summaryrefslogtreecommitdiff
path: root/src/head.c
blob: 7639ab97407b0643831606da0b47607ecfabd75b (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
/* head -- output first part of file(s)
   Copyright (C) 1989-2017 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 3 of the License, 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, see <http://www.gnu.org/licenses/>.  */

/* Options: (see usage)
   Reads from standard input if no files are given or when a filename of
   ''-'' is encountered.
   By default, filename headers are printed only if more than one file
   is given.
   By default, prints the first 10 lines (head -n 10).

   David MacKenzie <djm@gnu.ai.mit.edu> */

#include <config.h>

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

#include "system.h"

#include "die.h"
#include "error.h"
#include "full-read.h"
#include "quote.h"
#include "safe-read.h"
#include "stat-size.h"
#include "xfreopen.h"
#include "xdectoint.h"

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

#define AUTHORS \
  proper_name ("David MacKenzie"), \
  proper_name ("Jim Meyering")

/* Number of lines/chars/blocks to head. */
#define DEFAULT_NUMBER 10

/* Useful only when eliding tail bytes or lines.
   If true, skip the is-regular-file test used to determine whether
   to use the lseek optimization.  Instead, use the more general (and
   more expensive) code unconditionally. Intended solely for testing.  */
static bool presume_input_pipe;

/* If true, print filename headers. */
static bool print_headers;

/* Character to split lines by. */
static char line_end;

/* When to print the filename banners. */
enum header_mode
{
  multiple_files, always, never
};

/* Have we ever read standard input?  */
static bool have_read_stdin;

enum Copy_fd_status
  {
    COPY_FD_OK = 0,
    COPY_FD_READ_ERROR,
    COPY_FD_UNEXPECTED_EOF
  };

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

static struct option const long_options[] =
{
  {"bytes", required_argument, NULL, 'c'},
  {"lines", required_argument, NULL, 'n'},
  {"-presume-input-pipe", no_argument, NULL,
   PRESUME_INPUT_PIPE_OPTION}, /* do not document */
  {"quiet", no_argument, NULL, 'q'},
  {"silent", no_argument, NULL, 'q'},
  {"verbose", no_argument, NULL, 'v'},
  {"zero-terminated", no_argument, NULL, 'z'},
  {GETOPT_HELP_OPTION_DECL},
  {GETOPT_VERSION_OPTION_DECL},
  {NULL, 0, NULL, 0}
};

void
usage (int status)
{
  if (status != EXIT_SUCCESS)
    emit_try_help ();
  else
    {
      printf (_("\
Usage: %s [OPTION]... [FILE]...\n\
"),
              program_name);
      printf (_("\
Print the first %d lines of each FILE to standard output.\n\
With more than one FILE, precede each with a header giving the file name.\n\
"), DEFAULT_NUMBER);

      emit_stdin_note ();
      emit_mandatory_arg_note ();

      printf (_("\
  -c, --bytes=[-]NUM       print the first NUM bytes of each file;\n\
                             with the leading '-', print all but the last\n\
                             NUM bytes of each file\n\
  -n, --lines=[-]NUM       print the first NUM lines instead of the first %d;\n\
                             with the leading '-', print all but the last\n\
                             NUM lines of each file\n\
"), DEFAULT_NUMBER);
      fputs (_("\
  -q, --quiet, --silent    never print headers giving file names\n\
  -v, --verbose            always print headers giving file names\n\
"), stdout);
      fputs (_("\
  -z, --zero-terminated    line delimiter is NUL, not newline\n\
"), stdout);
      fputs (HELP_OPTION_DESCRIPTION, stdout);
      fputs (VERSION_OPTION_DESCRIPTION, stdout);
      fputs (_("\
\n\
NUM may have a multiplier suffix:\n\
b 512, kB 1000, K 1024, MB 1000*1000, M 1024*1024,\n\
GB 1000*1000*1000, G 1024*1024*1024, and so on for T, P, E, Z, Y.\n\
"), stdout);
      emit_ancillary_info (PROGRAM_NAME);
    }
  exit (status);
}

static void
diagnose_copy_fd_failure (enum Copy_fd_status err, char const *filename)
{
  switch (err)
    {
    case COPY_FD_READ_ERROR:
      error (0, errno, _("error reading %s"), quoteaf (filename));
      break;
    case COPY_FD_UNEXPECTED_EOF:
      error (0, errno, _("%s: file has shrunk too much"), quotef (filename));
      break;
    default:
      abort ();
    }
}

static void
write_header (const char *filename)
{
  static bool first_file = true;

  printf ("%s==> %s <==\n", (first_file ? "" : "\n"), filename);
  first_file = false;
}

