IO::Compress::FAQ(3p) Perl Programmers Reference Guide IO::Compress::FAQ(3p) #
IO::Compress::FAQ(3p) Perl Programmers Reference Guide IO::Compress::FAQ(3p)
NNAAMMEE #
IO::Compress::FAQ -- Frequently Asked Questions about IO::Compress
DDEESSCCRRIIPPTTIIOONN #
Common questions answered.
GGEENNEERRAALL #
CCoommppaattiibbiilliittyy wwiitthh UUnniixx ccoommpprreessss//uunnccoommpprreessss.. Although “Compress::Zlib” has a pair of functions called “compress” and “uncompress”, they are _n_o_t related to the Unix programs of the same name. The “Compress::Zlib” module is not compatible with Unix “compress”.
If you have the "uncompress" program available, you can use this to read
compressed files
open F, "uncompress -c $filename |";
while (<F>)
{
...
Alternatively, if you have the "gunzip" program available, you can use
this to read compressed files
open F, "gunzip -c $filename |";
while (<F>)
{
...
and this to write compress files, if you have the "compress" program
available
open F, "| compress -c $filename ";
print F "data";
...
close F ;
AAcccceessssiinngg ..ttaarr..ZZ ffiilleess The “Archive::Tar” module can optionally use “Compress::Zlib” (via the “IO::Zlib” module) to access tar files that have been compressed with “gzip”. Unfortunately tar files compressed with the Unix “compress” utility cannot be read by “Compress::Zlib” and so cannot be directly accessed by “Archive::Tar”.
If the "uncompress" or "gunzip" programs are available, you can use one
of these workarounds to read ".tar.Z" files from "Archive::Tar"
Firstly with "uncompress"
use strict;
use warnings;
use Archive::Tar;
open F, "uncompress -c $filename |";
my $tar = Archive::Tar->new(*F);
...
and this with "gunzip"
use strict;
use warnings;
use Archive::Tar;
open F, "gunzip -c $filename |";
my $tar = Archive::Tar->new(*F);
...
Similarly, if the "compress" program is available, you can use this to
write a ".tar.Z" file
use strict;
use warnings;
use Archive::Tar;
use IO::File;
my $fh = IO::File->new( "| compress -c >$filename" );
my $tar = Archive::Tar->new();
...
$tar->write($fh);
$fh->close ;
HHooww ddoo II rreeccoommpprreessss uussiinngg aa ddiiffffeerreenntt ccoommpprreessssiioonn?? This is easier that you might expect if you realise that all the “IO::Compress::” objects are derived from “IO::File” and that all the “IO::Uncompress::” modules can read from an “IO::File” filehandle.
So, for example, say you have a file compressed with gzip that you want
to recompress with bzip2. Here is all that is needed to carry out the
recompression.
use IO::Uncompress::Gunzip ':all';
use IO::Compress::Bzip2 ':all';
my $gzipFile = "somefile.gz";
my $bzipFile = "somefile.bz2";
my $gunzip = IO::Uncompress::Gunzip->new( $gzipFile )
or die "Cannot gunzip $gzipFile: $GunzipError\n" ;
bzip2 $gunzip => $bzipFile
or die "Cannot bzip2 to $bzipFile: $Bzip2Error\n" ;
Note, there is a limitation of this technique. Some compression file
formats store extra information along with the compressed data payload.
For example, gzip can optionally store the original filename and Zip
stores a lot of information about the original file. If the original
compressed file contains any of this extra information, it will not be
transferred to the new compressed file using the technique above.
ZZIIPP #
WWhhaatt CCoommpprreessssiioonn TTyyppeess ddoo IIOO::::CCoommpprreessss::::ZZiipp && IIOO::::UUnnccoommpprreessss::::UUnnzziipp ssuuppppoorrtt?? The following compression formats are supported by “IO::Compress::Zip” and “IO::Uncompress::Unzip”
• Store (method 0)
No compression at all.
• Deflate (method 8)
This is the default compression used when creating a zip file with
"IO::Compress::Zip".
• Bzip2 (method 12)
Only supported if the "IO-Compress-Bzip2" module is installed.
• Lzma (method 14)
Only supported if the "IO-Compress-Lzma" module is installed.
