Zlib

Section: User Contributed Perl Documentation (3)
Updated: 2007-01-21
Index Return to Main Contents
 

NAME

Compress::Zlib - Interface to zlib compression library  

SYNOPSIS

    use Compress::Zlib ;

    ($d, $status) = deflateInit( [OPT] ) ;
    ($out, $status) = $d->deflate($buffer) ;
    $status = $d->deflateParams([OPT]) ;
    ($out, $status) = $d->flush() ;
    $d->dict_adler() ;
    $d->total_in() ;
    $d->total_out() ;
    $d->msg() ;

    ($i, $status) = inflateInit( [OPT] ) ;
    ($out, $status) = $i->inflate($buffer) ;
    $status = $i->inflateSync($buffer) ;
    $i->dict_adler() ;
    $i->total_in() ;
    $i->total_out() ;
    $i->msg() ;

    $dest = compress($source, [$level]) ;
    $dest = uncompress($source) ;

    $gz = gzopen($filename or filehandle, $mode) ;
    $bytesread = $gz->gzread($buffer [,$size]) ;
    $bytesread = $gz->gzreadline($line) ;
    $byteswritten = $gz->gzwrite($buffer) ;
    $status = $gz->gzflush($flush) ;
    $status = $gz->gzclose() ;
    $status = $gz->gzeof() ;
    $status = $gz->gzsetparams($level, $strategy) ;
    $errstring = $gz->gzerror() ; 
    $gzerrno

    $dest = Compress::Zlib::memGzip($buffer) ;
    $dest = Compress::Zlib::memGunzip($buffer) ;

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

    ZLIB_VERSION

 

DESCRIPTION

The Compress::Zlib module provides a Perl interface to the zlib compression library (see ``AUTHOR'' for details about where to get zlib). Most of the functionality provided by zlib is available in Compress::Zlib.

The module can be split into two general areas of functionality, namely in-memory compression/decompression and read/write access to gzip files. Each of these areas will be discussed separately below.  

DEFLATE

The interface Compress::Zlib provides to the in-memory deflate (and inflate) functions has been modified to fit into a Perl model.

The main difference is that for both inflation and deflation, the Perl interface will always consume the complete input buffer before returning. Also the output buffer returned will be automatically grown to fit the amount of output available.

Here is a definition of the interface available:  

($d, $status) = deflateInit( [OPT] )

Initialises a deflation stream.

It combines the features of the zlib functions deflateInit, deflateInit2 and deflateSetDictionary.

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

If not successful, the returned deflation stream ($d) will be undef and $status will hold the exact 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.

The function takes one optional parameter, a reference to a hash. The contents of the hash allow the deflation interface to be tailored.

Here 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 "-Level =>Z_DEFAULT_COMPRESSION".

-Method
Defines the compression method. The only valid value at present (and the default) is "-Method =>Z_DEFLATED".
-WindowBits
For a definition of the meaning and valid values for WindowBits refer to the zlib documentation for deflateInit2.

Defaults to "-WindowBits =>MAX_WBITS".

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

Defaults to "-MemLevel =>MAX_MEM_LEVEL".

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

The default is "-Strategy =>Z_DEFAULT_STRATEGY".

-Dictionary
When a dictionary is specified Compress::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 deflation buffer. If the buffer has to be reallocated to increase the size, it will grow in increments of Bufsize.

The default is 4096.

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

    deflateInit( -Bufsize => 300, 
                 -Level => Z_BEST_SPEED  ) ;

 

($out, $status) = $d->deflate($buffer)

Deflates the contents of $buffer. The buffer can either be a scalar or a scalar reference. When finished, $buffer will be completely processed (assuming there were no errors). If the deflation was successful it returns the deflated output, $out, and a status value, $status, of "Z_OK".

On error, $out will be undef and $status will contain the zlib error code.

In a scalar context deflate will return $out only.

As with the deflate function in zlib, it is not necessarily the case that any output will be produced by this method. So don't rely on the fact that $out is empty for an error test.  