/* Write N_BYTES from BUFFER to stdout.
   Exit immediately on error with a single diagnostic.  */

static void
xwrite_stdout (char const *buffer, size_t n_bytes)
{
  if (n_bytes > 0 && fwrite (buffer, 1, n_bytes, stdout) < n_bytes)
    {
      clearerr (stdout); /* To avoid redundant close_stdout diagnostic.  */
      die (EXIT_FAILURE, errno, _("error writing %s"),
           quoteaf ("standard output"));
    }
}

/* Copy no more than N_BYTES from file descriptor SRC_FD to stdout.
   Return an appropriate indication of success or read failure.  */

static enum Copy_fd_status
copy_fd (int src_fd, uintmax_t n_bytes)
{
  char buf[BUFSIZ];
  const size_t buf_size = sizeof (buf);

  /* Copy the file contents.  */
  while (0 < n_bytes)
    {
      size_t n_to_read = MIN (buf_size, n_bytes);
      size_t n_read = safe_read (src_fd, buf, n_to_read);
      if (n_read == SAFE_READ_ERROR)
        return COPY_FD_READ_ERROR;

      n_bytes -= n_read;

      if (n_read == 0 && n_bytes != 0)
        return COPY_FD_UNEXPECTED_EOF;

      xwrite_stdout (buf, n_read);
    }

  return COPY_FD_OK;
}

/* Call lseek (FD, OFFSET, WHENCE), where file descriptor FD
   corresponds to the file FILENAME.  WHENCE must be SEEK_SET or
   SEEK_CUR.  Return the resulting offset.  Give a diagnostic and
   return -1 if lseek fails.  */

static off_t
elseek (int fd, off_t offset, int whence, char const *filename)
{
  off_t new_offset = lseek (fd, offset, whence);
  char buf[INT_BUFSIZE_BOUND (offset)];

  if (new_offset < 0)
    error (0, errno,
           _(whence == SEEK_SET
             ? N_("%s: cannot seek to offset %s")
             : N_("%s: cannot seek to relative offset %s")),
           quotef (filename),
           offtostr (offset, buf));

  return new_offset;
}

/* For an input file with name FILENAME and descriptor FD,
   output all but the last N_ELIDE_0 bytes.
   If CURRENT_POS is nonnegative, assume that the input file is
   positioned at CURRENT_POS and that it should be repositioned to
   just before the elided bytes before returning.
   Return true upon success.
   Give a diagnostic and return false upon error.  */
static bool
elide_tail_bytes_pipe (const char *filename, int fd, uintmax_t n_elide_0,
                       off_t current_pos)
{
  size_t n_elide = n_elide_0;
  uintmax_t desired_pos = current_pos;
  bool ok = true;

#ifndef HEAD_TAIL_PIPE_READ_BUFSIZE
# define HEAD_TAIL_PIPE_READ_BUFSIZE BUFSIZ
#endif
#define READ_BUFSIZE HEAD_TAIL_PIPE_READ_BUFSIZE

  /* If we're eliding no more than this many bytes, then it's ok to allocate
     more memory in order to use a more time-efficient algorithm.
     FIXME: use a fraction of available memory instead, as in sort.
     FIXME: is this even worthwhile?  */
#ifndef HEAD_TAIL_PIPE_BYTECOUNT_THRESHOLD
# define HEAD_TAIL_PIPE_BYTECOUNT_THRESHOLD 1024 * 1024
#endif

