Home

Introduction

zlib is the reference implementation for the deflate compression algorithm. Deflate is the algorithm used by the gzip container format, the the zip archive format, and HTTP compression.

RB-zlib is a zlib binding for Realbasic and Xojo projects.

The minimum supported zlib version is 1.2.8. The minimum supported Xojo version is RS2009R3.

Hilights

• Read and write compressed file or memory streams using a simple BinaryStream work-alike.
• Read and write zip archives (.zip)
• Read and write tape archives (.tar), with or without gzip compression.
• Supports gzip, deflate, and raw deflate compressed streams

Getting started

The following section covers using zlib for general purpose compression. Refer to the PKZip and USTAR modules for information on working with archives.

Utility methods

The zlib module provides several utility methods for basic compression or decompression of data:

• Deflate
• Inflate
• GZip
• GUnZip

All of these methods are overloaded with several useful variations on input and output parameters. All variations follow either this signature:

 function(source, destination, options[...]) As Boolean

or this signature:

 function(source, options[...]) As MemoryBlock

where source is a MemoryBlock, FolderItem, or an object which implements the Readable interface; and destination (when provided) is a FolderItem or an object which implements the Writeable interface. Methods which do not have a Destination parameter return output as a MemoryBlock instead. Refer to the examples below for demonstrations of some of these functions.

Additional optional arguments may be passed, to control the compression level, strategy, dictionary, and encoding. For example, GZip and GUnZip are just wrappers around Deflate and Inflate with options that specify the gzip format.

ZStream class

The second way to compress or decompress data is with the ZStream class. The ZStream is a BinaryStream work-alike, and implements both the Readable and Writeable interfaces. Anything written to a ZStream is compressed and emitted to the output stream (another Writeable); reading from a ZStream decompresses data from the input stream (another Readable).

Instances of ZStream can be created from MemoryBlocks, FolderItems, and objects that implement the Readable and/or Writeable interfaces. For example, creating an in-memory compression stream from a zero-length MemoryBlock and writing a string to it:

  Dim output As New MemoryBlock(0)
  Dim z As New zlib.ZStream(output) ' zero-length creates a compressor
  z.Write("Hello, world!")
  z.Close

The string will be processed through the compressor and written to the output MemoryBlock. To create a decompressor pass a MemoryBlock whose size is > 0 (continuing from above):

  z = New zlib.ZStream(output) ' output contains the compressed string
  MsgBox(z.ReadAll) ' read the decompressed string

Inflater and Deflater classes

The third and final way to use zlib is through the Inflater and Deflater classes. These classes provide a low-level wrapper to the zlib API. All compression and decompression done using the ZStream class or the utility methods is ultimately carried out by an instance of Deflater and Inflater, respectively.

  Dim d As New zlib.Deflater()
  Dim data As MemoryBlock = d.Deflate("H")
  data = data + d.Deflate("el")
  data = data + d.Deflate("lo", zlib.Z_FINISH)
  
  Dim i As New zlib.Inflater()
  data = i.Inflate(data)
  MsgBox(data)

More examples

This example compresses and decompresses a MemoryBlock using deflate compression:

  Dim data As MemoryBlock = "Potentially very large MemoryBlock goes here!"
  Dim comp As MemoryBlock = zlib.Deflate(data)
  Dim dcmp As MemoryBlock = zlib.Inflate(comp)

This example compresses and decompresses a MemoryBlock using GZip:

  Dim data As MemoryBlock = "Potentially very large MemoryBlock goes here!"
  Dim comp As MemoryBlock = zlib.GZip(data)
  Dim dcmp As MemoryBlock = zlib.GUnZip(comp)

This example gzips a file:

  Dim src As FolderItem = GetOpenFolderItem("") ' a file to be gzipped
  Dim dst As FolderItem = src.Parent.Child(src.Name + ".gz")
  If zlib.GZip(src, dst) Then 
    MsgBox("Compression succeeded!")
  Else
    MsgBox("Compression failed!")
  End If

This example opens an existing gzip file and decompresses it into a MemoryBlock:

  Dim f As FolderItem = GetOpenFolderItem("") ' the gzip file to open
  Dim data As MemoryBlock = zlib.GUnZip(f)
  If data <> Nil Then
    MsgBox("Decompression succeeded!")
  Else
    MsgBox("Decompression failed!")
  End If

This example extracts a zip archive into a directory:

  Dim src As FolderItem = GetOpenFolderItem("") ' a zip file to extract
  Dim dst As FolderItem = SelectFolder() ' the destination directory
  Dim extracted() As FolderItem ' the list of extracted files/folders
  extracted = PKZip.ReadZip(src, dst)

This example performs an HTTP request that asks for compression, and decompresses the response:

  Dim h As New HTTPSocket
  h.SetRequestHeader("Accept-Encoding", "gzip, deflate")
  Dim page As String = h.Get("http://www.example.com", 10)
  If h.PageHeaders.CommaSeparatedValues("Content-Encoding") = "gzip" Then
    page = zlib.GUnZip(page)
  ElseIf h.PageHeaders.CommaSeparatedValues("Content-Encoding") = "deflate" Then
    page = zlib.Inflate(page) ' assume DEFLATE_ENCODING; some servers send RAW_ENCODING
  End If

This example performs a hand-rolled HTTP request using a TCPSocket, and demonstrates how the ZStream can be used with any object that implements the Readable and/or Writeable interfaces:

  Static CRLF As String = EndOfLine.Windows
  Dim sock As New TCPSocket
  sock.Address = "www.example.com"
  sock.Port = 80
  sock.Connect()
  Do Until sock.IsConnected
    sock.Poll
  Loop Until sock.LastErrorCode <> 0
  sock.Write("GET / HTTP/1.0" + CRLF + "Accept-Encoding: gzip" + CRLF + "Connection: close" + CRLF + "Host: www.example.com" + CRLF + CRLF)
  Do
    sock.Poll
  Loop Until Not sock.IsConnected
  
  Dim headers As String = sock.Read(InStrB(sock.Lookahead, CRLF + CRLF) + 3)
  Dim z As zlib.ZStream = zlib.ZStream.Open(sock)
  Dim webpage As String = z.ReadAll ' read/decompress from the socket
  z.Close

How to incorporate zlib into your Realbasic/Xojo project

Import the zlib, USTAR, and PKZip modules

1. Download the RB-zlib project either in ZIP archive format or by cloning the repository with your Git client.
2. Open the RB-zlib project in REALstudio or Xojo. Open your project in a separate window.
3. Copy the zlib, USTAR, and PKZip modules into your project and save.

Ensure the zlib shared library is installed

zlib is installed by default on most Unix-like operating systems, including OS X and most Linux distributions, however at least zlib version 1.2.8 is needed.

Windows does not have it installed by default, you will need to ship the DLL with your application. You can use pre-built DLL available here (Win32x86), or you can build them yourself from source.

RB-zlib will raise a PlatformNotSupportedException when used if all required DLLs/SOs/DyLibs are not available at runtime.

The PKZip and USTAR modules may be used independently of the zlib module for reading and writing uncompressed archives. To do so set the PKZip.USE_ZLIB and/or USTAR.USE_ZLIB constants to False.