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

Typically used to finish the deflation. Any pending output will be returned via $out. $status will have a value "Z_OK" if successful.

In a scalar context flush will return $out only.

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.  

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

Change settings for the deflate stream $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".
 

$d->dict_adler()

Returns the adler32 value for the dictionary.  

$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.  

Example

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

    use strict ;
    use warnings ;

    use Compress::Zlib ;

    binmode STDIN;
    binmode STDOUT;
    my $x = deflateInit()
       or die "Cannot create a deflation stream\n" ;

    my ($output, $status) ;
    while (<>)
    {
        ($output, $status) = $x->deflate($_) ;

        $status == Z_OK
            or die "deflation failed\n" ;

        print $output ;
    }

    ($output, $status) = $x->flush() ;

    $status == Z_OK
        or die "deflation failed\n" ;

    print $output ;

 

INFLATE

Here is a definition of the interface:  

($i, $status) = inflateInit()

Initialises an inflation stream.

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

If successful, $i will hold the inflation stream 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.

The function takes one optional parameter, a reference to a hash. The contents of the hash allow the deflation interface to be tailored.

Here is a list of the valid options:

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

Defaults to "-WindowBits =>MAX_WBITS".

-Bufsize
Sets the initial size for the inflation buffer. If the buffer has to be reallocated to increase the size, it will grow in increments of Bufsize.

Default is 4096.

-Dictionary
The default is no dictionary.

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

    inflateInit( -Bufsize => 300 ) ;

 

($out, $status) = $i->inflate($buffer)

Inflates the complete contents of $buffer. The buffer can either be a scalar or a scalar reference.

Returns "Z_OK" if successful and "Z_STREAM_END" if the end of the compressed data has been successfully reached. If not successful, $out will be undef and $status will hold the zlib error code.

The $buffer parameter is modified by "inflate". On completion it will contain what remains of the input buffer after inflation. This means that $buffer will be an empty string when the return status is "Z_OK". When the return status is "Z_STREAM_END" the $buffer 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).  

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

Scans $buffer 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 $buffer will be have all data up to the flush point removed. This can then be passed to the "deflate" method.

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.  

$i->dict_adler()

Returns the adler32 value for the dictionary.  

$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.  

Example

Here is an example of using inflate.

    use strict ;
    use warnings ;

    use Compress::Zlib ;

    my $x = inflateInit()
       or die "Cannot create a inflation stream\n" ;

    my $input = '' ;
    binmode STDIN;
    binmode STDOUT;

    my ($output, $status) ;
    while (read(STDIN, $input, 4096))
    {
        ($output, $status) = $x->inflate(\$input) ;

        print $output 
            if $status == Z_OK or $status == Z_STREAM_END ;

        last if $status != Z_OK ;
    }

    die "inflation failed\n"
        unless $status == Z_STREAM_END ;

 

COMPRESS/UNCOMPRESS

Two high-level functions are provided by zlib to perform in-memory compression/uncompression of RFC1950 data streams. They are called compress and uncompress.

The two Perl subs defined below provide the equivalent functionality.

$dest = compress($source [, $level] ) ;
Compresses $source. If successful it returns the compressed data. Otherwise it returns undef.

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

The $level paramter defines the compression level. Valid values are 0 through 9, "Z_NO_COMPRESSION", "Z_BEST_SPEED", "Z_BEST_COMPRESSION", and "Z_DEFAULT_COMPRESSION". If $level is not specified "Z_DEFAULT_COMPRESSION" will be used.

$dest = uncompress($source) ;
Uncompresses $source. If successful it returns the uncompressed data. Otherwise it returns undef.

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

Please note: the two functions defined above are not compatible with the Unix commands of the same name.  

