p5-File-Tail/File-Tail-1.3/Tail.pm

package File::Tail;

use strict;
use vars qw($VERSION @ISA @EXPORT @EXPORT_OK $Is_Win32);

$Is_Win32 = ($^O =~ /win32/i) ? 1 : 0;

require Exporter;

@ISA = qw(Exporter);
# Items to export into callers namespace by default. Note: do not export
# names by default without a very good reason. Use EXPORT_OK instead.
# Do not simply export all your public functions/methods/constants.
$VERSION = '1.3';


# Preloaded methods go here.

use FileHandle;
#use IO::Seekable; # does not define SEEK_SET in 5005.02
use File::stat;
use Carp;
use Time::HiRes qw ( time sleep ); #import hires microsecond timers

sub SEEK_SET   () {0;}
sub SEEK_CUR () {1;}
sub SEEK_END   () {2;}


sub interval {
    my $object=shift @_;
    if (@_) {
	$object->{interval}=shift;
	$object->{interval}=$object->{maxinterval} if
	    $object->{interval}>$object->{maxinterval};
    }
    $object->{interval};
}

sub logit {
    my $object=shift;
    my @call=caller(1);
    print # STDERR
#	time()." ".
	"\033[7m".
	    $call[3]." ".$object->{"input"}." ".join("",@_).
		"\033[0m".
		    "\n"
	if $object->debug;
}

sub adjustafter {
    my $self=shift;
    $self->{adjustafter}=shift if @_;
    return $self->{adjustafter};
}

sub debug {
    my $self=shift;
    $self->{"debug"}=shift if @_;
    return $self->{"debug"};
}

sub errmode {
    my($self, $mode) = @_;
    my($prev) = $self->{errormode};

    if (@_ >= 2) {
        ## Set the error mode.
	defined $mode or $mode = '';
	if (ref($mode) eq 'CODE') {
	    $self->{errormode} = $mode;
	} elsif (ref($mode) eq 'ARRAY') {
	    unless (ref($mode->[0]) eq 'CODE') {
		croak 'bad errmode: first item in list must be a code ref';
		$mode = 'die';
	    }
	    $self->{errormode} = $mode;
	} else {
	    $self->{errormode} = lc $mode;
	}
    }
     $prev;
}

sub errmsg {
    my($self, @errmsgs) = @_;
    my($prev) = $self->{errormsg};

    if (@_ > 0) {
        $self->{errormsg} = join '', @errmsgs;
    }

    $prev;
} # end sub errmsg


sub error {
    my($self, @errmsg) = @_;
    my(
       $errmsg,
       $func,
       $mode,
       @args,
       );

    if (@_ >= 1) {
        ## Put error message in the object.
        $errmsg = join '', @errmsg;
        $self->{"errormsg"} = $errmsg;

        ## Do the error action as described by error mode.
        $mode = $self->{"errormode"};
        if (ref($mode) eq 'CODE') {
            &$mode($errmsg);
            return;
        } elsif (ref($mode) eq 'ARRAY') {
            ($func, @args) = @$mode;
            &$func(@args);
            return;
        } elsif ($mode eq "return") {
            return;
	} elsif ($mode eq "warn") {
	    carp $errmsg;
        } else {  # die
	    croak $errmsg;
	}
    } else {
        return $self->{"errormsg"} ne '';
    }
} # end sub error


sub copy {
    my $self=shift;
    $self->{copy}=shift if @_;
    return $self->{copy};
}

sub tail {
    my $self=shift;
    $self->{"tail"}=shift if @_;
    return $self->{"tail"};
}

sub reset_tail {
    my $self=shift;
    $self->{reset_tail}=shift if @_;
    return $self->{reset_tail};
}

sub nowait {
    my $self=shift;
    $self->{nowait}=shift if @_;
    return $self->{nowait};
}

sub method {
    my $self=shift;
    $self->{method}=shift if @_;
    return $self->{method};
}

sub input {
    my $self=shift;
    $self->{input}=shift if @_;
    return $self->{input};
}

sub maxinterval {
    my $self=shift;
    $self->{maxinterval}=shift if @_;
    return $self->{maxinterval};
}

sub resetafter {
    my $self=shift;
    $self->{resetafter}=shift if @_;
    return $self->{resetafter};
}

sub ignore_nonexistant {
    my $self=shift;
    $self->{ignore_nonexistant}=shift if @_;
    return $self->{ignore_nonexistant};
}

sub name_changes {
    my $self=shift;
    $self->{name_changes_callback}=shift if @_;
    return $self->{name_changes_callback};
}