#if HEAD_TAIL_PIPE_BYTECOUNT_THRESHOLD < 2 * READ_BUFSIZE
  "HEAD_TAIL_PIPE_BYTECOUNT_THRESHOLD must be at least 2 * READ_BUFSIZE"
#endif

  if (SIZE_MAX < n_elide_0 + READ_BUFSIZE)
    {
      char umax_buf[INT_BUFSIZE_BOUND (n_elide_0)];
      die (EXIT_FAILURE, 0, _("%s: number of bytes is too large"),
           umaxtostr (n_elide_0, umax_buf));
    }

  /* Two cases to consider...
     1) n_elide is small enough that we can afford to double-buffer:
        allocate 2 * (READ_BUFSIZE + n_elide) bytes
     2) n_elide is too big for that, so we allocate only
        (READ_BUFSIZE + n_elide) bytes

     FIXME: profile, to see if double-buffering is worthwhile

     CAUTION: do not fail (out of memory) when asked to elide
     a ridiculous amount, but when given only a small input.  */

  if (n_elide <= HEAD_TAIL_PIPE_BYTECOUNT_THRESHOLD)
    {
      bool first = true;
      bool eof = false;
      size_t n_to_read = READ_BUFSIZE + n_elide;
      bool i;
      char *b[2];
      b[0] = xnmalloc (2, n_to_read);
      b[1] = b[0] + n_to_read;

      for (i = false; ! eof ; i = !i)
        {
          size_t n_read = full_read (fd, b[i], n_to_read);
          size_t delta = 0;
          if (n_read < n_to_read)
            {
              if (errno != 0)
                {
                  error (0, errno, _("error reading %s"), quoteaf (filename));
                  ok = false;
                  break;
                }

              /* reached EOF */
              if (n_read <= n_elide)
                {
                  if (first)
                    {
                      /* The input is no larger than the number of bytes
                         to elide.  So there's nothing to output, and
                         we're done.  */
                    }
                  else
                    {
                      delta = n_elide - n_read;
                    }
                }
              eof = true;
            }

          /* Output any (but maybe just part of the) elided data from
             the previous round.  */
          if (! first)
            {
              desired_pos += n_elide - delta;
              xwrite_stdout (b[!i] + READ_BUFSIZE, n_elide - delta);
            }
          first = false;

          if (n_elide < n_read)
            {
              desired_pos += n_read - n_elide;
              xwrite_stdout (b[i], n_read - n_elide);
            }
        }

      free (b[0]);
    }
  else
    {
      /* Read blocks of size READ_BUFSIZE, until we've read at least n_elide
         bytes.  Then, for each new buffer we read, also write an old one.  */

      bool eof = false;
      size_t n_read;
      bool buffered_enough;
      size_t i, i_next;
      char **b = NULL;
      /* Round n_elide up to a multiple of READ_BUFSIZE.  */
      size_t rem = READ_BUFSIZE - (n_elide % READ_BUFSIZE);
      size_t n_elide_round = n_elide + rem;
      size_t n_bufs = n_elide_round / READ_BUFSIZE + 1;
      size_t n_alloc = 0;
      size_t n_array_alloc = 0;

      buffered_enough = false;
      for (i = 0, i_next = 1; !eof; i = i_next, i_next = (i_next + 1) % n_bufs)
        {
          if (n_array_alloc == i)
            {
              /* reallocate between 16 and n_bufs entries.  */
              if (n_array_alloc == 0)
                n_array_alloc = MIN (n_bufs, 16);
              else if (n_array_alloc <= n_bufs / 2)
                n_array_alloc *= 2;
              else
                n_array_alloc = n_bufs;
              b = xnrealloc (b, n_array_alloc, sizeof *b);
            }

          if (! buffered_enough)
            {
              b[i] = xmalloc (READ_BUFSIZE);
              n_alloc = i + 1;
            }
          n_read = full_read (fd, b[i], READ_BUFSIZE);
          if (n_read < READ_BUFSIZE)
            {
              if (errno != 0)
                {
                  error (0, errno, _("error reading %s"), quoteaf (filename));
                  ok = false;
                  goto free_mem;
                }
              eof = true;
            }

          if (i + 1 == n_bufs)
            buffered_enough = true;

          if (buffered_enough)
            {
              desired_pos += n_read;
              xwrite_stdout (b[i_next], n_read);
            }
        }