CCaann II RReeaadd//WWrriittee ZZiipp ffiilleess llaarrggeerr tthhee 44 GGiigg?? Yes, both the “IO-Compress-Zip” and “IO-Uncompress-Unzip” modules support the zip feature called _Z_i_p_6_4. That allows them to read/write files/buffers larger than 4Gig.
If you are creating a Zip file using the one-shot interface, and any of
the input files is greater than 4Gig, a zip64 complaint zip file will be
created.
zip "really-large-file" => "my.zip";
Similarly with the one-shot interface, if the input is a buffer larger
than 4 Gig, a zip64 complaint zip file will be created.
zip \$really_large_buffer => "my.zip";
The one-shot interface allows you to force the creation of a zip64 zip
file by including the "Zip64" option.
zip $filehandle => "my.zip", Zip64 => 1;
If you want to create a zip64 zip file with the OO interface you must
specify the "Zip64" option.
my $zip = IO::Compress::Zip->new( "whatever", Zip64 => 1 );
When uncompressing with "IO-Uncompress-Unzip", it will automatically
detect if the zip file is zip64.
If you intend to manipulate the Zip64 zip files created with
"IO-Compress-Zip" using an external zip/unzip, make sure that it supports
Zip64.
In particular, if you are using Info-Zip you need to have zip version 3.x
or better to update a Zip64 archive and unzip version 6.x to read a zip64
archive.
CCaann II wwrriittee mmoorree tthhaatt 6644KK eennttrriieess iiss aa ZZiipp ffiilleess?? Yes. Zip64 allows this. See previous question.
ZZiipp RReessoouurrcceess The primary reference for zip files is the “appnote” document available at http://www.pkware.com/documents/casestudies/APPNOTE.TXT
An alternatively is the Info-Zip appnote. This is available from
<ftp://ftp.info-zip.org/pub/infozip/doc/>
GGZZIIPP #
GGzziipp RReessoouurrcceess The primary reference for gzip files is RFC 1952 https://datatracker.ietf.org/doc/html/rfc1952
The primary site for gzip is <http://www.gzip.org>.
DDeeaalliinngg wwiitthh ccoonnccaatteennaatteedd ggzziipp ffiilleess If the gunzip program encounters a file containing multiple gzip files concatenated together it will automatically uncompress them all. The example below illustrates this behaviour
$ echo abc | gzip -c >x.gz
$ echo def | gzip -c >>x.gz
$ gunzip -c x.gz
abc
def
By default "IO::Uncompress::Gunzip" will _n_o_t behave like the gunzip
program. It will only uncompress the first gzip data stream in the file,
as shown below
$ perl -MIO::Uncompress::Gunzip=:all -e 'gunzip "x.gz" => \*STDOUT'
abc
To force "IO::Uncompress::Gunzip" to uncompress all the gzip data
streams, include the "MultiStream" option, as shown below
$ perl -MIO::Uncompress::Gunzip=:all -e 'gunzip "x.gz" => \*STDOUT, MultiStream => 1'
abc
def
RReeaaddiinngg bbggzziipp ffiilleess wwiitthh IIOO::::UUnnccoommpprreessss::::GGuunnzziipp A “bgzip” file consists of a series of valid gzip-compliant data streams concatenated together. To read a file created by “bgzip” with “IO::Uncompress::Gunzip” use the “MultiStream” option as shown in the previous section.
See the section titled "The BGZF compression format" in
<http://samtools.github.io/hts-specs/SAMv1.pdf> for a definition of
"bgzip".
ZZLLIIBB #
ZZlliibb RReessoouurrcceess The primary site for the _z_l_i_b compression library is http://www.zlib.org.
BBzziipp22 BBzziipp22 RReessoouurrcceess The primary site for bzip2 is http://www.bzip.org.
DDeeaalliinngg wwiitthh CCoonnccaatteennaatteedd bbzziipp22 ffiilleess If the bunzip2 program encounters a file containing multiple bzip2 files concatenated together it will automatically uncompress them all. The example below illustrates this behaviour
$ echo abc | bzip2 -c >x.bz2
$ echo def | bzip2 -c >>x.bz2
$ bunzip2 -c x.bz2
abc
def
By default "IO::Uncompress::Bunzip2" will _n_o_t behave like the bunzip2
program. It will only uncompress the first bunzip2 data stream in the
file, as shown below
$ perl -MIO::Uncompress::Bunzip2=:all -e 'bunzip2 "x.bz2" => \*STDOUT'
abc
To force "IO::Uncompress::Bunzip2" to uncompress all the bzip2 data
streams, include the "MultiStream" option, as shown below
$ perl -MIO::Uncompress::Bunzip2=:all -e 'bunzip2 "x.bz2" => \*STDOUT, MultiStream => 1'
abc
def
IInntteerrooppeerraattiinngg wwiitthh PPbbzziipp22 Pbzip2 (http://compression.ca/pbzip2/) is a parallel implementation of bzip2. The output from pbzip2 consists of a series of concatenated bzip2 data streams.
