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Compress::Raw::Zlib

Perl 5 version 10.1 documentation
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Compress::Raw::Zlib

NAME

Compress::Raw::Zlib - Low-Level Interface to zlib compression library

SYNOPSIS

  1.     use Compress::Raw::Zlib ;
  3.     ($d, $status) = new Compress::Raw::Zlib::Deflate( [OPT] ) ;
  4.     $status = $d->deflate($input, $output) ;
  5.     $status = $d->flush($output [, $flush_type]) ;
  6.     $d->deflateReset() ;
  7.     $d->deflateParams(OPTS) ;
  8.     $d->deflateTune(OPTS) ;
  9.     $d->dict_adler() ;
  10.     $d->crc32() ;
  11.     $d->adler32() ;
  12.     $d->total_in() ;
  13.     $d->total_out() ;
  14.     $d->msg() ;
  15.     $d->get_Strategy();
  16.     $d->get_Level();
  17.     $d->get_BufSize();
  19.     ($i, $status) = new Compress::Raw::Zlib::Inflate( [OPT] ) ;
  20.     $status = $i->inflate($input, $output [, $eof]) ;
  21.     $status = $i->inflateSync($input) ;
  22.     $i->dict_adler() ;
  23.     $d->crc32() ;
  24.     $d->adler32() ;
  25.     $i->total_in() ;
  26.     $i->total_out() ;
  27.     $i->msg() ;
  28.     $d->get_BufSize();
  30.     $crc = adler32($buffer [,$crc]) ;
  31.     $crc = crc32($buffer [,$crc]) ;
  33.     $crc = adler32_combine($crc1, $crc2, $len2)l
  34.     $crc = crc32_combine($adler1, $adler2, $len2)
  36.     my $version = Compress::Raw::Zlib::zlib_version();

DESCRIPTION

The Compress::Raw::Zlib module provides a Perl interface to the zlib compression library (see AUTHOR for details about where to get zlib).

Compress::Raw::Zlib::Deflate

This section defines an interface that allows in-memory compression using the deflate interface provided by zlib.

Here is a definition of the interface available:

($d, $status) = new Compress::Raw::Zlib::Deflate( [OPT] )

Initialises a deflation object.

If you are familiar with the zlib library, it combines the features of the zlib functions deflateInit , deflateInit2 and deflateSetDictionary .

If successful, it will return the initialised deflation object, $d and a $status of Z_OK in a list context. In scalar context it returns the deflation object, $d , only.

If not successful, the returned deflation object, $d , will be undef and $status will hold the a zlib error code.

The function optionally takes a number of named options specified as Name => value pairs. This allows individual options to be tailored without having to specify them all in the parameter list.

For backward compatibility, it is also possible to pass the parameters as a reference to a hash containing the name=>value pairs.

Below is a list of the valid options:

  • -Level

    Defines the compression level. Valid values are 0 through 9, Z_NO_COMPRESSION , Z_BEST_SPEED , Z_BEST_COMPRESSION , and Z_DEFAULT_COMPRESSION .

    The default is Z_DEFAULT_COMPRESSION .

  • -Method

    Defines the compression method. The only valid value at present (and the default) is Z_DEFLATED.

  • -WindowBits

    To compress an RFC 1950 data stream, set WindowBits to a positive number between 8 and 15.

    To compress an RFC 1951 data stream, set WindowBits to -MAX_WBITS .

    To compress an RFC 1952 data stream (i.e. gzip), set WindowBits to WANT_GZIP .

    For a definition of the meaning and valid values for WindowBits refer to the zlib documentation for deflateInit2.

    Defaults to MAX_WBITS .

  • -MemLevel

    For a definition of the meaning and valid values for MemLevel refer to the zlib documentation for deflateInit2.

    Defaults to MAX_MEM_LEVEL.

  • -Strategy

    Defines the strategy used to tune the compression. The valid values are Z_DEFAULT_STRATEGY , Z_FILTERED , Z_RLE , Z_FIXED and Z_HUFFMAN_ONLY .

    The default is Z_DEFAULT_STRATEGY.