      /* Output any remainder: rem bytes from b[i] + n_read.  */
      if (rem)
        {
          if (buffered_enough)
            {
              size_t n_bytes_left_in_b_i = READ_BUFSIZE - n_read;
              desired_pos += rem;
              if (rem < n_bytes_left_in_b_i)
                {
                  xwrite_stdout (b[i] + n_read, rem);
                }
              else
                {
                  xwrite_stdout (b[i] + n_read, n_bytes_left_in_b_i);
                  xwrite_stdout (b[i_next], rem - n_bytes_left_in_b_i);
                }
            }
          else if (i + 1 == n_bufs)
            {
              /* This happens when n_elide < file_size < n_elide_round.

                 |READ_BUF.|
                 |                      |  rem |
                 |---------!---------!---------!---------|
                 |---- n_elide ---------|
                 |                      | x |
                 |                   |y |
                 |---- file size -----------|
                 |                   |n_read|
                 |---- n_elide_round ----------|
               */
              size_t y = READ_BUFSIZE - rem;
              size_t x = n_read - y;
              desired_pos += x;
              xwrite_stdout (b[i_next], x);
            }
        }

    free_mem:
      for (i = 0; i < n_alloc; i++)
        free (b[i]);
      free (b);
    }

  if (0 <= current_pos && elseek (fd, desired_pos, SEEK_SET, filename) < 0)
    ok = false;
  return ok;
}

/* For the file FILENAME with descriptor FD, output all but the last N_ELIDE
   bytes.  If SIZE is nonnegative, this is a regular file positioned
   at CURRENT_POS with SIZE bytes.  Return true on success.
   Give a diagnostic and return false upon error.  */

/* NOTE: if the input file shrinks by more than N_ELIDE bytes between
   the length determination and the actual reading, then head fails.  */

static bool
elide_tail_bytes_file (const char *filename, int fd, uintmax_t n_elide,
                       struct stat const *st, off_t current_pos)
{
  off_t size = st->st_size;
  if (presume_input_pipe || current_pos < 0 || size <= ST_BLKSIZE (*st))
    return elide_tail_bytes_pipe (filename, fd, n_elide, current_pos);
  else
    {
      /* Be careful here.  The current position may actually be
         beyond the end of the file.  */
      off_t diff = size - current_pos;
      off_t bytes_remaining = diff < 0 ? 0 : diff;

      if (bytes_remaining <= n_elide)
        return true;

      enum Copy_fd_status err = copy_fd (fd, bytes_remaining - n_elide);
      if (err == COPY_FD_OK)
        return true;

      diagnose_copy_fd_failure (err, filename);
      return false;
    }
}

/* For an input file with name FILENAME and descriptor FD,
   output all but the last N_ELIDE_0 bytes.
   If CURRENT_POS is nonnegative, the input file is positioned there
   and should be repositioned to just before the elided bytes.
   Buffer the specified number of lines as a linked list of LBUFFERs,
   adding them as needed.  Return true if successful.  */

static bool
elide_tail_lines_pipe (const char *filename, int fd, uintmax_t n_elide,
                       off_t current_pos)
{
  struct linebuffer
  {
    char buffer[BUFSIZ];
    size_t nbytes;
    size_t nlines;
    struct linebuffer *next;
  };
  uintmax_t desired_pos = current_pos;
  typedef struct linebuffer LBUFFER;
  LBUFFER *first, *last, *tmp;
  size_t total_lines = 0;	/* Total number of newlines in all buffers.  */
  bool ok = true;
  size_t n_read;		/* Size in bytes of most recent read */

  first = last = xmalloc (sizeof (LBUFFER));
  first->nbytes = first->nlines = 0;
  first->next = NULL;
  tmp = xmalloc (sizeof (LBUFFER));

  /* Always read into a fresh buffer.
     Read, (producing no output) until we've accumulated at least
     n_elide newlines, or until EOF, whichever comes first.  */
  while (1)
    {
      n_read = safe_read (fd, tmp->buffer, BUFSIZ);
      if (n_read == 0 || n_read == SAFE_READ_ERROR)
        break;

      if (! n_elide)
        {
          desired_pos += n_read;
          xwrite_stdout (tmp->buffer, n_read);
          continue;
        }

      tmp->nbytes = n_read;
      tmp->nlines = 0;
      tmp->next = NULL;

