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Network Working Group P. Deutsch |
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Request for Comments: 1952 Aladdin Enterprises |
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Category: Informational May 1996 |
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GZIP file format specification version 4.3 |
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|
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Status of This Memo |
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|
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This memo provides information for the Internet community. This memo |
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does not specify an Internet standard of any kind. Distribution of |
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this memo is unlimited. |
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|
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IESG Note: |
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|
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The IESG takes no position on the validity of any Intellectual |
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Property Rights statements contained in this document. |
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|
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Notices |
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|
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Copyright (c) 1996 L. Peter Deutsch |
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|
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Permission is granted to copy and distribute this document for any |
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purpose and without charge, including translations into other |
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languages and incorporation into compilations, provided that the |
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copyright notice and this notice are preserved, and that any |
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substantive changes or deletions from the original are clearly |
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marked. |
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|
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A pointer to the latest version of this and related documentation in |
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HTML format can be found at the URL |
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<ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>. |
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|
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Abstract |
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|
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This specification defines a lossless compressed data format that is |
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compatible with the widely used GZIP utility. The format includes a |
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cyclic redundancy check value for detecting data corruption. The |
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format presently uses the DEFLATE method of compression but can be |
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easily extended to use other compression methods. The format can be |
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implemented readily in a manner not covered by patents. |
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Deutsch Informational [Page 1] |
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|
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RFC 1952 GZIP File Format Specification May 1996 |
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|
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|
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Table of Contents |
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|
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1. Introduction ................................................... 2 |
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1.1. Purpose ................................................... 2 |
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1.2. Intended audience ......................................... 3 |
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1.3. Scope ..................................................... 3 |
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1.4. Compliance ................................................ 3 |
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1.5. Definitions of terms and conventions used ................. 3 |
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1.6. Changes from previous versions ............................ 3 |
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2. Detailed specification ......................................... 4 |
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2.1. Overall conventions ....................................... 4 |
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2.2. File format ............................................... 5 |
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2.3. Member format ............................................. 5 |
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2.3.1. Member header and trailer ........................... 6 |
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2.3.1.1. Extra field ................................... 8 |
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2.3.1.2. Compliance .................................... 9 |
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3. References .................................................. 9 |
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4. Security Considerations .................................... 10 |
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5. Acknowledgements ........................................... 10 |
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6. Author's Address ........................................... 10 |
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7. Appendix: Jean-Loup Gailly's gzip utility .................. 11 |
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8. Appendix: Sample CRC Code .................................. 11 |
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|
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1. Introduction |
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|
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1.1. Purpose |
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|
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The purpose of this specification is to define a lossless |
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compressed data format that: |
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|
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* Is independent of CPU type, operating system, file system, |
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and character set, and hence can be used for interchange; |
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* Can compress or decompress a data stream (as opposed to a |
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randomly accessible file) to produce another data stream, |
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using only an a priori bounded amount of intermediate |
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storage, and hence can be used in data communications or |
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similar structures such as Unix filters; |
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* Compresses data with efficiency comparable to the best |
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currently available general-purpose compression methods, |
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and in particular considerably better than the "compress" |
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program; |
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* Can be implemented readily in a manner not covered by |
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patents, and hence can be practiced freely; |
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* Is compatible with the file format produced by the current |
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widely used gzip utility, in that conforming decompressors |
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will be able to read data produced by the existing gzip |
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compressor. |
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Deutsch Informational [Page 2] |
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|
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RFC 1952 GZIP File Format Specification May 1996 |
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|
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The data format defined by this specification does not attempt to: |
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|
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* Provide random access to compressed data; |
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* Compress specialized data (e.g., raster graphics) as well as |
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the best currently available specialized algorithms. |
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|
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1.2. Intended audience |