sub TIEHANDLE {
    my $ref=new(@_);
}

sub READLINE {
    $_[0]->read();
}

sub PRINT {
  $_[0]->error("PRINT makes no sense in File::Tail");
}

sub PRINTF {
  $_[0]->error("PRINTF makes no sense in File::Tail");
}

sub READ {
  $_[0]->error("READ not implemented in File::Tail -- use READLINE (<HANDLE>) instead");
}

sub GETC {
  $_[0]->error("GETC not (yet) implemented in File::Tail -- use READLINE (<HANDLE>) instead");
}

sub DESTROY {
  my($this) = $_[0];
  close($this->{"handle"}) if (defined($this) && defined($this->{'handle'}));
#  undef $_[0];
  return;
}

sub CLOSE {
    &DESTROY(@_);
}

sub new {
    my ($pkg)=shift @_;
    $pkg=ref($pkg) || $pkg;
    unless ($pkg) {
	$pkg="File::Tail";
    }
    my %params;
    if ($#_ == 0)  {
	$params{"name"}=$_[0];
    } else {
	if (($#_ % 2) != 1) {
	    croak "Odd number of parameters for new";
	    return;
	}
	%params=@_;
    }
    my $object = {};
    bless $object,$pkg;
    unless (defined($params{'name'})) {
	croak "No file name given. Pass filename as \"name\" parameter";
	return;
    }
    $object->input($params{'name'});
    $object->copy($params{'cname'});
    $object->method($params{'method'} || "tail");
    $object->{buffer}="";
    $object->maxinterval($params{'maxinterval'} || 60);
    $object->interval($params{'interval'} || 10);
    $object->adjustafter($params{'adjustafter'} || 10);
    $object->errmode($params{'errmode'} || "die");
    $object->resetafter($params{'resetafter'} ||
			 ($object->maxinterval*$object->adjustafter));
    $object->{"debug"}=($params{'debug'} || 0);
    $object->{"tail"}=($params{'tail'} || 0);
    $object->{"nowait"}=($params{'nowait'} || 0);
    $object->{"maxbuf"}=($params{'maxbuf'} || 16384);
    warn "maxbuf should be big enough to hold at least one longest probable line, and preferably several\n"
      unless $object->{"maxbuf"}>1024;
    $object->{"name_changes_callback"}=($params{'name_changes'} || undef);
    if (defined $params{'reset_tail'}) {
        $object->{"reset_tail"} = $params{'reset_tail'};
    } else {
        $object->{"reset_tail"} =  -1;
    }
    $object->{'ignore_nonexistant'}=($params{'ignore_nonexistant'} || 0);
    $object->{"lastread"}=0;
    $object->{"sleepcount"}=0;
    $object->{"lastcheck"}=0;
    $object->{"lastreset"}=0;
    $object->{"nextcheck"}=time();
    if ($object->{"method"} eq "tail") {
	$object->reset_pointers;
    }
#    $object->{curpos}=0;        # ADDED 25May01: undef warnings when
#    $object->{endpos}=0;        #   starting up on a nonexistant file
    return $object;
}

# Sets position in file when first opened or after that when reset:
# Sets {endpos} and {curpos} for current {handle} based on {tail}.
# Sets {tail} to value of {reset_tail}; effect is that first call
# uses {tail} and subsequent calls use {reset_tail}.
sub position {
  my $object=shift;
  $object->{"endpos"}=sysseek($object->{handle},0,SEEK_END);
  unless ($object->{"tail"}) {
    $object->{endpos}=$object->{curpos}=
      sysseek($object->{handle},0,SEEK_END);
  } elsif ($object->{"tail"}<0) {
    $object->{endpos}=sysseek($object->{handle},0,SEEK_END);
    $object->{curpos}=sysseek($object->{handle},0,SEEK_SET);
  } else {
    my $crs=0;
    my $maxlen=sysseek($object->{handle},0,SEEK_END);
    while ($crs<$object->{"tail"}+1) {
      my $avlen=length($object->{"buffer"})/($crs+1);
      $avlen=80 unless $avlen;
      my $calclen=$avlen*$object->{"tail"};
      $calclen=length($object->{buffer})+1024 if int($calclen)<=length($object->{"buffer"});
      $calclen=$maxlen if $calclen>$maxlen;
      $object->{curpos}=sysseek($object->{handle},-$calclen,SEEK_END);
      sysread($object->{handle},$object->{"buffer"},
              $calclen);
      $object->{"buffer"} =~ s/\015\012/\n/g if $Is_Win32;
      $object->{curpos}=sysseek($object->{handle},0,SEEK_CUR);
      $crs=$object->{"buffer"}=~tr/\n//;
      last if ($calclen>=$maxlen);
    }
    $object->{curpos}=sysseek($object->{handle},0,SEEK_CUR);
    $object->{endpos}=sysseek($object->{handle},0,SEEK_END);
    if ($crs>$object->{"tail"}) {
      my $toskip=$crs-$object->{"tail"};
      my $pos;
      $pos=index($object->{"buffer"},"\n");
      while (--$toskip) {
        $pos=index($object->{"buffer"},"\n",$pos+1);
      }
      $object->{"buffer"}=substr($object->{"buffer"},$pos+1);
    }
  }
  $object->{"tail"}=$object->{"reset_tail"};
}