  • -Dictionary

    When a dictionary is specified Compress::Raw::Zlib will automatically call deflateSetDictionary directly after calling deflateInit . The Adler32 value for the dictionary can be obtained by calling the method $d->dict_adler() .

    The default is no dictionary.

  • -Bufsize

    Sets the initial size for the output buffer used by the $d->deflate and $d->flush methods. If the buffer has to be reallocated to increase the size, it will grow in increments of Bufsize .

    The default buffer size is 4096.

  • -AppendOutput

    This option controls how data is written to the output buffer by the $d->deflate and $d->flush methods.

    If the AppendOutput option is set to false, the output buffers in the $d->deflate and $d->flush methods will be truncated before uncompressed data is written to them.

    If the option is set to true, uncompressed data will be appended to the output buffer in the $d->deflate and $d->flush methods.

    This option defaults to false.

  • -CRC32

    If set to true, a crc32 checksum of the uncompressed data will be calculated. Use the $d->crc32 method to retrieve this value.

    This option defaults to false.

  • -ADLER32

    If set to true, an adler32 checksum of the uncompressed data will be calculated. Use the $d->adler32 method to retrieve this value.

    This option defaults to false.

Here is an example of using the Compress::Raw::Zlib::Deflate optional parameter list to override the default buffer size and compression level. All other options will take their default values.

  1.     my $d = new Compress::Raw::Zlib::Deflate ( -Bufsize => 300, 
  2.                                                -Level   => Z_BEST_SPEED ) ;

$status = $d->deflate($input, $output)

Deflates the contents of $input and writes the compressed data to $output .

The $input and $output parameters can be either scalars or scalar references.

When finished, $input will be completely processed (assuming there were no errors). If the deflation was successful it writes the deflated data to $output and returns a status value of Z_OK .

On error, it returns a zlib error code.

If the AppendOutput option is set to true in the constructor for the $d object, the compressed data will be appended to $output . If it is false, $output will be truncated before any compressed data is written to it.

Note: This method will not necessarily write compressed data to $output every time it is called. So do not assume that there has been an error if the contents of $output is empty on returning from this method. As long as the return code from the method is Z_OK , the deflate has succeeded.

$status = $d->flush($output [, $flush_type])

Typically used to finish the deflation. Any pending output will be written to $output .

Returns Z_OK if successful.

Note that flushing can seriously degrade the compression ratio, so it should only be used to terminate a decompression (using Z_FINISH ) or when you want to create a full flush point (using Z_FULL_FLUSH ).

By default the flush_type used is Z_FINISH . Other valid values for flush_type are Z_NO_FLUSH , Z_PARTIAL_FLUSH , Z_SYNC_FLUSH and Z_FULL_FLUSH . It is strongly recommended that you only set the flush_type parameter if you fully understand the implications of what it does. See the zlib documentation for details.

If the AppendOutput option is set to true in the constructor for the $d object, the compressed data will be appended to $output . If it is false, $output will be truncated before any compressed data is written to it.

$status = $d->deflateReset()

This method will reset the deflation object $d . It can be used when you are compressing multiple data streams and want to use the same object to compress each of them. It should only be used once the previous data stream has been flushed successfully, i.e. a call to $d->flush(Z_FINISH) has returned Z_OK .

Returns Z_OK if successful.

$status = $d->deflateParams([OPT])

Change settings for the deflate object $d .

The list of the valid options is shown below. Options not specified will remain unchanged.

  • -Level

    Defines the compression level. Valid values are 0 through 9, Z_NO_COMPRESSION , Z_BEST_SPEED , Z_BEST_COMPRESSION , and Z_DEFAULT_COMPRESSION .

  • -Strategy

    Defines the strategy used to tune the compression. The valid values are Z_DEFAULT_STRATEGY , Z_FILTERED and Z_HUFFMAN_ONLY .

  • -BufSize

    Sets the initial size for the output buffer used by the $d->deflate and $d->flush methods. If the buffer has to be reallocated to increase the size, it will grow in increments of Bufsize .

$status = $d->deflateTune($good_length, $max_lazy, $nice_length, $max_chain)

Tune the internal settings for the deflate object $d . This option is only available if you are running zlib 1.2.2.3 or better.