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|
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This specification is intended for use by implementors of software |
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to compress data into gzip format and/or decompress data from gzip |
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format. |
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|
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The text of the specification assumes a basic background in |
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programming at the level of bits and other primitive data |
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representations. |
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|
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1.3. Scope |
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|
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The specification specifies a compression method and a file format |
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(the latter assuming only that a file can store a sequence of |
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arbitrary bytes). It does not specify any particular interface to |
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a file system or anything about character sets or encodings |
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(except for file names and comments, which are optional). |
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|
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1.4. Compliance |
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|
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Unless otherwise indicated below, a compliant decompressor must be |
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able to accept and decompress any file that conforms to all the |
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specifications presented here; a compliant compressor must produce |
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files that conform to all the specifications presented here. The |
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material in the appendices is not part of the specification per se |
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and is not relevant to compliance. |
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|
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1.5. Definitions of terms and conventions used |
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|
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byte: 8 bits stored or transmitted as a unit (same as an octet). |
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(For this specification, a byte is exactly 8 bits, even on |
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machines which store a character on a number of bits different |
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from 8.) See below for the numbering of bits within a byte. |
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|
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1.6. Changes from previous versions |
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|
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There have been no technical changes to the gzip format since |
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version 4.1 of this specification. In version 4.2, some |
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terminology was changed, and the sample CRC code was rewritten for |
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clarity and to eliminate the requirement for the caller to do pre- |
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and post-conditioning. Version 4.3 is a conversion of the |
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specification to RFC style. |
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Deutsch Informational [Page 3] |
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|
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RFC 1952 GZIP File Format Specification May 1996 |
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|
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|
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2. Detailed specification |
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|
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2.1. Overall conventions |
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|
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In the diagrams below, a box like this: |
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|
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+---+ |
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| | <-- the vertical bars might be missing |
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+---+ |
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|
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represents one byte; a box like this: |
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+==============+ |
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| | |
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+==============+ |
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represents a variable number of bytes. |
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|
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Bytes stored within a computer do not have a "bit order", since |
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they are always treated as a unit. However, a byte considered as |
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an integer between 0 and 255 does have a most- and least- |
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significant bit, and since we write numbers with the most- |
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significant digit on the left, we also write bytes with the most- |
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significant bit on the left. In the diagrams below, we number the |
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bits of a byte so that bit 0 is the least-significant bit, i.e., |
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the bits are numbered: |
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|
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+--------+ |
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|76543210| |
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+--------+ |
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|
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This document does not address the issue of the order in which |
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bits of a byte are transmitted on a bit-sequential medium, since |
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the data format described here is byte- rather than bit-oriented. |
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Within a computer, a number may occupy multiple bytes. All |
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multi-byte numbers in the format described here are stored with |
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the least-significant byte first (at the lower memory address). |
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For example, the decimal number 520 is stored as: |
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|
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0 1 |
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+--------+--------+ |
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|00001000|00000010| |
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+--------+--------+ |
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^ ^ |
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| | |
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| + more significant byte = 2 x 256 |
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+ less significant byte = 8 |
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Deutsch Informational [Page 4] |
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|
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RFC 1952 GZIP File Format Specification May 1996 |
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|
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2.2. File format |
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|
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A gzip file consists of a series of "members" (compressed data |
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sets). The format of each member is specified in the following |
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section. The members simply appear one after another in the file, |
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with no additional information before, between, or after them. |
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2.3. Member format |
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Each member has the following structure: |
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+---+---+---+---+---+---+---+---+---+---+ |
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|ID1|ID2|CM |FLG| MTIME |XFL|OS | (more-->) |
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+---+---+---+---+---+---+---+---+---+---+ |
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|
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(if FLG.FEXTRA set) |
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|
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+---+---+=================================+ |
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| XLEN |...XLEN bytes of "extra field"...| (more-->) |