By default "IO::Uncompress::Bzip2" will only uncompress the first bzip2
data stream in a pbzip2 file. To uncompress the complete pbzip2 file you
must include the "MultiStream" option, like this.
bunzip2 $input => \$output, MultiStream => 1
or die "bunzip2 failed: $Bunzip2Error\n";
HHTTTTPP && NNEETTWWOORRKK #
AAppaacchhee::::GGZZiipp RReevviissiitteedd Below is a mod_perl Apache compression module, called “Apache::GZip”, taken from http://perl.apache.org/docs/tutorials/tips/mod_perl_tricks/mod_perl_tricks.html#On_the_Fly_Compression
package Apache::GZip;
#File: Apache::GZip.pm
use strict vars;
use Apache::Constants ':common';
use Compress::Zlib;
use IO::File;
use constant GZIP_MAGIC => 0x1f8b;
use constant OS_MAGIC => 0x03;
sub handler {
my $r = shift;
my ($fh,$gz);
my $file = $r->filename;
return DECLINED unless $fh=IO::File->new($file);
$r->header_out('Content-Encoding'=>'gzip');
$r->send_http_header;
return OK if $r->header_only;
tie *STDOUT,'Apache::GZip',$r;
print($_) while <$fh>;
untie *STDOUT;
return OK;
}
sub TIEHANDLE {
my($class,$r) = @_;
# initialize a deflation stream
my $d = deflateInit(-WindowBits=>-MAX_WBITS()) || return undef;
# gzip header -- don't ask how I found out
$r->print(pack("nccVcc",GZIP_MAGIC,Z_DEFLATED,0,time(),0,OS_MAGIC));
return bless { r => $r,
crc => crc32(undef),
d => $d,
l => 0
},$class;
}
sub PRINT {
my $self = shift;
foreach (@_) {
# deflate the data
my $data = $self->{d}->deflate($_);
$self->{r}->print($data);
# keep track of its length and crc
$self->{l} += length($_);
$self->{crc} = crc32($_,$self->{crc});
}
}
sub DESTROY {
my $self = shift;
# flush the output buffers
my $data = $self->{d}->flush;
$self->{r}->print($data);
# print the CRC and the total length (uncompressed)
$self->{r}->print(pack("LL",@{$self}{qw/crc l/}));
}
1;
Here's the Apache configuration entry you'll need to make use of it.
Once set it will result in everything in the /compressed directory will
be compressed automagically.
<Location /compressed>
SetHandler perl-script
PerlHandler Apache::GZip
</Location>
Although at first sight there seems to be quite a lot going on in
"Apache::GZip", you could sum up what the code was doing as follows --
read the contents of the file in "$r->filename", compress it and write
the compressed data to standard output. That's all.
This code has to jump through a few hoops to achieve this because
1. The gzip support in "Compress::Zlib" version 1.x can only work with a
real filesystem filehandle. The filehandles used by Apache modules
are not associated with the filesystem.
2. That means all the gzip support has to be done by hand - in this case
by creating a tied filehandle to deal with creating the gzip header
and trailer.
"IO::Compress::Gzip" doesn't have that filehandle limitation (this was
one of the reasons for writing it in the first place). So if
"IO::Compress::Gzip" is used instead of "Compress::Zlib" the whole tied
filehandle code can be removed. Here is the rewritten code.
package Apache::GZip;
use strict vars;
use Apache::Constants ':common';
use IO::Compress::Gzip;
use IO::File;
sub handler {
my $r = shift;
my ($fh,$gz);
my $file = $r->filename;
return DECLINED unless $fh=IO::File->new($file);
$r->header_out('Content-Encoding'=>'gzip');
$r->send_http_header;
return OK if $r->header_only;
my $gz = IO::Compress::Gzip->new( '-', Minimal => 1 )
or return DECLINED ;
print $gz $_ while <$fh>;
return OK;
}
or even more succinctly, like this, using a one-shot gzip
package Apache::GZip;
use strict vars;
use Apache::Constants ':common';
use IO::Compress::Gzip qw(gzip);
sub handler {
my $r = shift;
$r->header_out('Content-Encoding'=>'gzip');
$r->send_http_header;
return OK if $r->header_only;
gzip $r->filename => '-', Minimal => 1
or return DECLINED ;
return OK;
}
1;
The use of one-shot "gzip" above just reads from "$r->filename" and
writes the compressed data to standard output.