# Tries to open or reopen the file; failure is an error unless
# {ignore_nonexistant} is set.
#
# For a new file (ie, first time opened) just does some book-keeping
# and calls position for initial position setup.  Otherwise does some
# checks whether file has been replaced, and if so changes to the new
# file.  (Calls position for reset setup).
#
# Always updates {lastreset} to current time.
#
sub reset_pointers {
    my $object=shift @_;
    $object->{lastreset} = time();

    my $st;

    my $oldhandle=$object->{handle};
    my $newhandle=FileHandle->new;

    my $newname;
    if ($oldhandle && $$object{'name_changes_callback'}) {
	$newname=$$object{'name_changes_callback'}();
        $object->{"input"}= $newname;
    } else {
	$newname=$object->input;
    }

    unless (open($newhandle,"<$newname")) {
	if ($object->{'ignore_nonexistant'}) {
         # If we have an oldhandle, leave endpos and curpos to what they
         # were, since oldhandle will still be the "current" handle elsewhere,
         # eg, checkpending.  This also allows tailing a file which is removed
         # but still being written to.
            if (!$oldhandle) {
                $object->{'endpos'}=0;
                $object->{'curpos'}=0;
            }
	    return;
	}
	$object->error("Error opening ".$object->input.": $!");
	$object->{'endpos'}=0 unless defined($object->{'endpos'});
	$object->{'curpos'}=0 unless defined($object->{'curpos'});
	return;
    }
    binmode($newhandle);

    if (defined($oldhandle)) {
	# If file has not been changed since last OK read do not do anything
	$st=stat($newhandle);
	# lastread uses fractional time, stat doesn't. This can cause false
        # negatives.
        # If the file was changed the same second as it was last read,
        # we only reopen it if it's length has changed. The alternative is that
        # sometimes, files would be reopened needlessly, and with reset_tail
	# set to -1, we would see the whole file again.
	# Of course, if the file was removed the same second as when it was
        # last read, and replaced (within that second) with a file of equal
        # length, we're out of luck. I don't see how to fix this.
	if ($st->mtime<=int($object->{'lastread'})) {
	    if (($st->size==$object->{"curpos"}) && ($st->ino == $object->{"inode"})) {
		$object->{lastread} = $st->mtime;
		return;
	    } else {
		# will continue further to reset
	    }
	} else {
	}
	$object->{handle}=$newhandle;
        $object->{inode} = $st->ino;
	$object->position;
	$object->{lastread} = $st->mtime;
	close($oldhandle);
    } else {                  # This is the first time we are opening this file
	$st=stat($newhandle);
	$object->{handle}=$newhandle;
	$object->position;
	$object->{lastread}=$st->mtime; # for better estimate on initial read
    }

}


sub checkpending {
   my $object=shift @_;

   my $old_lastcheck = $object->{lastcheck};
   $object->{"lastcheck"}=time;
   unless ($object->{handle}) {
       $object->reset_pointers;
       unless ($object->{handle}) { # This try did not open the file either
	   return 0;
       }
   }