      /* Count the number of newlines just read.  */
      {
        char const *buffer_end = tmp->buffer + n_read;
        char const *p = tmp->buffer;
        while ((p = memchr (p, line_end, buffer_end - p)))
          {
            ++p;
            ++tmp->nlines;
          }
      }
      total_lines += tmp->nlines;

      /* If there is enough room in the last buffer read, just append the new
         one to it.  This is because when reading from a pipe, 'n_read' can
         often be very small.  */
      if (tmp->nbytes + last->nbytes < BUFSIZ)
        {
          memcpy (&last->buffer[last->nbytes], tmp->buffer, tmp->nbytes);
          last->nbytes += tmp->nbytes;
          last->nlines += tmp->nlines;
        }
      else
        {
          /* If there's not enough room, link the new buffer onto the end of
             the list, then either free up the oldest buffer for the next
             read if that would leave enough lines, or else malloc a new one.
             Some compaction mechanism is possible but probably not
             worthwhile.  */
          last = last->next = tmp;
          if (n_elide < total_lines - first->nlines)
            {
              desired_pos += first->nbytes;
              xwrite_stdout (first->buffer, first->nbytes);
              tmp = first;
              total_lines -= first->nlines;
              first = first->next;
            }
          else
            tmp = xmalloc (sizeof (LBUFFER));
        }
    }

  free (tmp);

  if (n_read == SAFE_READ_ERROR)
    {
      error (0, errno, _("error reading %s"), quoteaf (filename));
      ok = false;
      goto free_lbuffers;
    }

  /* If we read any bytes at all, count the incomplete line
     on files that don't end with a newline.  */
  if (last->nbytes && last->buffer[last->nbytes - 1] != line_end)
    {
      ++last->nlines;
      ++total_lines;
    }

  for (tmp = first; n_elide < total_lines - tmp->nlines; tmp = tmp->next)
    {
      desired_pos += tmp->nbytes;
      xwrite_stdout (tmp->buffer, tmp->nbytes);
      total_lines -= tmp->nlines;
    }

  /* Print the first 'total_lines - n_elide' lines of tmp->buffer.  */
  if (n_elide < total_lines)
    {
      size_t n = total_lines - n_elide;
      char const *buffer_end = tmp->buffer + tmp->nbytes;
      char const *p = tmp->buffer;
      while (n && (p = memchr (p, line_end, buffer_end - p)))
        {
          ++p;
          ++tmp->nlines;
          --n;
        }
      desired_pos += p - tmp->buffer;
      xwrite_stdout (tmp->buffer, p - tmp->buffer);
    }

free_lbuffers:
  while (first)
    {
      tmp = first->next;
      free (first);
      first = tmp;
    }

  if (0 <= current_pos && elseek (fd, desired_pos, SEEK_SET, filename) < 0)
    ok = false;
  return ok;
}

/* Output all but the last N_LINES lines of the input stream defined by
   FD, START_POS, and SIZE.
   START_POS is the starting position of the read pointer for the file
   associated with FD (may be nonzero).
   SIZE is the file size in bytes.
   Return true upon success.
   Give a diagnostic and return false upon error.

   NOTE: this code is very similar to that of tail.c's file_lines function.
   Unfortunately, factoring out some common core looks like it'd result
   in a less efficient implementation or a messy interface.  */
static bool
elide_tail_lines_seekable (const char *pretty_filename, int fd,
                           uintmax_t n_lines,
                           off_t start_pos, off_t size)
{
  char buffer[BUFSIZ];
  size_t bytes_read;
  off_t pos = size;

  /* Set 'bytes_read' to the size of the last, probably partial, buffer;
     0 < 'bytes_read' <= 'BUFSIZ'.  */
  bytes_read = (pos - start_pos) % BUFSIZ;
  if (bytes_read == 0)
    bytes_read = BUFSIZ;
  /* Make 'pos' a multiple of 'BUFSIZ' (0 if the file is short), so that all
     reads will be on block boundaries, which might increase efficiency.  */
  pos -= bytes_read;
  if (elseek (fd, pos, SEEK_SET, pretty_filename) < 0)
    return false;
  bytes_read = safe_read (fd, buffer, bytes_read);
  if (bytes_read == SAFE_READ_ERROR)
    {
      error (0, errno, _("error reading %s"), quoteaf (pretty_filename));
      return false;
    }

