ReadBackwards(3) User Contributed Perl Documentation ReadBackwards(3)NAME
File::ReadBackwards.pm -- Read a file backwards by lines.
SYNOPSIS
use File::ReadBackwards ;
# Object interface
$bw = File::ReadBackwards->new( 'log_file' ) or
die "can't read 'log_file' $!" ;
while( defined( $log_line = $bw->readline ) ) {
print $log_line ;
}
# ... or the alternative way of reading
until ( $bw->eof ) {
print $bw->readline ;
}
# Tied Handle Interface
tie *BW, 'File::ReadBackwards', 'log_file' or
die "can't read 'log_file' $!" ;
while( <BW> ) {
print ;
}
DESCRIPTION
This module reads a file backwards line by line. It is simple to use,
memory efficient and fast. It supports both an object and a tied handle
interface.
It is intended for processing log and other similar text files which
typically have their newest entries appended to them. By default files
are assumed to be plain text and have a line ending appropriate to the
OS. But you can set the input record separator string on a per file
basis.
OBJECT INTERFACE
These are the methods in "File::ReadBackwards"' object interface:
new( $file, [$rec_sep], [$sep_is_regex] )
"new" takes as arguments a filename, an optional record separator and
an optional flag that marks the record separator as a regular
expression. It either returns the object on a successful open or undef
upon failure. $! is set to the error code if any.
readline
"readline" takes no arguments and it returns the previous line in the
file or undef when there are no more lines in the file. If the file is
a non-seekable file (e.g. a pipe), then undef is returned.
getline
"getline" is an alias for the readline method. It is here for
compatibilty with the IO::* classes which has a getline method.
eof
"eof" takes no arguments and it returns true when readline() has
iterated through the whole file.
close
"close" takes no arguments and it closes the handle
tell
"tell" takes no arguments and it returns the current filehandle
position. This value may be used to seek() back to this position using
a normal file handle.
get_handle
"get_handle" takes no arguments and it returns the internal Perl
filehandle used by the File::ReadBackwards object. This handle may be
used to read the file forward. Its seek position will be set to the
position that is returned by the tell() method. Note that interleaving
forward and reverse reads may produce unpredictable results. The only
use supported at present is to read a file backward to a certain point,
then use 'handle' to extract the handle, and read forward from that
point.
TIED HANDLE INTERFACE
tie( *HANDLE, 'File::ReadBackwards', $file, [$rec_sep], [$sep_is_regex] )
The TIEHANDLE, READLINE, EOF, CLOSE and TELL methods are aliased to the
new, readline, eof, close and tell methods respectively so refer to
them for their arguments and API. Once you have tied a handle to
File::ReadBackwards the only I/O operation permissible is <> which will
read the previous line. You can call eof() and close() on the tied
handle as well. All other tied handle operations will generate an
unknown method error. Do not seek, write or perform any other
unsupported operations on the tied handle.
LINE AND RECORD ENDINGS
Since this module needs to use low level I/O for efficiency, it can't
portably seek and do block I/O without managing line ending
conversions. This module supports the default record separators of
normal line ending strings used by the OS. You can also set the
separator on a per file basis.
The record separator is a regular expression by default, which differs
from the behavior of $/.
Only if the record separator is not specified and it defaults to CR/LF
(e.g, VMS, redmondware) will it will be converted to a single newline.
Unix and MacOS files systems use only a single character for line
endings and the lines are left unchanged. This means that for native
text files, you should be able to process their lines backwards without
any problems with line endings. If you specify a record separator, no
conversions will be done and you will get the records as if you read
them in binary mode.
DESIGN
It works by reading a large (8kb) block of data from the end of the
file. It then splits them on the record separator and stores a list of
records in the object. Each call to readline returns the top record of
the list and if the list is empty it refills it by reading the previous
block from the file and splitting it. When the beginning of the file
is reached and there are no more lines, undef is returned. All
boundary conditions are handled correctly i.e. if there is a trailing
partial line (no newline) it will be the first line returned and lines
larger than the read buffer size are handled properly.
NOTES
There is no support for list context in either the object or tied
interfaces. If you want to slurp all of the lines into an array in
backwards order (and you don't care about memory usage) just do:
@back_lines = reverse <FH>.
This module is only intended to read one line at a time from the end of
a file to the beginning.
AUTHOR
Uri Guttman, uri@stemsystems.com
COPYRIGHT
Copyright (C) 2003 by Uri Guttman. All rights reserved. This program
is free software; you can redistribute it and/or modify it under the
same terms as Perl itself.
perl v5.14.0 2005-05-05 ReadBackwards(3)