Refer to the documentation in zlib.h for instructions on how to fly deflateTune .

$d->dict_adler()

Returns the adler32 value for the dictionary.

$d->crc32()

Returns the crc32 value for the uncompressed data to date.

If the CRC32 option is not enabled in the constructor for this object, this method will always return 0;

$d->adler32()

Returns the adler32 value for the uncompressed data to date.

$d->msg()

Returns the last error message generated by zlib.

$d->total_in()

Returns the total number of bytes uncompressed bytes input to deflate.

$d->total_out()

Returns the total number of compressed bytes output from deflate.

$d->get_Strategy()

Returns the deflation strategy currently used. Valid values are Z_DEFAULT_STRATEGY , Z_FILTERED and Z_HUFFMAN_ONLY .

$d->get_Level()

Returns the compression level being used.

$d->get_BufSize()

Returns the buffer size used to carry out the compression.

Example

Here is a trivial example of using deflate . It simply reads standard input, deflates it and writes it to standard output.

  1.     use strict ;
  2.     use warnings ;
  4.     use Compress::Raw::Zlib ;
  6.     binmode STDIN;
  7.     binmode STDOUT;
  8.     my $x = new Compress::Raw::Zlib::Deflate
  9.        or die "Cannot create a deflation stream\n" ;
  11.     my ($output, $status) ;
  12.     while (<>)
  13.     {
  14.         $status = $x->deflate($_, $output) ;
  15.     
  16.         $status == Z_OK
  17.             or die "deflation failed\n" ;
  18.     
  19.         print $output ;
  20.     }
  21.     
  22.     $status = $x->flush($output) ;
  23.     
  24.     $status == Z_OK
  25.         or die "deflation failed\n" ;
  26.     
  27.     print $output ;

Compress::Raw::Zlib::Inflate

This section defines an interface that allows in-memory uncompression using the inflate interface provided by zlib.

Here is a definition of the interface:

($i, $status) = new Compress::Raw::Zlib::Inflate( [OPT] )

Initialises an inflation object.

In a list context it returns the inflation object, $i , and the zlib status code ($status ). In a scalar context it returns the inflation object only.

If successful, $i will hold the inflation object and $status will be Z_OK .

If not successful, $i will be undef and $status will hold the zlib error code.

The function optionally takes a number of named options specified as -Name => value pairs. This allows individual options to be tailored without having to specify them all in the parameter list.

For backward compatibility, it is also possible to pass the parameters as a reference to a hash containing the name=>value pairs.

Here is a list of the valid options:

  • -WindowBits

    To uncompress an RFC 1950 data stream, set WindowBits to a positive number between 8 and 15.

    To uncompress an RFC 1951 data stream, set WindowBits to -MAX_WBITS .

    To uncompress an RFC 1952 data stream (i.e. gzip), set WindowBits to WANT_GZIP .

    To auto-detect and uncompress an RFC 1950 or RFC 1952 data stream (i.e. gzip), set WindowBits to WANT_GZIP_OR_ZLIB .

    For a full definition of the meaning and valid values for WindowBits refer to the zlib documentation for inflateInit2.

    Defaults to MAX_WBITS .

  • -Bufsize

    Sets the initial size for the output buffer used by the $i->inflate method. If the output buffer in this method has to be reallocated to increase the size, it will grow in increments of Bufsize .

    Default is 4096.

  • -Dictionary

    The default is no dictionary.

  • -AppendOutput

    This option controls how data is written to the output buffer by the $i->inflate method.

    If the option is set to false, the output buffer in the $i->inflate method will be truncated before uncompressed data is written to it.

    If the option is set to true, uncompressed data will be appended to the output buffer by the $i->inflate method.

    This option defaults to false.

  • -CRC32

    If set to true, a crc32 checksum of the uncompressed data will be calculated. Use the $i->crc32 method to retrieve this value.

    This option defaults to false.

  • -ADLER32

    If set to true, an adler32 checksum of the uncompressed data will be calculated. Use the $i->adler32 method to retrieve this value.

    This option defaults to false.

  • -ConsumeInput

    If set to true, this option will remove compressed data from the input buffer of the $i->inflate method as the inflate progresses.