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+---+---+=================================+ |
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(if FLG.FNAME set) |
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+=========================================+ |
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|...original file name, zero-terminated...| (more-->) |
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+=========================================+ |
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(if FLG.FCOMMENT set) |
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+===================================+ |
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|...file comment, zero-terminated...| (more-->) |
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+===================================+ |
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(if FLG.FHCRC set) |
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+---+---+ |
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| CRC16 | |
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+---+---+ |
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+=======================+ |
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|...compressed blocks...| (more-->) |
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+=======================+ |
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0 1 2 3 4 5 6 7 |
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+---+---+---+---+---+---+---+---+ |
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| CRC32 | ISIZE | |
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+---+---+---+---+---+---+---+---+ |
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Deutsch Informational [Page 5] |
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|
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RFC 1952 GZIP File Format Specification May 1996 |
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|
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|
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2.3.1. Member header and trailer |
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|
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ID1 (IDentification 1) |
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ID2 (IDentification 2) |
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These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139 |
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(0x8b, \213), to identify the file as being in gzip format. |
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|
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CM (Compression Method) |
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This identifies the compression method used in the file. CM |
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= 0-7 are reserved. CM = 8 denotes the "deflate" |
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compression method, which is the one customarily used by |
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gzip and which is documented elsewhere. |
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|
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FLG (FLaGs) |
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This flag byte is divided into individual bits as follows: |
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|
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bit 0 FTEXT |
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bit 1 FHCRC |
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bit 2 FEXTRA |
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bit 3 FNAME |
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bit 4 FCOMMENT |
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bit 5 reserved |
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bit 6 reserved |
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bit 7 reserved |
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|
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If FTEXT is set, the file is probably ASCII text. This is |
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an optional indication, which the compressor may set by |
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checking a small amount of the input data to see whether any |
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non-ASCII characters are present. In case of doubt, FTEXT |
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is cleared, indicating binary data. For systems which have |
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different file formats for ascii text and binary data, the |
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decompressor can use FTEXT to choose the appropriate format. |
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We deliberately do not specify the algorithm used to set |
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this bit, since a compressor always has the option of |
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leaving it cleared and a decompressor always has the option |
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of ignoring it and letting some other program handle issues |
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of data conversion. |
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|
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If FHCRC is set, a CRC16 for the gzip header is present, |
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immediately before the compressed data. The CRC16 consists |
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of the two least significant bytes of the CRC32 for all |
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bytes of the gzip header up to and not including the CRC16. |
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[The FHCRC bit was never set by versions of gzip up to |
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1.2.4, even though it was documented with a different |
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meaning in gzip 1.2.4.] |
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|
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If FEXTRA is set, optional extra fields are present, as |
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described in a following section. |
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|
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|
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|
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Deutsch Informational [Page 6] |
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|
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RFC 1952 GZIP File Format Specification May 1996 |
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|
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|
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If FNAME is set, an original file name is present, |
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terminated by a zero byte. The name must consist of ISO |
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8859-1 (LATIN-1) characters; on operating systems using |
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EBCDIC or any other character set for file names, the name |
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must be translated to the ISO LATIN-1 character set. This |
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is the original name of the file being compressed, with any |
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directory components removed, and, if the file being |
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compressed is on a file system with case insensitive names, |
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forced to lower case. There is no original file name if the |
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data was compressed from a source other than a named file; |
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for example, if the source was stdin on a Unix system, there |
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is no file name. |
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|
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If FCOMMENT is set, a zero-terminated file comment is |
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present. This comment is not interpreted; it is only |
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intended for human consumption. The comment must consist of |
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ISO 8859-1 (LATIN-1) characters. Line breaks should be |
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denoted by a single line feed character (10 decimal). |
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|
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Reserved FLG bits must be zero. |
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|
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MTIME (Modification TIME) |
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This gives the most recent modification time of the original |
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file being compressed. The time is in Unix format, i.e., |
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seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this |
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may cause problems for MS-DOS and other systems that use |
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local rather than Universal time.) If the compressed data |