  /* n_lines == 0 case needs special treatment. */
  const bool all_lines = !n_lines;

  /* Count the incomplete line on files that don't end with a newline.  */
  if (n_lines && bytes_read && buffer[bytes_read - 1] != line_end)
    --n_lines;

  while (1)
    {
      /* Scan backward, counting the newlines in this bufferfull.  */

      size_t n = bytes_read;
      while (n)
        {
          if (all_lines)
            n -= 1;
          else
            {
              char const *nl;
              nl = memrchr (buffer, line_end, n);
              if (nl == NULL)
                break;
              n = nl - buffer;
            }
          if (n_lines-- == 0)
            {
              /* Found it.  */
              /* If necessary, restore the file pointer and copy
                 input to output up to position, POS.  */
              if (start_pos < pos)
                {
                  enum Copy_fd_status err;
                  if (elseek (fd, start_pos, SEEK_SET, pretty_filename) < 0)
                    return false;

                  err = copy_fd (fd, pos - start_pos);
                  if (err != COPY_FD_OK)
                    {
                      diagnose_copy_fd_failure (err, pretty_filename);
                      return false;
                    }
                }

              /* Output the initial portion of the buffer
                 in which we found the desired newline byte.  */
              xwrite_stdout (buffer, n + 1);

              /* Set file pointer to the byte after what we've output.  */
              return 0 <= elseek (fd, pos + n + 1, SEEK_SET, pretty_filename);
            }
        }

      /* Not enough newlines in that bufferfull.  */
      if (pos == start_pos)
        {
          /* Not enough lines in the file.  */
          return true;
        }
      pos -= BUFSIZ;
      if (elseek (fd, pos, SEEK_SET, pretty_filename) < 0)
        return false;

      bytes_read = safe_read (fd, buffer, BUFSIZ);
      if (bytes_read == SAFE_READ_ERROR)
        {
          error (0, errno, _("error reading %s"), quoteaf (pretty_filename));
          return false;
        }

      /* FIXME: is this dead code?
         Consider the test, pos == start_pos, above. */
      if (bytes_read == 0)
        return true;
    }
}

/* For the file FILENAME with descriptor FD, output all but the last N_ELIDE
   lines.  If SIZE is nonnegative, this is a regular file positioned
   at START_POS with SIZE bytes.  Return true on success.
   Give a diagnostic and return nonzero upon error.  */

static bool
elide_tail_lines_file (const char *filename, int fd, uintmax_t n_elide,
                       struct stat const *st, off_t current_pos)
{
  off_t size = st->st_size;
  if (presume_input_pipe || current_pos < 0 || size <= ST_BLKSIZE (*st))
    return elide_tail_lines_pipe (filename, fd, n_elide, current_pos);
  else
    {
      /* Find the offset, OFF, of the Nth newline from the end,
         but not counting the last byte of the file.
         If found, write from current position to OFF, inclusive.
         Otherwise, just return true.  */

      return (size <= current_pos
              || elide_tail_lines_seekable (filename, fd, n_elide,
                                            current_pos, size));
    }
}

static bool
head_bytes (const char *filename, int fd, uintmax_t bytes_to_write)
{
  char buffer[BUFSIZ];
  size_t bytes_to_read = BUFSIZ;

  while (bytes_to_write)
    {
      size_t bytes_read;
      if (bytes_to_write < bytes_to_read)
        bytes_to_read = bytes_to_write;
      bytes_read = safe_read (fd, buffer, bytes_to_read);
      if (bytes_read == SAFE_READ_ERROR)
        {
          error (0, errno, _("error reading %s"), quoteaf (filename));
          return false;
        }
      if (bytes_read == 0)
        break;
      xwrite_stdout (buffer, bytes_read);
      bytes_to_write -= bytes_read;
    }
  return true;
}