GZIP INTERFACE

A number of functions are supplied in zlib for reading and writing gzip files. This module provides an interface to most of them. In general the interface provided by this module operates identically to the functions provided by zlib. Any differences are explained below.
$gz = gzopen(filename or filehandle, mode)
This function operates identically to the zlib equivalent except that it returns an object which is used to access the other gzip methods.

As with the zlib equivalent, the mode parameter is used to specify both whether the file is opened for reading or writing and to optionally specify a a compression level. Refer to the zlib documentation for the exact format of the mode parameter.

If a reference to an open filehandle is passed in place of the filename, gzdopen will be called behind the scenes. The third example at the end of this section, gzstream, uses this feature.

$bytesread = $gz->gzread($buffer [, $size]) ;
Reads $size bytes from the compressed file into $buffer. If $size is not specified, it will default to 4096. If the scalar $buffer is not large enough, it will be extended automatically.

Returns the number of bytes actually read. On EOF it returns 0 and in the case of an error, -1.

$bytesread = $gz->gzreadline($line) ;
Reads the next line from the compressed file into $line.

Returns the number of bytes actually read. On EOF it returns 0 and in the case of an error, -1.

It is legal to intermix calls to gzread and gzreadline.

At this time gzreadline ignores the variable $/ ($INPUT_RECORD_SEPARATOR or $RS when "English" is in use). The end of a line is denoted by the C character '\n'.

$byteswritten = $gz->gzwrite($buffer) ;
Writes the contents of $buffer to the compressed file. Returns the number of bytes actually written, or 0 on error.
$status = $gz->gzflush($flush) ;
Flushes all pending output to the compressed file. Works identically to the zlib function it interfaces to. Note that the use of gzflush can degrade compression.

Returns "Z_OK" if $flush is "Z_FINISH" and all output could be flushed. Otherwise the zlib error code is returned.

Refer to the zlib documentation for the valid values of $flush.

$status = $gz->gzeof() ;
Returns 1 if the end of file has been detected while reading the input file, otherwise returns 0.
$gz->gzclose
Closes the compressed file. Any pending data is flushed to the file before it is closed.
$gz->gzsetparams($level, $strategy
Change settings for the deflate stream $gz.

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

Note: This method is only available if you are running zlib 1.0.6 or better.

$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".
$gz->gzerror
Returns the zlib error message or number for the last operation associated with $gz. The return value will be the zlib error number when used in a numeric context and the zlib error message when used in a string context. The zlib error number constants, shown below, are available for use.

    Z_OK
    Z_STREAM_END
    Z_ERRNO
    Z_STREAM_ERROR
    Z_DATA_ERROR
    Z_MEM_ERROR
    Z_BUF_ERROR

$gzerrno
The $gzerrno scalar holds the error code associated with the most recent gzip routine. Note that unlike gzerror(), the error is not associated with a particular file.

As with gzerror() it returns an error number in numeric context and an error message in string context. Unlike gzerror() though, the error message will correspond to the zlib message when the error is associated with zlib itself, or the UNIX error message when it is not (i.e. zlib returned "Z_ERRORNO").

As there is an overlap between the error numbers used by zlib and UNIX, $gzerrno should only be used to check for the presence of an error in numeric context. Use gzerror() to check for specific zlib errors. The gzcat example below shows how the variable can be used safely.

 

Examples

Here is an example script which uses the interface. It implements a gzcat function.

    use strict ;
    use warnings ;

    use Compress::Zlib ;

    die "Usage: gzcat file...\n"
        unless @ARGV ;

    my $file ;

    foreach $file (@ARGV) {
        my $buffer ;

        my $gz = gzopen($file, "rb") 
             or die "Cannot open $file: $gzerrno\n" ;

        print $buffer while $gz->gzread($buffer) > 0 ;

        die "Error reading from $file: $gzerrno" . ($gzerrno+0) . "\n" 
            if $gzerrno != Z_STREAM_END ;

        $gz->gzclose() ;
    }

Below is a script which makes use of gzreadline. It implements a very simple grep like script.