   $object->{"endpos"}=sysseek($object->{handle},0,SEEK_END);
   if ($object->{"endpos"}<$object->{curpos}) {  # file was truncated
       $object->position;
   } elsif (($object->{curpos}==$object->{"endpos"})
	       && (time()-$object->{lastread})>$object->{'resetafter'}) {
       $object->reset_pointers;
       $object->{"endpos"}=sysseek($object->{handle},0,SEEK_END);
   }

   if ($object->{"endpos"}-$object->{curpos}) {
       sysseek($object->{handle},$object->{curpos},SEEK_SET);
       readin($object,$object->{"endpos"}-$object->{curpos});
   }
   return ($object->{"endpos"}-$object->{curpos});
}

sub predict {
    my $object=shift;
    my $crs=$object->{"buffer"}=~tr/\n//; # Count newlines in buffer
    my @call=caller(1);
    return 0 if $crs;
    my $ttw=$object->{"nextcheck"}-time();
    return $ttw if $ttw>0;
    if (my $len=$object->checkpending) {
	readin($object,$len);
	return 0;
    }
    if ($object->{"sleepcount"}>$object->adjustafter) {
	$object->{"sleepcount"}=0;
	$object->interval($object->interval*10);
    }
    $object->{"sleepcount"}++;
    $object->{"nextcheck"}=time()+$object->interval;
    return ($object->interval);
}

sub bitprint {
    return "undef" unless defined($_[0]);
    return unpack("b*",$_[0]);
}

sub select {
    my $object=shift @_  if ref($_[0]);
    my ($timeout,@fds)=splice(@_,3);
    $object=$fds[0] unless defined($object);
    my ($savein,$saveout,$saveerr)=@_;
    my ($minpred,$mustreturn);
    if (defined($timeout)) {
	$minpred=$timeout;
	$mustreturn=time()+$timeout;
    } else {
	$minpred=$fds[0]->predict;
    }
    foreach (@fds) {
	my $val=$_->predict;
	$minpred=$val if $minpred>$val;
    }
    my ($nfound,$timeleft);
    my @retarr;
    while (defined($timeout)?(!$nfound && (time()<$mustreturn)):!$nfound) {
# Restore bitmaps in case we called select before
	splice(@_,0,3,$savein,$saveout,$saveerr);


	($nfound,$timeleft)=select($_[0],$_[1],$_[2],$minpred);


	if (defined($timeout)) {
	    $minpred=$timeout;
	} else {
	    $minpred=$fds[0]->predict;
	}
	undef @retarr;
	foreach (@fds) {
	    my $val=$_->predict;
	    $nfound++ unless $val;
	    $minpred=$val if $minpred>$val;
	    push(@retarr,$_) unless $val;
	}
    }
    if (wantarray) {
	return ($nfound,$timeleft,@retarr);
    } else {
	return $nfound;
    }
}

sub readin {
    my $crs;
    my ($object,$len)=@_;
    if (length($object->{"buffer"})) {
	# this means the file was reset AND a tail -n was active
	$crs=$object->{"buffer"}=~tr/\n//; # Count newlines in buffer
	return $crs if $crs;
    }
    $len=$object->{"maxbuf"} if ($len>$object->{"maxbuf"});
    my $nlen=$len;
    while ($nlen>0) {
	$len=sysread($object->{handle},$object->{"buffer"},
		     $nlen,length($object->{"buffer"}));
        $object->{"buffer"} =~ s/\015\012/\n/g if $Is_Win32;

	last if $len==0; # Some busy filesystems return 0 sometimes,
                             # and never give anything more from then on if
                             # you don't give them time to rest. This return
                             # allows File::Tail to use the usual exponential
                             # backoff.
	$nlen=$nlen-$len;
    }
    $object->{curpos}=sysseek($object->{handle},0,SEEK_CUR);

    $crs=$object->{"buffer"}=~tr/\n//;

    if ($crs) {
	my $tmp=time;
	$object->{lastread}=$tmp if $object->{lastread}>$tmp; #???
	$object->interval(($tmp-($object->{lastread}))/$crs);
	$object->{lastread}=$tmp;
    }
    return ($crs);
}

sub read {
    my $object=shift @_;
    my $len;
    my $pending=$object->{"endpos"}-$object->{"curpos"};
    my $crs=$object->{"buffer"}=~m/\n/;
    while (!$pending && !$crs) {
	$object->{"sleepcount"}=0;
	while ($object->predict) {
	    if ($object->nowait) {
		if (wantarray) {
		    return ();
		} else {
		    return "";
		}
	    }
	    sleep($object->interval) if ($object->interval>0);
	}
	$pending=$object->{"endpos"}-$object->{"curpos"};
	$crs=$object->{"buffer"}=~m/\n/;
    }

    if (!length($object->{"buffer"}) || index($object->{"buffer"},"\n")<0) {
	readin($object,$pending);
    }
    unless (wantarray) {
	my $str=substr($object->{"buffer"},0,
		       1+index($object->{"buffer"},"\n"));
	$object->{"buffer"}=substr($object->{"buffer"},
				   1+index($object->{"buffer"},"\n"));
	return $str;
    } else {
	my @str;
	while (index($object->{"buffer"},"\n")>-1) {
	    push(@str,substr($object->{"buffer"},0,
			     1+index($object->{"buffer"},"\n")));
	    $object->{"buffer"}=substr($object->{"buffer"},
				       1+index($object->{"buffer"},"\n"));

