view toys/posix/tail.c @ 694:786841fdb1e0

Reindent to two spaces per level. Remove vi: directives that haven't worked right in years (ubuntu broke its' vim implementation). Remove trailing spaces. Add/remove blank lines. Re-wordwrap in places. Update documentation with new coding style. The actual code should be the same afterward, this is just cosmetic refactoring.
author Rob Landley <>
date Tue, 13 Nov 2012 17:14:08 -0600
parents 7e846e281e38
children ed026abefd63
line wrap: on
line source

/* tail.c - copy last lines from input to stdout.
 * Copyright 2012 Timothy Elliott <>
 * See


config TAIL
  bool "tail"
  default y
    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 X lines (default 10), +X counts from start.
    -c	output the last X bytes, +X counts from start
    -f	follow file, waiting for more data to be appended

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

#define FOR_tail
#include "toys.h"

  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);

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

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

  return line;

static void dump_chunk(void *ptr)
  struct line_list *list = ptr;
  xwrite(1, list->data, list->len);

// 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");
    if (!(temp = get_chunk(fd, chunk))) break;
    if (list) list->next = temp;
    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) {


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


  // 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;

  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));

    // Read data until we run out, keep a trailing buffer
    else for (;;) {
      int len, count;
      char *try;

      if (!(new = get_chunk(fd, sizeof(toybuf)))) break;
      // append in order
      dlist_add_nomalloc((struct double_list **)&list,
                 (struct double_list *)new);

      // Measure new chunk, discarding extra data from buffer
      len = new->len;
      try = new->data;
      for (count=0; count<len; count++) {
        if ((toys.optflags & FLAG_c) && bytes) {

        if (lines) {
          if(try[count] != '\n' && count != len-1) continue;
          if (lines<0) {
            if (!++lines) ++lines;

        // Time to discard data; given that bytes and lines were
        // nonzero coming in, we can't discard too much if we're
        // measuring right.
        do {
          char c = *(list->data++);
          if (!(--list->len)) {
            struct line_list *next = list->next;
            list->prev->next = next;
            list->next->prev = list->prev;
            list = next;
          if (c == '\n') break;
        } while (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) {
      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