    This option can be useful when you are processing compressed data that is embedded in another file/buffer. In this case the data that immediately follows the compressed stream will be left in the input buffer.

    This option defaults to true.

  • -LimitOutput

    The LimitOutput option changes the behavior of the $i->inflate method so that the amount of memory used by the output buffer can be limited.

    When LimitOutput is used the size of the output buffer used will either be the value of the Bufsize option or the amount of memory already allocated to $output , whichever is larger. Predicting the output size available is tricky, so don't rely on getting an exact output buffer size.

    When LimitOutout is not specified $i->inflate will use as much memory as it takes to write all the uncompressed data it creates by uncompressing the input buffer.

    If LimitOutput is enabled, the ConsumeInput option will also be enabled.

    This option defaults to false.

    See The LimitOutput option for a discussion on why LimitOutput is needed and how to use it.

Here is an example of using an optional parameter to override the default buffer size.

  1.     my ($i, $status) = new Compress::Raw::Zlib::Inflate( -Bufsize => 300 ) ;

$status = $i->inflate($input, $output [,$eof])

Inflates the complete contents of $input and writes the uncompressed data to $output . The $input and $output parameters can either be scalars or scalar references.

Returns Z_OK if successful and Z_STREAM_END if the end of the compressed data has been successfully reached.

If not successful $status will hold the zlib error code.

If the ConsumeInput option has been set to true when the Compress::Raw::Zlib::Inflate object is created, the $input parameter is modified by inflate . On completion it will contain what remains of the input buffer after inflation. In practice, this means that when the return status is Z_OK the $input parameter will contain an empty string, and when the return status is Z_STREAM_END the $input parameter will contains what (if anything) was stored in the input buffer after the deflated data stream.

This feature is useful when processing a file format that encapsulates a compressed data stream (e.g. gzip, zip) and there is useful data immediately after the deflation stream.

If the AppendOutput option is set to true in the constructor for this object, the uncompressed data will be appended to $output . If it is false, $output will be truncated before any uncompressed data is written to it.

The $eof parameter needs a bit of explanation.

Prior to version 1.2.0, zlib assumed that there was at least one trailing byte immediately after the compressed data stream when it was carrying out decompression. This normally isn't a problem because the majority of zlib applications guarantee that there will be data directly after the compressed data stream. For example, both gzip (RFC 1950) and zip both define trailing data that follows the compressed data stream.

The $eof parameter only needs to be used if all of the following conditions apply

1

You are either using a copy of zlib that is older than version 1.2.0 or you want your application code to be able to run with as many different versions of zlib as possible.

2

You have set the WindowBits parameter to -MAX_WBITS in the constructor for this object, i.e. you are uncompressing a raw deflated data stream (RFC 1951).

3

There is no data immediately after the compressed data stream.

If all of these are the case, then you need to set the $eof parameter to true on the final call (and only the final call) to $i->inflate .

If you have built this module with zlib >= 1.2.0, the $eof parameter is ignored. You can still set it if you want, but it won't be used behind the scenes.

$status = $i->inflateSync($input)

This method can be used to attempt to recover good data from a compressed data stream that is partially corrupt. It scans $input until it reaches either a full flush point or the end of the buffer.

If a full flush point is found, Z_OK is returned and $input will be have all data up to the flush point removed. This data can then be passed to the $i->inflate method to be uncompressed.

Any other return code means that a flush point was not found. If more data is available, inflateSync can be called repeatedly with more compressed data until the flush point is found.

Note full flush points are not present by default in compressed data streams. They must have been added explicitly when the data stream was created by calling Compress::Deflate::flush with Z_FULL_FLUSH .

$i->dict_adler()

Returns the adler32 value for the dictionary.

$i->crc32()

Returns the crc32 value for the uncompressed data to date.

If the CRC32 option is not enabled in the constructor for this object, this method will always return 0;

$i->adler32()

Returns the adler32 value for the uncompressed data to date.

If the ADLER32 option is not enabled in the constructor for this object, this method will always return 0;

$i->msg()

Returns the last error message generated by zlib.

