IO::Co...ss::FAQ(3perl) Perl Programmers Reference GuideIO::Co...ss::FAQ(3perl)
NAME
IO::Compress::FAQ -- Frequently Asked Questions about IO::Compress
DESCRIPTION
Common questions answered.
GENERAL
Compatibility with Unix compress/uncompress.
Although "Compress::Zlib" has a pair of functions called "compress" and
"uncompress", they are not 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 ;
Accessing .tar.Z files
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 ;
How do I recompress using a different compression?
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.
ZIP
What Compression Types do IO::Compress::Zip & IO::Uncompress::Unzip support?
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.
Can I Read/Write Zip files larger the 4 Gig?
Yes, both the "IO-Compress-Zip" and "IO-Uncompress-Unzip" modules
support the zip feature called Zip64. 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.
Can I write more that 64K entries is a Zip files?
Yes. Zip64 allows this. See previous question.
Zip Resources
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/>
GZIP
Gzip Resources
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>.
Dealing with concatenated gzip files
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 not 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
Reading bgzip files with IO::Uncompress::Gunzip
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".
ZLIB
Zlib Resources
The primary site for the zlib compression library is
<http://www.zlib.org>.
Bzip2
Bzip2 Resources
The primary site for bzip2 is <http://www.bzip.org>.
Dealing with Concatenated bzip2 files
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 not 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
Interoperating with Pbzip2
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";
HTTP & NETWORK
Apache::GZip Revisited
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 always 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.
Compressed files and Net::FTP
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";
MISC
Using "InputLength" to uncompress data embedded in a larger file/buffer.
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
at most $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.
SUPPORT
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=>.
SEE ALSO
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
AUTHOR
This module was written by Paul Marquess, "pmqs@cpan.org".
MODIFICATION HISTORY
See the Changes file.
COPYRIGHT AND LICENSE
Copyright (c) 2005-2024 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 v5.40.1 2026-08-30 IO::Co...ss::FAQ(3perl)
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