NAME

IO::Compress::Bzip2::FAQ -- Frequently Asked Questions about IO::Compress::Bzip2

DESCRIPTION

Common questions answered.

Compatibility with Unix compress/uncompress.

This 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

See previous FAQ item.

If the Archive::Tar module is installed and either the uncompress or gunzip programs are available, you can use one of these workarounds to read .tar.Z files.

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 $fh = new IO::File "| compress -c >$filename";
    my $tar = Archive::Tar->new();
    ...
    $tar->write($fh);
    $fh->close ;

Accessing Zip Files

This module does not support reading/writing zip files.

Support for reading/writing zip files is included with the IO::Compress::Zip and IO::Uncompress::Unzip modules.

The primary focus of the IO::Compress::Zip and IO::Uncompress::Unzip modules is to provide an IO::File compatible streaming read/write interface to zip files/buffers. They are not fully flegged archivers. If you are looking for an archiver check out the Archive::Zip module. You can find it on CPAN at

    http://www.cpan.org/modules/by-module/Archive/Archive-Zip-*.tar.gz

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::Bunzip2 qw(:all);

    my $ftp = new Net::FTP ...

    my $retr_fh = $ftp->retr($compressed_filename);
    bunzip2 $retr_fh => $outFilename, AutoClose => 1
        or die "Cannot uncompress '$compressed_file': $Bunzip2Error\n";

and this to compress a file as it is written to the FTP Server

    use Net::FTP;
    use IO::Compress::Bzip2 qw(:all);

    my $stor_fh = $ftp->stor($filename);
    bzip2 "filename" => $stor_fh, AutoClose => 1
        or die "Cannot compress '$filename': $Bzip2Error\n";

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 = new IO::Uncompress::Gunzip $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 usign the technique above.

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 )

    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 $fh = new IO::File "<$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 = new IO::Uncompress::RawInflate $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 = new IO::Uncompress::RawInflate $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 = new IO::Uncompress::RawInflate \$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.

AUTHOR

This module was written by Paul Marquess, pmqs@cpan.org.

MODIFICATION HISTORY

See the Changes file.

COPYRIGHT AND LICENSE

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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