	}
	return @str;
    }
}

1;

__END__

=head1 NAME

File::Tail - Perl extension for reading from continously updated files

=head1 SYNOPSIS

  use File::Tail;
  $file=File::Tail->new("/some/log/file");
  while (defined($line=$file->read)) {
      print "$line";
  }

  use File::Tail;
  $file=File::Tail->new(name=>$name, maxinterval=>300, adjustafter=>7);
  while (defined($line=$file->read)) {
      print "$line";
  }

OR, you could use tie (additional parameters can be passed with the name, or
can be set using $ref):

    use File::Tail;
    my $ref=tie *FH,"File::Tail",(name=>$name);
    while (<FH>) {
        print "$_";
    }


Note that the above script will never exit. If there is nothing being written
to the file, it will simply block.

You can find more synopsii in the file logwatch, which is included
in the distribution.

Note: Select functionality was added in version 0.9, and it required
some reworking of all routines. ***PLEASE*** let me know if you see anything
strange happening.

You can find two way of using select in the file select_demo which is included
in the ditribution.

=head1 DESCRIPTION

The primary purpose of File::Tail is reading and analysing log files while
they are being written, which is especialy usefull if you are monitoring
the logging process with a tool like Tobias Oetiker's MRTG.

The module tries very hard NOT to "busy-wait" on a file that has little
traffic. Any time it reads new data from the file, it counts the number
of new lines, and divides that number by the time that passed since data
were last written to the file before that. That is considered the average
time before new data will be written. When there is no new data to read,
C<File::Tail> sleeps for that number of seconds. Thereafter, the waiting
time is recomputed dynamicaly. Note that C<File::Tail> never sleeps for
more than the number of seconds set by C<maxinterval>.

If the file does not get altered for a while, C<File::Tail> gets suspicious
and startschecking if the file was truncated, or moved and recreated. If
anything like that had happened, C<File::Tail> will quietly reopen the file,
and continue reading. The only way to affect what happens on reopen is by
setting the reset_tail parameter (see below). The effect of this is that
the scripts need not be aware when the logfiles were rotated, they will
just quietly work on.

Note that the sleep and time used are from Time::HiRes, so this module
should do the right thing even if the time to sleep is less than one second.

The logwatch script (also included) demonstrates several ways of calling
the methods.

=head1 CONSTRUCTOR

=head2 new ([ ARGS ])

Creates a C<File::Tail>. If it has only one paramter, it is assumed to
be the filename. If the open fails, the module performs a croak. I
am currently looking for a way to set $! and return undef.

You can pass several parameters to new:

=over 4

=item name

This is the name of the file to open. The file will be opened for reading.
This must be a regular file, not a pipe or a terminal (i.e. it must be
seekable).

=item maxinterval

The maximum number of seconds (real number) that will be spent sleeping.
Default is 60, meaning C<File::Tail> will never spend more than sixty
seconds without checking the file.

=item interval

The initial number of seconds (real number) that will be spent sleeping,
before the file is first checked. Default is ten seconds, meaning C<File::Tail>
will sleep for 10 seconds and then determine, how many new lines have appeared
in the file.

=item adjustafter

The number of C<times> C<File::Tail> waits for the current interval,
before adjusting the interval upwards. The default is 10.

=item resetafter

The number of seconds after last change when C<File::Tail> decides
the file may have been closed and reopened. The default is
adjustafter*maxinterval.

=item maxbuf

The maximum size of the internal buffer. When File::Tail
suddenly found an enormous ammount of information in the file
(for instance if the retry parameters were set to very
infrequent checking and the file was rotated), File::Tail
sometimes slurped way too much file into memory.  This sets
the maximum size of File::Tail's buffer.

Default value is 16384 (bytes).

A large internal buffer may result in worse performance (as well as
increased memory usage), since File::Tail will have to do more work
processing the internal buffer.

=item nowait

Does not block on read, but returns an empty string if there is nothing
to read. DO NOT USE THIS unless you know what you are doing. If you
are using it in a loop, you probably DON'T know what you are doing.
If you want to read tails from multiple files, use select.