Note the use of the "Minimal" option in the code above. When using gzip
for Content-Encoding you should _a_l_w_a_y_s use this option. In the example
above it will prevent the filename being included in the gzip header and
make the size of the gzip data stream a slight bit smaller.
CCoommpprreesssseedd ffiilleess aanndd NNeett::::FFTTPP The “Net::FTP” module provides two low-level methods called “stor” and “retr” that both return filehandles. These filehandles can used with the “IO::Compress/Uncompress” modules to compress or uncompress files read from or written to an FTP Server on the fly, without having to create a temporary file.
Firstly, here is code that uses "retr" to uncompressed a file as it is
read from the FTP Server.
use Net::FTP;
use IO::Uncompress::Gunzip qw(:all);
my $ftp = Net::FTP->new( ... )
my $retr_fh = $ftp->retr($compressed_filename);
gunzip $retr_fh => $outFilename, AutoClose => 1
or die "Cannot uncompress '$compressed_file': $GunzipError\n";
and this to compress a file as it is written to the FTP Server
use Net::FTP;
use IO::Compress::Gzip qw(:all);
my $stor_fh = $ftp->stor($filename);
gzip "filename" => $stor_fh, AutoClose => 1
or die "Cannot compress '$filename': $GzipError\n";
MMIISSCC #
UUssiinngg “"IInnppuuttLLeennggtthh"” ttoo uunnccoommpprreessss ddaattaa eemmbbeeddddeedd iinn aa llaarrggeerr ffiillee//bbuuffffeerr.. A fairly common use-case is where compressed data is embedded in a larger file/buffer and you want to read both.
As an example consider the structure of a zip file. This is a well-
defined file format that mixes both compressed and uncompressed sections
of data in a single file.
For the purposes of this discussion you can think of a zip file as
sequence of compressed data streams, each of which is prefixed by an
uncompressed local header. The local header contains information about
the compressed data stream, including the name of the compressed file
and, in particular, the length of the compressed data stream.
To illustrate how to use "InputLength" here is a script that walks a zip
file and prints out how many lines are in each compressed file (if you
intend write code to walking through a zip file for real see "Walking
through a zip file" in IO::Uncompress::Unzip ). Also, although this
example uses the zlib-based compression, the technique can be used by the
other "IO::Uncompress::*" modules.
use strict;
use warnings;
use IO::File;
use IO::Uncompress::RawInflate qw(:all);
use constant ZIP_LOCAL_HDR_SIG => 0x04034b50;
use constant ZIP_LOCAL_HDR_LENGTH => 30;
my $file = $ARGV[0] ;
my $fh = IO::File->new( "<$file" )
or die "Cannot open '$file': $!\n";
while (1)
{
my $sig;
my $buffer;
my $x ;
($x = $fh->read($buffer, ZIP_LOCAL_HDR_LENGTH)) == ZIP_LOCAL_HDR_LENGTH
or die "Truncated file: $!\n";
my $signature = unpack ("V", substr($buffer, 0, 4));
last unless $signature == ZIP_LOCAL_HDR_SIG;
# Read Local Header
my $gpFlag = unpack ("v", substr($buffer, 6, 2));
my $compressedMethod = unpack ("v", substr($buffer, 8, 2));
my $compressedLength = unpack ("V", substr($buffer, 18, 4));
my $uncompressedLength = unpack ("V", substr($buffer, 22, 4));
my $filename_length = unpack ("v", substr($buffer, 26, 2));
my $extra_length = unpack ("v", substr($buffer, 28, 2));
my $filename ;
$fh->read($filename, $filename_length) == $filename_length
or die "Truncated file\n";
$fh->read($buffer, $extra_length) == $extra_length
or die "Truncated file\n";
if ($compressedMethod != 8 && $compressedMethod != 0)
{
warn "Skipping file '$filename' - not deflated $compressedMethod\n";
$fh->read($buffer, $compressedLength) == $compressedLength
or die "Truncated file\n";
next;
}
if ($compressedMethod == 0 && $gpFlag & 8 == 8)
{
die "Streamed Stored not supported for '$filename'\n";
}
next if $compressedLength == 0;
# Done reading the Local Header
my $inf = IO::Uncompress::RawInflate->new( $fh,
Transparent => 1,
InputLength => $compressedLength )
or die "Cannot uncompress $file [$filename]: $RawInflateError\n" ;
my $line_count = 0;
while (<$inf>)
{
++ $line_count;
}
print "$filename: $line_count\n";
}
The majority of the code above is concerned with reading the zip local
header data. The code that I want to focus on is at the bottom.
