tail.c

/* 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
 *
 * Deviations from posix: -f waits for pipe/fifo on stdin (nonblock?).

USE_TAIL(NEWTOY(tail, "?fFs:c(bytes)-n(lines)-[-cn][-fF]", TOYFLAG_USR|TOYFLAG_BIN|TOYFLAG_LINEBUF))

config TAIL
  bool "tail"
  default y
  help
    usage: tail [-n|c NUMBER] [-f|F] [-s SECONDS] [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) by descriptor, waiting for more data to be appended
    -F	Follow FILE(s) by filename, waiting for more data, and retrying
    -s	Used with -F, sleep SECONDS between retries (default 1)
*/

#define FOR_tail
#include "toys.h"

GLOBALS(
  long n, c;
  char *s;

  int file_no, last_fd, ss;
  struct xnotify *not;
  struct {
    char *path;
    int fd;
    struct dev_ino di;
  } *F;
)

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

static struct line_list *read_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 write_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);
  off_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 = read_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, write_chunk);

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

// For -f and -F
static void tail_continue()
{
  long long pos;
  char *path;
  struct stat sb;
  int i = 0, fd, len;

  for (i = 0; ; i++) {
    if (FLAG(f)) fd = xnotify_wait(TT.not, &path);
    else {
      if (i == TT.file_no) {
        i = 0;
        msleep(TT.ss);
      }
      fd = TT.F[i].fd;
      path = TT.F[i].path;

      if (stat(TT.F[i].path, &sb)) {
        if (fd >= 0) {
          close(fd);
          TT.F[i].fd = -1;
          error_msg("file inaccessible: %s\n", TT.F[i].path);
        }
        continue;
      }

      if (fd<0 || !same_dev_ino(&sb, &TT.F[i].di)) {
        if (fd>=0) close(fd);
        if (-1 == (TT.F[i].fd = fd = open(path, O_RDONLY))) continue;
        error_msg("following new file: %s\n", path);
        TT.F[i].di.dev = sb.st_dev;
        TT.F[i].di.ino = sb.st_ino;
      } else if (sb.st_size <= (pos = lseek(fd, 0, SEEK_CUR))) {
        if (pos == sb.st_size) continue;
        error_msg("file truncated: %s\n", path);
        lseek(fd, 0, SEEK_SET);
      }
    }

    while ((len = read(fd, toybuf, sizeof(toybuf)))>0) {
      if (TT.file_no>1 && TT.last_fd != fd) {
        TT.last_fd = fd;
        xprintf("\n==> %s <==\n", path);
      }
      xwrite(1, toybuf, len);
    }
  }
}

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

  if (FLAG(F)) {
    if (!fd) perror_exit("no -F with '-'");
  } else if (fd == -1) return;
  if (FLAG(f) || FLAG(F)) {
    char *s = name;
    struct stat sb;

    if (!fd) sprintf(s = toybuf, "/proc/self/fd/%d", fd);

    if (FLAG(f)) xnotify_add(TT.not, fd, s);
    if (FLAG(F)) {
      if (fd != -1) {
        if (fstat(fd, &sb)) perror_exit("%s", name);
        TT.F[TT.file_no].di.dev = sb.st_dev;
        TT.F[TT.file_no].di.ino = sb.st_ino;
      }
      TT.F[TT.file_no].fd = fd;
      TT.F[TT.file_no].path = s;
    }
  }

  if (TT.file_no++) xputc('\n');
  TT.last_fd = fd;
  if (toys.optc > 1) 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 (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 = read_chunk(fd, sizeof(toybuf)))) break;
      dlist_add_nomalloc((void *)&list, (void *)new);

      // If tracing bytes, add until we have enough, discarding overflow.
      if (TT.c) {
        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, write_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);
  }
}

void tail_main(void)
{
  char **args = toys.optargs;

  if (!FLAG(n) && !FLAG(c)) {
    char *arg = *args;

    // handle old "-42" / "+42" style arguments, else default to last 10 lines
    if (arg && (*arg == '-' || *arg == '+') && arg[1]) {
      TT.n = atolx(*(args++));
      toys.optc--;
    } else TT.n = -10;
  }

  if (FLAG(F)) TT.F = xzalloc(toys.optc*sizeof(*TT.F));
  else if (FLAG(f)) TT.not = xnotify_init(toys.optc);
  TT.ss = TT.s ? xparsemillitime(TT.s) : 1000;

  loopfiles_rw(args,
    O_RDONLY|WARN_ONLY|LOOPFILES_ANYWAY|O_CLOEXEC*!(FLAG(f) || FLAG(F)),
    0, do_tail);

  // Wait for more data when following files
  if (TT.file_no && (FLAG(F) || FLAG(f))) tail_continue();
}