex matching, we can back up.  */
 367           if (sentinel_length > 0)
 368             match_start -= match_length - 1;
 369         }
 370     }
 371 }
 372
 373 /* Copy from file descriptor INPUT_FD (corresponding to the named FILE) to
 374    a temporary file, and set *G_TMP and *G_TEMPFILE to the resulting stream
 375    and file name.  Return the number of bytes copied, or -1 on error.  */
 376
 377 static off_t
 378 copy_to_temp (FILE **g_tmp, char **g_tempfile, int input_fd, char const *file)
 379 {
 380   FILE *fp;
 381   char *file_name;
 382   uintmax_t bytes_copied = 0;
 383   if (!temp_stream (&fp, &file_name))
 384     return -1;
 385
 386   while (true)
 387     {
 388       ptrdiff_t bytes_read = safe_read (input_fd, G_buffer, read_size);
 389       if (bytes_read == 0)
 390         break;
 391       if (bytes_read < 0)
 392         {
 393           error (0, errno, _("%s: read error"), quotef (file));
 394           return -1;
 395         }
 396
 397       if (fwrite (G_buffer, 1, bytes_read, fp) != bytes_read)
 398         {
 399           error (0, errno, _("%s: write error"), quotef (file_name));
 400           return -1;
 401         }
 402
 403       /* Implicitly <= OFF_T_MAX due to preceding fwrite(),
 404          but unsigned type used to avoid compiler warnings
 405          not aware of this fact.  */
 406       bytes_copied += bytes_read;
 407     }
 408
 409   if (fflush (fp) != 0)
 410     {
 411       error (0, errno, _("%s: write error"), quotef (file_name));
 412       return -1;
 413     }
 414
 415   *g_tmp = fp;
 416   *g_tempfile = file_name;
 417   return bytes_copied;
 418 }
 419
 420 /* Copy INPUT_FD to a temporary, then tac that file.
 421    Return true if successful.  */
 422
 423 static bool
 424 tac_nonseekable (int input_fd, char const *file)
 425 {
 426   FILE *tmp_stream;
 427   char *tmp_file;
 428   off_t bytes_copied = copy_to_temp (&tmp_stream, &tmp_file, input_fd, file);
 429   if (bytes_copied < 0)
 430     return false;
 431
 432   bool ok = tac_seekable (fileno (tmp_stream), tmp_file, bytes_copied);
 433   return ok;
 434 }
 435
 436 /* Print FILE in reverse, copying it to a temporary
 437    file first if it is not seekable.
 438    Return true if successful.  */
 439
 440 static bool
 441 tac_file (char const *filename)
 442 {
 443   bool ok;
 444   off_t file_size;
 445   int fd;
 446   bool is_stdin = STREQ (filename, "-");
 447
 448   if (is_stdin)
 449     {
 450       have_read_stdin = true;
 451       fd = STDIN_FILENO;
 452       filename = _("standard input");
 453       xset_binary_mode (STDIN_FILENO, O_BINARY);
 454     }
 455   else
 456     {
 457       fd = open (filename, O_RDONLY | O_BINARY);
 458       if (fd < 0)
 459         {
 460           error (0, errno, _("failed to open %s for reading"),
 461                  quoteaf (filename));
 462           return false;
 463         }
 464     }
 465
 466   file_size = lseek (fd, 0, SEEK_END);
 467
 468   ok = (file_size < 0 || isatty (fd)
 469         ? tac_nonseekable (fd, filename)
 470         : tac_seekable (fd, filename, file_size));
 471
 472   if (!is_stdin && close (fd) != 0)
 473     {
 474       error (0, errno, _("%s: read error"), quotef (filename));
 475       ok = false;
 476     }
 477   return ok;
 478 }
 479
 480 int
 481 main (int argc, char **argv)
 482 {
 483   char const *error_message;    /* Return value from re_compile_pattern. */
 484   int optc;
 485   bool ok;
 486   size_t half_buffer_size;
 487
 488   /* Initializer for file_list if no file-arguments
 489      were specified on the command line.  */
 490   static char const *const default_file_list[] = {"-", nullptr};
 491   char const *const *file;
 492
 493   initialize_main (&argc, &argv);
 494   set_program_name (argv[0]);
 495   setlocale (LC_ALL, "");
 496   bindtextdomain (PACKAGE, LOCALEDIR);
 497   textdomain (PACKAGE);
 498
 499   atexit (close_stdout);
 500
 501   separator = "\n";
 502   sentinel_length = 1;
 503   separator_ends_record = true;
 504
 505   while ((optc = getopt_long (argc, argv, "brs:", longopts, nullptr)) != -1)
 506     {
 507       switch (optc)
 508         {
 509         case 'b':
 510           separator_ends_record = false;
 511           break;
 512         case 'r':
 513           sentinel_length = 0;
 514           break;
 515         case 's':
 516           separator = optarg;
 517           break;
 518         case_GETOPT_HELP_CHAR;
 519         case_GETOPT_VERSION_CHAR (PROGRAM_NAME, AUTHORS);
 520         default:
 521           usage (EXIT_FAILURE);
 522         }
 523     }
 524
 525   if (sentinel_length == 0)
 526     {
 527       if (*separator == 0)
 528         error (EXIT_FAILURE, 0, _("separator cannot be empty"));
 529
 530       compiled_separator.buffer = nullptr;
 531       compiled_separator.allocated = 0;
 532       compiled_separator.fastmap = compiled_separator_fastmap;
 533       compiled_separator.translate = nullptr;
 534       error_message = re_compile_pattern (separator, strlen (separator),
 535                                           &compiled_separator);
 536       if (error_message)
 537         error (EXIT_FAILURE, 0, "%s", (error_message));
 538     }
 539   else
 540     match_length = sentinel_length = *separator ? strlen (separator) : 1;
 541
 542   read_size = INITIAL_READSIZE;
 543   while (sentinel_length >= read_size / 2)
 544     {
 545       if (SIZE_MAX / 2 < read_size)
 546         xalloc_die ();
 547       read_size *= 2;
 548     }
 549   half_buffer_size = read_size + sentinel_length + 1;
 550   G_buffer_size = 2 * half_buffer_size;
 551   if (! (read_size < half_buffer_size && half_buffer_size < G_buffer_size))
 552     xalloc_die ();
 553   G_buffer = xmalloc (G_buffer_size);
 554   if (sentinel_length)
 555     {
 556       memcpy (G_buffer, separator, sentinel_length + 1);
 557       G_buffer += sentinel_length;
 558     }
 559   else
 560     {
 561       ++G_buffer;
 562     }
 563
 564   file = (optind < argc
 565           ? (char const *const *) &argv[optind]
 566           : default_file_list);
 567
 568   xset_binary_mode (STDOUT_FILENO, O_BINARY);
 569
 570   {
 571     ok = true;
 572     for (size_t i = 0; file[i]; ++i)
 573       ok &= tac_file (file[i]);
 574   }
 575
 576   /* Flush the output buffer. */
 577   output ((char *) nullptr, (char *) nullptr);
 578
 579   if (have_read_stdin && close (STDIN_FILENO) < 0)
 580     {
 581       error (0, errno, "-");
 582       ok = false;
 583     }
 584
 585   main_exit (ok ? EXIT_SUCCESS : EXIT_FAILURE);
 586 }