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did not come from a file, MTIME is set to the time at which |
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compression started. MTIME = 0 means no time stamp is |
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available. |
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|
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XFL (eXtra FLags) |
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These flags are available for use by specific compression |
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methods. The "deflate" method (CM = 8) sets these flags as |
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follows: |
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|
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XFL = 2 - compressor used maximum compression, |
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slowest algorithm |
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XFL = 4 - compressor used fastest algorithm |
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|
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OS (Operating System) |
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This identifies the type of file system on which compression |
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took place. This may be useful in determining end-of-line |
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convention for text files. The currently defined values are |
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as follows: |
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|
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|
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Deutsch Informational [Page 7] |
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|
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RFC 1952 GZIP File Format Specification May 1996 |
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|
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|
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0 - FAT filesystem (MS-DOS, OS/2, NT/Win32) |
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1 - Amiga |
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2 - VMS (or OpenVMS) |
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3 - Unix |
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4 - VM/CMS |
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5 - Atari TOS |
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6 - HPFS filesystem (OS/2, NT) |
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7 - Macintosh |
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8 - Z-System |
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9 - CP/M |
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10 - TOPS-20 |
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11 - NTFS filesystem (NT) |
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12 - QDOS |
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13 - Acorn RISCOS |
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255 - unknown |
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|
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XLEN (eXtra LENgth) |
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If FLG.FEXTRA is set, this gives the length of the optional |
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extra field. See below for details. |
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|
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CRC32 (CRC-32) |
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This contains a Cyclic Redundancy Check value of the |
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uncompressed data computed according to CRC-32 algorithm |
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used in the ISO 3309 standard and in section 8.1.1.6.2 of |
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ITU-T recommendation V.42. (See http://www.iso.ch for |
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ordering ISO documents. See gopher://info.itu.ch for an |
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online version of ITU-T V.42.) |
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|
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ISIZE (Input SIZE) |
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This contains the size of the original (uncompressed) input |
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data modulo 2^32. |
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|
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2.3.1.1. Extra field |
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|
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If the FLG.FEXTRA bit is set, an "extra field" is present in |
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the header, with total length XLEN bytes. It consists of a |
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series of subfields, each of the form: |
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|
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+---+---+---+---+==================================+ |
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|SI1|SI2| LEN |... LEN bytes of subfield data ...| |
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+---+---+---+---+==================================+ |
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|
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SI1 and SI2 provide a subfield ID, typically two ASCII letters |
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with some mnemonic value. Jean-Loup Gailly |
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<gzip@prep.ai.mit.edu> is maintaining a registry of subfield |
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IDs; please send him any subfield ID you wish to use. Subfield |
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IDs with SI2 = 0 are reserved for future use. The following |
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IDs are currently defined: |
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|
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|
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|
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Deutsch Informational [Page 8] |
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|
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RFC 1952 GZIP File Format Specification May 1996 |
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|
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|
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SI1 SI2 Data |
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---------- ---------- ---- |
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0x41 ('A') 0x70 ('P') Apollo file type information |
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|
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LEN gives the length of the subfield data, excluding the 4 |
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initial bytes. |
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|
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2.3.1.2. Compliance |
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|
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A compliant compressor must produce files with correct ID1, |
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ID2, CM, CRC32, and ISIZE, but may set all the other fields in |
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the fixed-length part of the header to default values (255 for |
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OS, 0 for all others). The compressor must set all reserved |
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bits to zero. |
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|
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A compliant decompressor must check ID1, ID2, and CM, and |
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provide an error indication if any of these have incorrect |
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values. It must examine FEXTRA/XLEN, FNAME, FCOMMENT and FHCRC |
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at least so it can skip over the optional fields if they are |
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present. It need not examine any other part of the header or |
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trailer; in particular, a decompressor may ignore FTEXT and OS |
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and always produce binary output, and still be compliant. A |
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compliant decompressor must give an error indication if any |
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reserved bit is non-zero, since such a bit could indicate the |
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presence of a new field that would cause subsequent data to be |
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interpreted incorrectly. |
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|
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3. References |
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|
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[1] "Information Processing - 8-bit single-byte coded graphic |
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character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987). |
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The ISO 8859-1 (Latin-1) character set is a superset of 7-bit |
487 |
ASCII. Files defining this character set are available as |