=item ignore_nonexistant

    Do not complain if the file doesn't exist when it is first
opened or when it is to be reopened. (File may be reopened after
resetafter seconds have passed since last data was found.)

=item tail

    When first started, read and return C<n> lines from the file.
If C<n> is zero, start at the end of file. If C<n> is negative,
return the whole file.

    Default is C<0>.

=item reset_tail

    Same as tail, but applies after reset. (i.e. after the
file has been automaticaly closed and reopened). Defaults to
C<-1>, i.e. does not skip any information present in the
file when it first checks it.

   Why would you want it otherwise? I've seen files which
have been cycled like this:

   grep -v lastmonth log >newlog
   mv log archive/lastmonth
   mv newlog log
   kill -HUP logger


Obviously, if this happens and you have reset_tail set to
c<-1>, you will suddenly get a whole bunch of lines - lines
you already saw. So in this case, reset_tail should probably
be set to a small positive number or even C<0>.

=item name_changes

Some logging systems change the name of the file
they are writing to, sometimes to include a date, sometimes a
sequence number, sometimes other, even more bizarre changes.

Instead of trying to implement various clever detection methods,
File::Tail will call the code reference defined in name_changes. The code reference should return the string which is the new name of the file to try opening.

Note that if the file does not exist, File::Tail will report a fatal error (unless ignore_nonexistant has also been specified).

=item debug

Set to nonzero if you want to see more about the inner workings of
File::Tail. Otherwise not useful.

=item errmode

Modeled after the methods from Net:Telnet, here you decide how the
errors should be handled. The parameter can be a code reference which
is called with the error string as a parameter, an array with a code
reference as the first parameter and other parameters to be passed to
handler subroutine, or one of the words:

return  - ignore any error (just put error message in errmsg).
warn    - output the error message but continue
die     - display error message and exit

Default is die.

=back

=head1 METHODS

=head2 read

C<read> returns one line from the input file. If there are no lines
ready, it blocks until there are.

=head2 select

C<select> is intended to enable the programmer to simoultaneously wait for
input on normal filehandles and File::Tail filehandles. Of course, you may
use it to simply read from more than one File::Tail filehandle at a time.


Basicaly, you call File::Tail::select just as you would normal select,
with fields for rbits, wbits and ebits, as well as a timeout, however, you
can tack any number of File::Tail objects (not File::Tail filehandles!)
to the end.

Usage example:

 foreach (@ARGV) {
     push(@files,File::Tail->new(name=>"$_",debug=>$debug));
 }
 while (1) {
   ($nfound,$timeleft,@pending)=
             File::Tail::select(undef,undef,undef,$timeout,@files);
   unless ($nfound) {
     # timeout - do something else here, if you need to
   } else {
     foreach (@pending) {
        print $_->{"input"}." (".localtime(time).") ".$_->read;
   }
 }

 #
 # There is a more elaborate example in select_demo in the distribution.
 #

When you do this, File::Tail's select emulates normal select, with two
exceptions:

a) it will return if there is input on any of the parameters
(i.e. normal filehandles) _or_ File::Tails.

b) In addition to C<($nfound, $timeleft)>, the return array will also contain
a list of File::Tail objects which are ready for reading. C<$nfound> will
contain the correct number of filehandles to be read (i.e. both normal
and File::Tails).

Once select returns, when you want to determine which File::Tail objects
have input ready, you can either use the list of objects select returned,
or you can check each individual object with $object->predict. This returns
the ammount of time (in fractional seconds) after which the handle expects
input. If it returns 0, there is input waiting. There is no guarantee that
there will be input waiting after the returned number of seconds has passed.
However, File::Tail won't do any I/O on the file until that time has passed.
Note that the value of $timeleft may or may not be correct - that depends on
the underlying operating system (and it's select), so you're better off NOT
relying on it.

Also note, if you are determining which files are ready for input by calling
each individual predict, the C<$nfound> value may be invalid, because one
or more of File::Tail object may have become ready between the time select
has returned and the time when you checked it.

=head1 TO BE DONE

Planned for 1.0: Using $/ instead of \n to
separate "lines" (which should make it possible to read wtmp type files).
Except that I discovered I have no need for that enhancement If you do,
feel free to send me the patches and I'll apply them - if I feel they don't
add too much processing time.

=head1 AUTHOR

Matija Grabnar, matija.grabnar@arnes.si

=head1 SEE ALSO

perl(1), tail (1),
MRTG (http://ee-staff.ethz.ch/~oetiker/webtools/mrtg/mrtg.html)

=cut