$i->total_in()

Returns the total number of bytes compressed bytes input to inflate.

$i->total_out()

Returns the total number of uncompressed bytes output from inflate.

$d->get_BufSize()

Returns the buffer size used to carry out the decompression.

Examples

Here is an example of using inflate .

  1.     use strict ;
  2.     use warnings ;
  3.     
  4.     use Compress::Raw::Zlib;
  5.     
  6.     my $x = new Compress::Raw::Zlib::Inflate()
  7.        or die "Cannot create a inflation stream\n" ;
  8.     
  9.     my $input = '' ;
  10.     binmode STDIN;
  11.     binmode STDOUT;
  12.     
  13.     my ($output, $status) ;
  14.     while (read(STDIN, $input, 4096))
  15.     {
  16.         $status = $x->inflate($input, $output) ;
  17.     
  18.         print $output ;
  19.     
  20.         last if $status != Z_OK ;
  21.     }
  22.     
  23.     die "inflation failed\n"
  24.         unless $status == Z_STREAM_END ;

The next example show how to use the LimitOutput option. Notice the use of two nested loops in this case. The outer loop reads the data from the input source - STDIN and the inner loop repeatedly calls inflate until $input is exhausted, we get an error, or the end of the stream is reached. One point worth remembering is by using the LimitOutput option you also get ConsumeInput set as well - this makes the code below much simpler.

  1.     use strict ;
  2.     use warnings ;
  3.     
  4.     use Compress::Raw::Zlib;
  5.     
  6.     my $x = new Compress::Raw::Zlib::Inflate(LimitOutput => 1)
  7.        or die "Cannot create a inflation stream\n" ;
  8.     
  9.     my $input = '' ;
  10.     binmode STDIN;
  11.     binmode STDOUT;
  12.     
  13.     my ($output, $status) ;
  15.   OUTER:
  16.     while (read(STDIN, $input, 4096))
  17.     {
  18.         do
  19.         {
  20.             $status = $x->inflate($input, $output) ;
  22.             print $output ;
  24.             last OUTER
  25.                 unless $status == Z_OK || $status == Z_BUF_ERROR ;
  26.         }
  27.         while ($status == Z_OK && length $input);
  28.     }
  29.     
  30.     die "inflation failed\n"
  31.         unless $status == Z_STREAM_END ;

CHECKSUM FUNCTIONS

Two functions are provided by zlib to calculate checksums. For the Perl interface, the order of the two parameters in both functions has been reversed. This allows both running checksums and one off calculations to be done.

  1.     $crc = adler32($buffer [,$crc]) ;
  2.     $crc = crc32($buffer [,$crc]) ;

The buffer parameters can either be a scalar or a scalar reference.

If the $crc parameters is undef, the crc value will be reset.

If you have built this module with zlib 1.2.3 or better, two more CRC-related functions are available.

  1.     $crc = adler32_combine($crc1, $crc2, $len2)l
  2.     $crc = crc32_combine($adler1, $adler2, $len2)

These functions allow checksums to be merged.

Misc

my $version = Compress::Raw::Zlib::zlib_version();

Returns the version of the zlib library.

The LimitOutput option.

By default $i->inflate($input, $output) will uncompress all data in $input and write all of the uncompressed data it has generated to $output . This makes the interface to inflate much simpler - if the method has uncompressed $input successfully all compressed data in $input will have been dealt with. So if you are reading from an input source and uncompressing as you go the code will look something like this

  1.     use strict ;
  2.     use warnings ;
  3.     
  4.     use Compress::Raw::Zlib;
  5.     
  6.     my $x = new Compress::Raw::Zlib::Inflate()
  7.        or die "Cannot create a inflation stream\n" ;
  8.     
  9.     my $input = '' ;
  10.     
  11.     my ($output, $status) ;
  12.     while (read(STDIN, $input, 4096))
  13.     {
  14.         $status = $x->inflate($input, $output) ;
  15.     
  16.         print $output ;
  17.     
  18.         last if $status != Z_OK ;
  19.     }
  20.     
  21.     die "inflation failed\n"
  22.         unless $status == Z_STREAM_END ;

The points to note are

  • The main processing loop in the code handles reading of compressed data from STDIN.

