gzip-4.3.doc

This is a resource based on j2me embedded,if you dont understand,you can connection with me .

INTERNET-DRAFT                                          L. Peter Deutsch
GZIP 4.3                                             Alladin Enterprises
Expires: 24 July 1996                                        24 Jan 1996

GZIP file format specification version 4.3

File draft-deutsch-gzip-4.3-00.txt

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet- Drafts
   Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

   Distribution of this memo is unlimited.

Notices

   Copyright (C) 1995 L. Peter Deutsch

   Permission is granted to copy and distribute this document for any
   purpose and without charge, including translations into other
   languages and incorporation into compilations, provided that it is
   copied as a whole (including the copyright notice and this notice)
   and with no changes.

   Questions about this specification can be sent by email to Jean-loup
   Gailly <[e-mail]>.

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be



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   implemented readily in a manner not covered by patents.

Table of contents

   1. Introduction ................................................... 2
      1.1 Purpose .................................................... 2
      1.2 Intended audience .......................................... 3
      1.3 Scope ...................................................... 3
      1.4 Compliance ................................................. 3
      1.5 Definitions of terms and conventions used .................. 3
      1.6 Changes from previous versions ............................. 3
   2. Detailed specification ......................................... 4
      2.1 Overall conventions ........................................ 4
      2.2 File format ................................................ 5
      2.3 Member format .............................................. 5
          2.3.1. Member header and trailer ........................... 6
          2.3.1.1. Extra field ....................................... 8
          2.3.1.2. Compliance ........................................ 9
   3. References ..................................................... 9
      3.1 Related standards .......................................... 9
      3.2 Other related publications ................................. 9
   4. Security considerations ....................................... 10
   5. Acknowledgements .............................................. 10
   6. Author's address .............................................. 10
   7. Appendix: Jean-loup Gailly's gzip utility ..................... 11
   8. Appendix: Sample CRC Code ..................................... 11

1. Introduction

   1.1. Purpose

      The purpose of this specification is to define a lossless
      compressed data format that:

          o Is independent of CPU type, operating system, file system,
            and character set, and hence can be used for interchange;
          o Can compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another data stream,
            using only an a priori bounded amount of intermediate
            storage, and hence can be used in data communications or
            similar structures such as Unix filters;
          o Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods, and
            in particular considerably better than the 'compress'
            program;
          o Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          o Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.



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      The data format defined by this specification does not attempt to:

          o Provide random access to compressed data;
          o Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      format.

      The text of the specification assumes a basic background in
      programming at the level of bits and other primitive data
      representations.

   1.3. Scope

      The specification specifies a compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   1.5. Definitions of terms and conventions used

      byte: 8 bits stored or transmitted as a unit (same as an octet).
      (For this specification, a byte is exactly 8 bits, even on
      machines which store a character on a number of bits different
      from 8.)  See Section 2.1, below for the numbering of bits within
      a byte.

   1.6. Changes from previous versions

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to Internet Draft style.

2. Detailed specification



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   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      represents one byte; a box like this:

         +==============+
         |              |
         +==============+

      represents a variable number of bytes.

      Bytes stored within a computer do not have a 'bit order', since
      they are always treated as a unit.  However, a byte considered as
      an integer between 0 and 255 does have a most- and least-
      significant bit, and since we write numbers with the most-
      significant digit on the left, we also write bytes with the most-
      significant bit on the left.  In the diagrams below, we number the
      bits of a byte so that bit 0 is the least-significant bit, i.e.,
      the bits are numbered:

         +--------+
         |76543210|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- rather than bit-oriented.

      Within a computer, a number may occupy multiple bytes.  All
      multi-byte numbers in the format described here are stored with
      the least-significant byte first (at the lower memory address).
      For example, the decimal number 520 is stored as:

             0        1
         +--------+--------+
         |00001000|00000010|
         +--------+--------+
          ^        ^
          |        |
          |        + more significant byte = 2 x 256
          + less significant byte = 8

   2.2. File format

      A gzip file consists of a series of "members" (compressed data
      sets).  The format of each member is specified in the following
      section.  The members simply appear one after another in the file,


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      with no additional information before, between, or after them.

   2.3. Member format

      Each member has the following structure:

         +---+---+---+---+---+---+---+---+---+---+
         |ID1|ID2|CM |FLG|     MTIME     |XFL|OS | (more-->)
         +---+---+---+---+---+---+---+---+---+---+

      (if FLG.FEXTRA set)

         +---+---+=================================+
         | XLEN  |...XLEN bytes of 'extra field'...| (more-->)
         +---+---+=================================+

      (if FLG.FNAME set)

         +=========================================+
         |...original file name, zero-terminated...| (more-->)
         +=========================================+

      (if FLG.FCOMMENT set)

         +===================================+
         |...file comment, zero-terminated...| (more-->)
         +===================================+

      (if FLG.FHCRC set)

         +---+---+
         | CRC16 |
         +---+---+

         +=======================+
         |...compressed blocks...| (more-->)
         +=======================+

           0   1   2   3   4   5   6   7
         +---+---+---+---+---+---+---+---+
         |     CRC32     |     ISIZE     |
         +---+---+---+---+---+---+---+---+

      2.3.1. Member header and trailer

         ID1 (IDentification 1)

         ID2 (IDentification 2)

            These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139
            (0x8b, \213), to identify the file as being in gzip format.



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         CM (Compression Method)

            This identifies the compression method used in the file.  CM
            = 0-7 are reserved.  CM = 8 denotes the 'deflate'
            compression method, which is the one customarily used by
            gzip and which is documented elsewhere.

         FLG (FLaGs)

            This flag byte is divided into individual bits as follows:

               bit 0   FTEXT
               bit 1   FHCRC
               bit 2   FEXTRA
               bit 3   FNAME
               bit 4   FCOMMENT
               bit 5   reserved
               bit 6   reserved
               bit 7   reserved

            If FTEXT is set, the file is probably ASCII text.  This is
            an optional indication, which the compressor may set by
            checking a small amount of the input data to see whether any
            non-ASCII characters are present.  In case of doubt, FTEXT
            is cleared, indicating binary data. For systems which have
            different file formats for ascii text and binary data, the
            decompressor can use FTEXT to choose the appropriate format.
            We deliberately do not specify the algorithm used to set
            this bit, since a compressor always has the option of
            leaving it cleared and a decompressor always has the option
            of ignoring it and letting some other program handle issues
            of data conversion.

            If FHCRC is set, a CRC16 for the gzip header is present,
            immediately before the compressed data. The CRC16 consists
            of the two least significant bytes of the CRC32 for all
            bytes of the gzip header up to and not including the CRC16.
            [The FHCRC bit was never set by versions of gzip up to
            1.2.4, even though it was documented with a different
            meaning in gzip 1.2.4.]

            If FEXTRA is set, optional extra fields are present, as
            described in a following section.

            If FNAME is set, an original file name is present,
            terminated by a zero byte.  The name must consist of ISO
            8859-1 (LATIN-1) characters; on operating systems using
            EBCDIC or any other character set for file names, the name
            must be translated to the ISO LATIN-1 character set.  This
            is the original name of the file being compressed, with any
            directory components removed, and, if the file being
            compressed is on a file system with case insensitive names,


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