    use strict ;
    use warnings ;

    use Compress::Zlib ;

    die "Usage: gzgrep pattern file...\n"
        unless @ARGV >= 2;

    my $pattern = shift ;

    my $file ;

    foreach $file (@ARGV) {
        my $gz = gzopen($file, "rb") 
             or die "Cannot open $file: $gzerrno\n" ;

        while ($gz->gzreadline($_) > 0) {
            print if /$pattern/ ;
        }

        die "Error reading from $file: $gzerrno\n" 
            if $gzerrno != Z_STREAM_END ;

        $gz->gzclose() ;
    }

This script, gzstream, does the opposite of the gzcat script above. It reads from standard input and writes a gzip file to standard output.

    use strict ;
    use warnings ;

    use Compress::Zlib ;

    binmode STDOUT;     # gzopen only sets it on the fd

    my $gz = gzopen(\*STDOUT, "wb")
          or die "Cannot open stdout: $gzerrno\n" ;

    while (<>) {
        $gz->gzwrite($_) 
        or die "error writing: $gzerrno\n" ;
    }

    $gz->gzclose ;

 

Compress::Zlib::memGzip

This function is used to create an in-memory gzip file. It creates a minimal gzip header.

    $dest = Compress::Zlib::memGzip($buffer) ;

If successful, it returns the in-memory gzip file, otherwise it returns undef.

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

Compress::Zlib::memGunzip

This function is used to uncompress an in-memory gzip file.

    $dest = Compress::Zlib::memGunzip($buffer) ;

If successful, it returns the uncompressed gzip file, otherwise it returns undef.

The buffer parameter can either be a scalar or a scalar reference. The contents of the buffer parameter are destroyed after calling this function.  

CHECKSUM FUNCTIONS

Two functions are provided by zlib to calculate a checksum. 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.

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

FAQ

 

Compatibility with Unix compress/uncompress.

Although "Compress::Zlib" has a pair of functions called "compress" and "uncompress", they are not the same as the Unix programs of the same name. The "Compress::Zlib" library is not compatable 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>)
    {
        ...

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

 

Accessing ZIP Files

Although it is possible to use this module to access .zip files, there is a module on CPAN that will do all the hard work for you. Check out

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

Assuming you don't want to use this module to access zip files there are a number of undocumented features in the zlib library you need to be aware of.

1.
When calling inflateInit or deflateInit the WindowBits parameter must be set to "-MAX_WBITS". This disables the creation of the zlib header.
2.
The zlib function inflate, and so the inflate method supplied in this module, assume that there is at least one trailing byte after the compressed data stream. Normally this isn't a problem because both the gzip and zip file formats will guarantee that there is data directly after the compressed data stream.
 

CONSTANTS

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

AUTHOR

The Compress::Zlib module was written by Paul Marquess, pmqs@cpan.org. The latest copy of the module can be found on CPAN in modules/by-module/Compress/Compress-Zlib-x.x.tar.gz.

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

MODIFICATION HISTORY

See the Changes file.


 

Index

NAME
SYNOPSIS
DESCRIPTION
DEFLATE
($d, $status) = deflateInit( [OPT] )
($out, $status) = $d->deflate($buffer)
($out, $status) = $d->flush([flush_type])
$status = $d->deflateParams([OPT])
$d->dict_adler()
$d->msg()
$d->total_in()
$d->total_out()
Example
INFLATE
($i, $status) = inflateInit()
($out, $status) = $i->inflate($buffer)
$status = $i->inflateSync($buffer)
$i->dict_adler()
$i->msg()
$i->total_in()
$i->total_out()
Example
COMPRESS/UNCOMPRESS
GZIP INTERFACE
Examples
Compress::Zlib::memGzip
Compress::Zlib::memGunzip
CHECKSUM FUNCTIONS
FAQ
Compatibility with Unix compress/uncompress.
Accessing .tar.Z files
Accessing ZIP Files
CONSTANTS
AUTHOR
MODIFICATION HISTORY

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