  • The status code returned from inflate will only trigger termination of the main processing loop if it isn't Z_OK . When LimitOutput has not been used the Z_OK status means means that the end of the compressed data stream has been reached or there has been an error in uncompression.

  • After the call to inflate all of the uncompressed data in $input will have been processed. This means the subsequent call to read can overwrite it's contents without any problem.

For most use-cases the behavior described above is acceptable (this module and it's predecessor, Compress::Zlib , have used it for over 10 years without an issue), but in a few very specific use-cases the amount of memory required for $output can prohibitively large. For example, if the compressed data stream contains the same pattern repeated thousands of times, a relatively small compressed data stream can uncompress into hundreds of megabytes. Remember inflate will keep allocating memory until all the uncompressed data has been written to the output buffer - the size of $output is unbounded.

The LimitOutput option is designed to help with this use-case.

The main difference in your code when using LimitOutput is having to deal with cases where the $input parameter still contains some uncompressed data that inflate hasn't processed yet. The status code returned from inflate will be Z_OK if uncompression took place and Z_BUF_ERROR if the output buffer is full.

Below is typical code that shows how to use LimitOutput .

  1.     use strict ;
  2.     use warnings ;
  3.     
  4.     use Compress::Raw::Zlib;
  5.     
  6.     my $x = new Compress::Raw::Zlib::Inflate(LimitOutput => 1)
  7.        or die "Cannot create a inflation stream\n" ;
  8.     
  9.     my $input = '' ;
  10.     binmode STDIN;
  11.     binmode STDOUT;
  12.     
  13.     my ($output, $status) ;
  15.   OUTER:
  16.     while (read(STDIN, $input, 4096))
  17.     {
  18.         do
  19.         {
  20.             $status = $x->inflate($input, $output) ;
  22.             print $output ;
  24.             last OUTER
  25.                 unless $status == Z_OK || $status == Z_BUF_ERROR ;
  26.         }
  27.         while ($status == Z_OK && length $input);
  28.     }
  29.     
  30.     die "inflation failed\n"
  31.         unless $status == Z_STREAM_END ;

Points to note this time:

  • There are now two nested loops in the code: the outer loop for reading the compressed data from STDIN, as before; and the inner loop to carry out the uncompression.

  • There are two exit points from the inner uncompression loop.

    Firstly when inflate has returned a status other than Z_OK or Z_BUF_ERROR . This means that either the end of the compressed data stream has been reached (Z_STREAM_END ) or there is an error in the compressed data. In either of these cases there is no point in continuing with reading the compressed data, so both loops are terminated.

    The second exit point tests if there is any data left in the input buffer, $input - remember that the ConsumeInput option is automatically enabled when LimitOutput is used. When the input buffer has been exhausted, the outer loop can run again and overwrite a now empty $input .

ACCESSING ZIP FILES

Although it is possible (with some effort on your part) to use this module to access .zip files, there are other perl modules available that will do all the hard work for you. Check out Archive::Zip , IO::Compress::Zip and IO::Uncompress::Unzip .

CONSTANTS

All the zlib constants are automatically imported when you make use of Compress::Raw::Zlib.

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::Lzop, IO::Uncompress::UnLzop, IO::Compress::Lzf, IO::Uncompress::UnLzf, IO::Uncompress::AnyInflate, IO::Uncompress::AnyUncompress

Compress::Zlib::FAQ

File::GlobMapper, Archive::Zip, Archive::Tar, IO::Zlib

For RFC 1950, 1951 and 1952 see http://www.faqs.org/rfcs/rfc1950.html, http://www.faqs.org/rfcs/rfc1951.html and http://www.faqs.org/rfcs/rfc1952.html

The zlib compression library was written by Jean-loup Gailly gzip@prep.ai.mit.edu and Mark Adler madler@alumni.caltech.edu.

The primary site for the zlib compression library is http://www.zlib.org.

The primary site for gzip is http://www.gzip.org.

AUTHOR

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

MODIFICATION HISTORY

See the Changes file.

COPYRIGHT AND LICENSE

Copyright (c) 2005-2009 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.

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