while (1) {
# read local zip header data
# get $filename
# get $compressedLength
my $inf = IO::Uncompress::RawInflate->new( $fh,
Transparent => 1,
InputLength => $compressedLength )
or die "Cannot uncompress $file [$filename]: $RawInflateError\n" ;
my $line_count = 0;
while (<$inf>)
{
++ $line_count;
}
print "$filename: $line_count\n";
}
The call to "IO::Uncompress::RawInflate" creates a new filehandle $inf
that can be used to read from the parent filehandle $fh, uncompressing it
as it goes. The use of the "InputLength" option will guarantee that _a_t
_m_o_s_t $compressedLength bytes of compressed data will be read from the $fh
filehandle (The only exception is for an error case like a truncated file
or a corrupt data stream).
This means that once RawInflate is finished $fh will be left at the byte
directly after the compressed data stream.
Now consider what the code looks like without "InputLength"
while (1) {
# read local zip header data
# get $filename
# get $compressedLength
# read all the compressed data into $data
read($fh, $data, $compressedLength);
my $inf = IO::Uncompress::RawInflate->new( \$data,
Transparent => 1 )
or die "Cannot uncompress $file [$filename]: $RawInflateError\n" ;
my $line_count = 0;
while (<$inf>)
{
++ $line_count;
}
print "$filename: $line_count\n";
}
The difference here is the addition of the temporary variable $data.
This is used to store a copy of the compressed data while it is being
uncompressed.
If you know that $compressedLength isn't that big then using temporary
storage won't be a problem. But if $compressedLength is very large or you
are writing an application that other people will use, and so have no
idea how big $compressedLength will be, it could be an issue.
Using "InputLength" avoids the use of temporary storage and means the
application can cope with large compressed data streams.
One final point -- obviously "InputLength" can only be used whenever you
know the length of the compressed data beforehand, like here with a zip
file.
SSUUPPPPOORRTT #
General feedback/questions/bug reports should be sent to
<https://github.com/pmqs//issues> (preferred) or
<https://rt.cpan.org/Public/Dist/Display.html?Name=>.
SSEEEE AALLSSOO #
Compress::Zlib, IO::Compress::Gzip, IO::Uncompress::Gunzip,
IO::Compress::Deflate, IO::Uncompress::Inflate, IO::Compress::RawDeflate,
IO::Uncompress::RawInflate, IO::Compress::Bzip2, IO::Uncompress::Bunzip2,
IO::Compress::Lzma, IO::Uncompress::UnLzma, IO::Compress::Xz,
IO::Uncompress::UnXz, IO::Compress::Lzip, IO::Uncompress::UnLzip,
IO::Compress::Lzop, IO::Uncompress::UnLzop, IO::Compress::Lzf,
IO::Uncompress::UnLzf, IO::Compress::Zstd, IO::Uncompress::UnZstd,
IO::Uncompress::AnyInflate, IO::Uncompress::AnyUncompress
IO::Compress::FAQ
File::GlobMapper, Archive::Zip, Archive::Tar, IO::Zlib
AAUUTTHHOORR #
This module was written by Paul Marquess, "pmqs@cpan.org".
MMOODDIIFFIICCAATTIIOONN HHIISSTTOORRYY #
See the Changes file.
CCOOPPYYRRIIGGHHTT AANNDD LLIICCEENNSSEE #
Copyright (c) 2005-2022 Paul Marquess. All rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.36.3 2023-02-15 IO::Compress::FAQ(3p)