static bool
head_lines (const char *filename, int fd, uintmax_t lines_to_write)
{
  char buffer[BUFSIZ];

  while (lines_to_write)
    {
      size_t bytes_read = safe_read (fd, buffer, BUFSIZ);
      size_t bytes_to_write = 0;

      if (bytes_read == SAFE_READ_ERROR)
        {
          error (0, errno, _("error reading %s"), quoteaf (filename));
          return false;
        }
      if (bytes_read == 0)
        break;
      while (bytes_to_write < bytes_read)
        if (buffer[bytes_to_write++] == line_end && --lines_to_write == 0)
          {
            off_t n_bytes_past_EOL = bytes_read - bytes_to_write;
            /* If we have read more data than that on the specified number
               of lines, try to seek back to the position we would have
               gotten to had we been reading one byte at a time.  */
            if (lseek (fd, -n_bytes_past_EOL, SEEK_CUR) < 0)
              {
                struct stat st;
                if (fstat (fd, &st) != 0 || S_ISREG (st.st_mode))
                  elseek (fd, -n_bytes_past_EOL, SEEK_CUR, filename);
              }
            break;
          }
      xwrite_stdout (buffer, bytes_to_write);
    }
  return true;
}

static bool
head (const char *filename, int fd, uintmax_t n_units, bool count_lines,
      bool elide_from_end)
{
  if (print_headers)
    write_header (filename);

  if (elide_from_end)
    {
      off_t current_pos = -1;
      struct stat st;
      if (fstat (fd, &st) != 0)
        {
          error (0, errno, _("cannot fstat %s"),
                 quoteaf (filename));
          return false;
        }
      if (! presume_input_pipe && usable_st_size (&st))
        {
          current_pos = elseek (fd, 0, SEEK_CUR, filename);
          if (current_pos < 0)
            return false;
        }
      if (count_lines)
        return elide_tail_lines_file (filename, fd, n_units, &st, current_pos);
      else
        return elide_tail_bytes_file (filename, fd, n_units, &st, current_pos);
    }
  if (count_lines)
    return head_lines (filename, fd, n_units);
  else
    return head_bytes (filename, fd, n_units);
}

static bool
head_file (const char *filename, uintmax_t n_units, bool count_lines,
           bool elide_from_end)
{
  int fd;
  bool ok;
  bool is_stdin = STREQ (filename, "-");

  if (is_stdin)
    {
      have_read_stdin = true;
      fd = STDIN_FILENO;
      filename = _("standard input");
      if (O_BINARY && ! isatty (STDIN_FILENO))
        xfreopen (NULL, "rb", stdin);
    }
  else
    {
      fd = open (filename, O_RDONLY | O_BINARY);
      if (fd < 0)
        {
          error (0, errno, _("cannot open %s for reading"), quoteaf (filename));
          return false;
        }
    }

  ok = head (filename, fd, n_units, count_lines, elide_from_end);
  if (!is_stdin && close (fd) != 0)
    {
      error (0, errno, _("failed to close %s"), quoteaf (filename));
      return false;
    }
  return ok;
}

/* Convert a string of decimal digits, N_STRING, with an optional suffix
   to an integral value.  Upon successful conversion,
   return that value.  If it cannot be converted, give a diagnostic and exit.
   COUNT_LINES indicates whether N_STRING is a number of bytes or a number
   of lines.  It is used solely to give a more specific diagnostic.  */

static uintmax_t
string_to_integer (bool count_lines, const char *n_string)
{
  return xdectoumax (n_string, 0, UINTMAX_MAX, "bkKmMGTPEZY0",
                     count_lines ? _("invalid number of lines")
                                 : _("invalid number of bytes"), 0);
}

int
main (int argc, char **argv)
{
  enum header_mode header_mode = multiple_files;
  bool ok = true;
  int c;
  size_t i;

  /* Number of items to print. */
  uintmax_t n_units = DEFAULT_NUMBER;

  /* If true, interpret the numeric argument as the number of lines.
     Otherwise, interpret it as the number of bytes.  */
  bool count_lines = true;