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iso_8859-1.* in ftp://ftp.uu.net/graphics/png/documents/ |
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|
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[2] ISO 3309 |
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|
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[3] ITU-T recommendation V.42 |
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|
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[4] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification", |
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available in ftp://ftp.uu.net/pub/archiving/zip/doc/ |
496 |
|
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[5] Gailly, J.-L., GZIP documentation, available as gzip-*.tar in |
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ftp://prep.ai.mit.edu/pub/gnu/ |
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|
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[6] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table |
501 |
Look-Up", Communications of the ACM, 31(8), pp.1008-1013. |
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|
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|
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|
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|
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Deutsch Informational [Page 9] |
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|
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RFC 1952 GZIP File Format Specification May 1996 |
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|
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|
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[7] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal, |
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pp.118-133. |
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|
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[8] ftp://ftp.adelaide.edu.au/pub/rocksoft/papers/crc_v3.txt, |
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describing the CRC concept. |
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|
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4. Security Considerations |
518 |
|
519 |
Any data compression method involves the reduction of redundancy in |
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the data. Consequently, any corruption of the data is likely to have |
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severe effects and be difficult to correct. Uncompressed text, on |
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the other hand, will probably still be readable despite the presence |
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of some corrupted bytes. |
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|
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It is recommended that systems using this data format provide some |
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means of validating the integrity of the compressed data, such as by |
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setting and checking the CRC-32 check value. |
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|
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5. Acknowledgements |
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|
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Trademarks cited in this document are the property of their |
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respective owners. |
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|
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Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler, |
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the related software described in this specification. Glenn |
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Randers-Pehrson converted this document to RFC and HTML format. |
537 |
|
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6. Author's Address |
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|
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L. Peter Deutsch |
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Aladdin Enterprises |
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203 Santa Margarita Ave. |
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Menlo Park, CA 94025 |
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|
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Phone: (415) 322-0103 (AM only) |
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FAX: (415) 322-1734 |
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EMail: <ghost@aladdin.com> |
548 |
|
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Questions about the technical content of this specification can be |
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sent by email to: |
551 |
|
552 |
Jean-Loup Gailly <gzip@prep.ai.mit.edu> and |
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Mark Adler <madler@alumni.caltech.edu> |
554 |
|
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Editorial comments on this specification can be sent by email to: |
556 |
|
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L. Peter Deutsch <ghost@aladdin.com> and |
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Glenn Randers-Pehrson <randeg@alumni.rpi.edu> |
559 |
|
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|
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|
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Deutsch Informational [Page 10] |
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|
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RFC 1952 GZIP File Format Specification May 1996 |
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|
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|
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7. Appendix: Jean-Loup Gailly's gzip utility |
568 |
|
569 |
The most widely used implementation of gzip compression, and the |
570 |
original documentation on which this specification is based, were |
571 |
created by Jean-Loup Gailly <gzip@prep.ai.mit.edu>. Since this |
572 |
implementation is a de facto standard, we mention some more of its |
573 |
features here. Again, the material in this section is not part of |
574 |
the specification per se, and implementations need not follow it to |
575 |
be compliant. |
576 |
|
577 |
When compressing or decompressing a file, gzip preserves the |
578 |
protection, ownership, and modification time attributes on the local |
579 |
file system, since there is no provision for representing protection |
580 |
attributes in the gzip file format itself. Since the file format |
581 |
includes a modification time, the gzip decompressor provides a |
582 |
command line switch that assigns the modification time from the file, |
583 |
rather than the local modification time of the compressed input, to |
584 |
the decompressed output. |
585 |
|
586 |
8. Appendix: Sample CRC Code |
587 |
|
588 |
The following sample code represents a practical implementation of |
589 |
the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42 |
590 |
for a formal specification.) |
591 |
|
592 |
The sample code is in the ANSI C programming language. Non C users |
593 |
may find it easier to read with these hints: |
594 |
|
595 |
& Bitwise AND operator. |
596 |
^ Bitwise exclusive-OR operator. |
597 |
>> Bitwise right shift operator. When applied to an |
598 |
unsigned quantity, as here, right shift inserts zero |
599 |
bit(s) at the left. |
600 |
! Logical NOT operator. |
601 |
++ "n++" increments the variable n. |
602 |
0xNNN 0x introduces a hexadecimal (base 16) constant. |
603 |
Suffix L indicates a long value (at least 32 bits). |
604 |
|
605 |
/* Table of CRCs of all 8-bit messages. */ |
606 |
unsigned long crc_table[256]; |
607 |
|
608 |
/* Flag: has the table been computed? Initially false. */ |
609 |
int crc_table_computed = 0; |
610 |
|
611 |
/* Make the table for a fast CRC. */ |
612 |
void make_crc_table(void) |
613 |
{ |
614 |
unsigned long c; |
615 |
|
616 |
|
617 |
|
618 |
Deutsch Informational [Page 11] |
619 |
|
620 |
RFC 1952 GZIP File Format Specification May 1996 |
621 |
|
622 |
|
623 |
int n, k; |
624 |
for (n = 0; n < 256; n++) { |
625 |
c = (unsigned long) n; |
626 |
for (k = 0; k < 8; k++) { |
627 |
if (c & 1) { |
628 |
c = 0xedb88320L ^ (c >> 1); |
629 |
} else { |
630 |
c = c >> 1; |
631 |
} |
632 |
} |
633 |
crc_table[n] = c; |
634 |
} |
635 |
crc_table_computed = 1; |
636 |
} |
637 |
|
638 |
/* |
639 |
Update a running crc with the bytes buf[0..len-1] and return |
640 |
the updated crc. The crc should be initialized to zero. Pre- and |
641 |
post-conditioning (one's complement) is performed within this |
642 |
function so it shouldn't be done by the caller. Usage example: |
643 |
|
644 |
unsigned long crc = 0L; |
645 |
|
646 |
while (read_buffer(buffer, length) != EOF) { |
647 |
crc = update_crc(crc, buffer, length); |
648 |
} |
649 |
if (crc != original_crc) error(); |
650 |
*/ |
651 |
unsigned long update_crc(unsigned long crc, |
652 |
unsigned char *buf, int len) |
653 |
{ |
654 |
unsigned long c = crc ^ 0xffffffffL; |
655 |
int n; |
656 |
|
657 |
if (!crc_table_computed) |
658 |
make_crc_table(); |
659 |
for (n = 0; n < len; n++) { |
660 |
c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8); |
661 |
} |
662 |
return c ^ 0xffffffffL; |
663 |
} |
664 |
|
665 |
/* Return the CRC of the bytes buf[0..len-1]. */ |
666 |
unsigned long crc(unsigned char *buf, int len) |
667 |
{ |
668 |
return update_crc(0L, buf, len); |
669 |
} |
670 |
|
671 |
|
672 |
|
673 |
|
674 |
Deutsch Informational [Page 12] |
675 |
|