view toys/posix/tail.c @ 1563:3d32f9523584 draft

Fix "tail -c 10" segfault spotted by Lukasz Szpakowski. Once we've read through the initial TT.bytes backlog we discard the extra data, meaning we adjust the remaining amount each time so the overflow is zero bytes. We were doing the adjustment right, but not zeroing out the overflow counter after we did so.
author Rob Landley <rob@landley.net>
date Wed, 19 Nov 2014 14:29:53 -0600
parents 1d996b0a11c0
children fa8f0a5dfc11
line wrap: on
line source

/* tail.c - copy last lines from input to stdout.
 *
 * Copyright 2012 Timothy Elliott <tle@holymonkey.com>
 *
 * See http://opengroup.org/onlinepubs/9699919799/utilities/tail.html

USE_TAIL(NEWTOY(tail, "fc-n-[-cn]", TOYFLAG_BIN))

config TAIL
  bool "tail"
  default y
  help
    usage: tail [-n|c NUMBER] [-f] [FILE...]

    Copy last lines from files to stdout. If no files listed, copy from
    stdin. Filename "-" is a synonym for stdin.

    -n	output the last NUMBER lines (default 10), +X counts from start.
    -c	output the last NUMBER bytes, +NUMBER counts from start
    -f	follow FILE(s), waiting for more data to be appended

config TAIL_SEEK
  bool "tail seek support"
  default y
  depends on TAIL
  help
    This version uses lseek, which is faster on large files.
*/

#define FOR_tail
#include "toys.h"

GLOBALS(
  long lines;
  long bytes;

  int file_no;
)

struct line_list {
  struct line_list *next, *prev;
  char *data;
  int len;
};

static struct line_list *get_chunk(int fd, int len)
{
  struct line_list *line = xmalloc(sizeof(struct line_list)+len);

  memset(line, 0, sizeof(struct line_list));
  line->data = ((char *)line) + sizeof(struct line_list);
  line->len = readall(fd, line->data, len);

  if (line->len < 1) {
    free(line);
    return 0;
  }

  return line;
}

static void dump_chunk(void *ptr)
{
  struct line_list *list = ptr;

  xwrite(1, list->data, list->len);
  free(list);
}

// Reading through very large files is slow.  Using lseek can speed things
// up a lot, but isn't applicable to all input (cat | tail).
// Note: bytes and lines are negative here.
static int try_lseek(int fd, long bytes, long lines)
{
  struct line_list *list = 0, *temp;
  int flag = 0, chunk = sizeof(toybuf);
  ssize_t pos = lseek(fd, 0, SEEK_END);

  // If lseek() doesn't work on this stream, return now.
  if (pos<0) return 0;

  // Seek to the right spot, output data from there.
  if (bytes) {
    if (lseek(fd, bytes, SEEK_END)<0) lseek(fd, 0, SEEK_SET);
    xsendfile(fd, 1);
    return 1;
  }

  // Read from end to find enough lines, then output them.

  bytes = pos;
  while (lines && pos) {
    int offset;

    // Read in next chunk from end of file
    if (chunk>pos) chunk = pos;
    pos -= chunk;
    if (pos != lseek(fd, pos, SEEK_SET)) {
      perror_msg("seek failed");
      break;
    }
    if (!(temp = get_chunk(fd, chunk))) break;
    temp->next = list;
    list = temp;

    // Count newlines in this chunk.
    offset = list->len;
    while (offset--) {
      // If the last line ends with a newline, that one doesn't count.
      if (!flag) flag++;

      // Start outputting data right after newline
      else if (list->data[offset] == '\n' && !++lines) {
        offset++;
        list->data += offset;
        list->len -= offset;

        break;
      }
    }
  }

  // Output stored data
  llist_traverse(list, dump_chunk);

  // In case of -f
  lseek(fd, bytes, SEEK_SET);
  return 1;
}

// Called for each file listed on command line, and/or stdin
static void do_tail(int fd, char *name)
{
  long bytes = TT.bytes, lines = TT.lines;
  int linepop = 1;

  if (toys.optc > 1) {
    if (TT.file_no++) xputc('\n');
    xprintf("==> %s <==\n", name);
  }

  // Are we measuring from the end of the file?

  if (bytes<0 || lines<0) {
    struct line_list *list = 0, *new;

    // The slow codepath is always needed, and can handle all input,
    // so make lseek support optional.
    if (CFG_TAIL_SEEK && try_lseek(fd, bytes, lines)) return;

    // Read data until we run out, keep a trailing buffer
    for (;;) {
      // Read next page of data, appending to linked list in order
      if (!(new = get_chunk(fd, sizeof(toybuf)))) break;
      dlist_add_nomalloc((void *)&list, (void *)new);

      // If tracing bytes, add until we have enough, discarding overflow.
      if (TT.bytes) {
        bytes += new->len;
        if (bytes > 0) {
          while (list->len <= bytes) {
            bytes -= list->len;
            free(dlist_pop(&list));
          }
          list->data += bytes;
          list->len -= bytes;
          bytes = 0;
        }
      } else {
        int len = new->len, count;
        char *try = new->data;

        // First character _after_ a newline starts a new line, which
        // works even if file doesn't end with a newline
        for (count=0; count<len; count++) {
          if (linepop) lines++;
          linepop = try[count] == '\n';

          if (lines > 0) {
            char c;

            do {
              c = *list->data;
              if (!--(list->len)) free(dlist_pop(&list));
              else list->data++;
            } while (c != '\n');
            lines--;
          }
        }
      }
    }

    // Output/free the buffer.
    llist_traverse(list, dump_chunk);

  // Measuring from the beginning of the file.
  } else for (;;) {
    int len, offset = 0;

    // Error while reading does not exit.  Error writing does.
    len = read(fd, toybuf, sizeof(toybuf));
    if (len<1) break;
    while (bytes > 1 || lines > 1) {
      bytes--;
      if (toybuf[offset++] == '\n') lines--;
      if (offset >= len) break;
    }
    if (offset<len) xwrite(1, toybuf+offset, len-offset);
  }

  // -f support: cache name/descriptor
}

void tail_main(void)
{
  // if nothing specified, default -n to -10
  if (!(toys.optflags&(FLAG_n|FLAG_c))) TT.lines = -10;

  loopfiles(toys.optargs, do_tail);

  // do -f stuff
}