Notes for users of Compress::Zlib version 1¶

The main change in "Compress::Zlib" version 2.x is that it does not now interface directly to the zlib library. Instead it uses the "IO::Compress::Gzip" and "IO::Uncompress::Gunzip" modules for reading/writing gzip files, and the "Compress::Raw::Zlib" module for some low-level zlib access.

The interface provided by version 2 of this module should be 100% backward compatible with version 1. If you find a difference in the expected behaviour please contact the author (See "AUTHOR"). See "GZIP INTERFACE"

With the creation of the "IO::Compress" and "IO::Uncompress" modules no new features are planned for "Compress::Zlib" - the new modules do everything that "Compress::Zlib" does and then some. Development on "Compress::Zlib" will be limited to bug fixes only.

If you are writing new code, your first port of call should be one of the new "IO::Compress" or "IO::Uncompress" modules.

Examples¶

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

    use strict ;
    use warnings ;
    
    use Compress::Zlib ;
    
    # use stdin if no files supplied
    @ARGV = '-' unless @ARGV ;
    
    foreach my $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 >= 1;
    
    my $pattern = shift ;
    
    # use stdin if no files supplied
    @ARGV = '-' unless @ARGV ;
    
    foreach my $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 data stream 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 with the minimum possible gzip header (exactly 10 bytes).

    $dest = Compress::Zlib::memGzip($buffer) 
        or die "Cannot compress: $gzerrno\n";

If successful, it returns the in-memory gzip file. Otherwise it returns "undef" and the $gzerrno variable will store the zlib error code.

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

See IO::Compress::Gzip for an alternative way to carry out in-memory gzip compression.

Compress::Zlib::memGunzip¶

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

    $dest = Compress::Zlib::memGunzip($buffer) 
        or die "Cannot uncompress: $gzerrno\n";

If successful, it returns the uncompressed gzip file. Otherwise it returns "undef" and the $gzerrno variable will store the zlib error code.

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

If $buffer consists of multiple concatenated gzip data streams only the first will be uncompressed. Use "gunzip" with the "MultiStream" option in the "IO::Uncompress::Gunzip" module if you need to deal with concatenated data streams.

See IO::Uncompress::Gunzip for an alternative way to carry out in-memory gzip uncompression.

($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 Z_DEFAULT_COMPRESSION.

-Method

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

-WindowBits

To create an RFC 1950 data stream, set "WindowBits" to a positive number.

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

For a full 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" and "Z_HUFFMAN_ONLY".

The default is 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) = $d->flush() =head2 ($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 ;

($i, $status) = inflateInit()¶

Initialises an inflation stream.

In a list context it returns the inflation stream, $i, and the zlib status code in $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

To uncompress an RFC 1950 data stream, set "WindowBits" to a positive number.

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

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 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 ;

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

Returns the version of the zlib library.