  /* Elide the specified number of lines or bytes, counting from
     the end of the file.  */
  bool elide_from_end = false;

  /* Initializer for file_list if no file-arguments
     were specified on the command line.  */
  static char const *const default_file_list[] = {"-", NULL};
  char const *const *file_list;

  initialize_main (&argc, &argv);
  set_program_name (argv[0]);
  setlocale (LC_ALL, "");
  bindtextdomain (PACKAGE, LOCALEDIR);
  textdomain (PACKAGE);

  atexit (close_stdout);

  have_read_stdin = false;

  print_headers = false;

  line_end = '\n';

  if (1 < argc && argv[1][0] == '-' && ISDIGIT (argv[1][1]))
    {
      char *a = argv[1];
      char *n_string = ++a;
      char *end_n_string;
      char multiplier_char = 0;

      /* Old option syntax; a dash, one or more digits, and one or
         more option letters.  Move past the number. */
      do ++a;
      while (ISDIGIT (*a));

      /* Pointer to the byte after the last digit.  */
      end_n_string = a;

      /* Parse any appended option letters. */
      for (; *a; a++)
        {
          switch (*a)
            {
            case 'c':
              count_lines = false;
              multiplier_char = 0;
              break;

            case 'b':
            case 'k':
            case 'm':
              count_lines = false;
              multiplier_char = *a;
              break;

            case 'l':
              count_lines = true;
              break;

            case 'q':
              header_mode = never;
              break;

            case 'v':
              header_mode = always;
              break;

            case 'z':
              line_end = '\0';
              break;

            default:
              error (0, 0, _("invalid trailing option -- %c"), *a);
              usage (EXIT_FAILURE);
            }
        }

      /* Append the multiplier character (if any) onto the end of
         the digit string.  Then add NUL byte if necessary.  */
      *end_n_string = multiplier_char;
      if (multiplier_char)
        *(++end_n_string) = 0;

      n_units = string_to_integer (count_lines, n_string);

      /* Make the options we just parsed invisible to getopt. */
      argv[1] = argv[0];
      argv++;
      argc--;
    }

  while ((c = getopt_long (argc, argv, "c:n:qvz0123456789", long_options, NULL))
         != -1)
    {
      switch (c)
        {
        case PRESUME_INPUT_PIPE_OPTION:
          presume_input_pipe = true;
          break;

        case 'c':
          count_lines = false;
          elide_from_end = (*optarg == '-');
          if (elide_from_end)
            ++optarg;
          n_units = string_to_integer (count_lines, optarg);
          break;

        case 'n':
          count_lines = true;
          elide_from_end = (*optarg == '-');
          if (elide_from_end)
            ++optarg;
          n_units = string_to_integer (count_lines, optarg);
          break;

        case 'q':
          header_mode = never;
          break;

        case 'v':
          header_mode = always;
          break;

        case 'z':
          line_end = '\0';
          break;

        case_GETOPT_HELP_CHAR;

        case_GETOPT_VERSION_CHAR (PROGRAM_NAME, AUTHORS);

        default:
          if (ISDIGIT (c))
            error (0, 0, _("invalid trailing option -- %c"), c);
          usage (EXIT_FAILURE);
        }
    }

  if (header_mode == always
      || (header_mode == multiple_files && optind < argc - 1))
    print_headers = true;

  if ( ! count_lines && elide_from_end && OFF_T_MAX < n_units)
    {
      char umax_buf[INT_BUFSIZE_BOUND (n_units)];
      die (EXIT_FAILURE, EOVERFLOW, "%s: %s", _("invalid number of bytes"),
           quote (umaxtostr (n_units, umax_buf)));
    }

  file_list = (optind < argc
               ? (char const *const *) &argv[optind]
               : default_file_list);

  if (O_BINARY && ! isatty (STDOUT_FILENO))
    xfreopen (NULL, "wb", stdout);

  for (i = 0; file_list[i]; ++i)
    ok &= head_file (file_list[i], n_units, count_lines, elide_from_end);

  if (have_read_stdin && close (STDIN_FILENO) < 0)
    die (EXIT_FAILURE, errno, "-");

  return ok ? EXIT_SUCCESS : EXIT_FAILURE;
}