< draft-deutsch-gzip-spec-01.txt   draft-deutsch-gzip-spec-02.txt >
INTERNET-DRAFT L. Peter Deutsch INTERNET-DRAFT L. Peter Deutsch
GZIP 4.3 Aladdin Enterprises GZIP 4.3 Aladdin Enterprises
Expires: 17 Aug 1996 12 Feb 1996 Expires: 16 Sep 1996 11 Mar 1996
GZIP file format specification version 4.3 GZIP file format specification version 4.3
File draft-deutsch-gzip-spec-01.txt File draft-deutsch-gzip-spec-02.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts. working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
skipping to change at line 32 skipping to change at line 32
To learn the current status of any Internet-Draft, please check the To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts ``1id-abstracts.txt'' listing contained in the Internet- Drafts
Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast). ftp.isi.edu (US West Coast).
Distribution of this memo is unlimited. Distribution of this memo is unlimited.
Notices Notices
Copyright (C) 1996 L. Peter Deutsch Copyright (c) 1996 L. Peter Deutsch
Permission is granted to copy and distribute this document for any Permission is granted to copy and distribute this document for any
purpose and without charge, including translations into other purpose and without charge, including translations into other
languages and incorporation into compilations, provided that it is languages and incorporation into compilations, provided that it is
copied as a whole (including the copyright notice and this notice) copied as a whole (including the copyright notice and this notice)
and with no changes. and with no changes.
Deutsch [Page 1]
Abstract Abstract
This specification defines a lossless compressed data format that is This specification defines a lossless compressed data format that is
compatible with the widely used GZIP utility. The format includes a compatible with the widely used GZIP utility. The format includes a
cyclic redundancy check value for detecting data corruption. The cyclic redundancy check value for detecting data corruption. The
format presently uses the DEFLATE method of compression but can be format presently uses the DEFLATE method of compression but can be
easily extended to use other compression methods. The format can be easily extended to use other compression methods. The format can be
implemented readily in a manner not covered by patents. implemented readily in a manner not covered by patents.
Table of contents Table of Contents
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1. Introduction ................................................... 2 1. Introduction ................................................... 2
1.1 Purpose .................................................... 2 1.1. Purpose ................................................... 2
1.2 Intended audience .......................................... 2 1.2. Intended audience ......................................... 3
1.3 Scope ...................................................... 3 1.3. Scope ..................................................... 3
1.4 Compliance ................................................. 3 1.4. Compliance ................................................ 3
1.5 Definitions of terms and conventions used .................. 3 1.5. Definitions of terms and conventions used ................. 3
1.6 Changes from previous versions ............................. 3 1.6. Changes from previous versions ............................ 3
2. Detailed specification ......................................... 3 2. Detailed specification ......................................... 4
2.1 Overall conventions ........................................ 3 2.1. Overall conventions ....................................... 4
2.2 File format ................................................ 4 2.2. File format ............................................... 5
2.3 Member format .............................................. 4 2.3. Member format ............................................. 5
2.3.1. Member header and trailer ........................... 5 2.3.1. Member header and trailer ........................... 5
2.3.1.1. Extra field ....................................... 8 * 2.3.1.1. Extra field ...................... 8
2.3.1.2. Compliance ........................................ 8 * 2.3.1.2. Compliance ....................... 9
3. References ..................................................... 9 3. References .................................................. 9
3.1 Related standards .......................................... 9 4. Security considerations .................................... 10
3.2 Other related publications ................................. 9 5. Acknowledgements ........................................... 10
4. Security considerations ........................................ 9 6. Author's address ........................................... 10
5. Acknowledgements .............................................. 10 7. Appendix: Jean-loup Gailly's gzip utility .................. 11
6. Author's address .............................................. 10 8. Appendix: Sample CRC Code .................................. 11
7. Appendix: Jean-loup Gailly's gzip utility ..................... 10
8. Appendix: Sample CRC Code ..................................... 11
1. Introduction 1. Introduction
1.1. Purpose 1.1. Purpose
The purpose of this specification is to define a lossless The purpose of this specification is to define a lossless
compressed data format that: compressed data format that:
o Is independent of CPU type, operating system, file system,
* Is independent of CPU type, operating system, file system,
and character set, and hence can be used for interchange; and character set, and hence can be used for interchange;
o Can compress or decompress a data stream (as opposed to a * Can compress or decompress a data stream (as opposed to a
randomly accessible file) to produce another data stream, randomly accessible file) to produce another data stream,
using only an a priori bounded amount of intermediate using only an a priori bounded amount of intermediate
storage, and hence can be used in data communications or storage, and hence can be used in data communications or
similar structures such as Unix filters; similar structures such as Unix filters;
o Compresses data with efficiency comparable to the best * Compresses data with efficiency comparable to the best
currently available general-purpose compression methods, and currently available general-purpose compression methods, and
in particular considerably better than the 'compress' in particular considerably better than the 'compress'
program; program;
o Can be implemented readily in a manner not covered by * Can be implemented readily in a manner not covered by
patents, and hence can be practiced freely; patents, and hence can be practiced freely;
o Is compatible with the file format produced by the current
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* Is compatible with the file format produced by the current
widely used gzip utility, in that conforming decompressors widely used gzip utility, in that conforming decompressors
will be able to read data produced by the existing gzip will be able to read data produced by the existing gzip
compressor. compressor.
The data format defined by this specification does not attempt to: 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 * Provide random access to compressed data;
* Compress specialized data (e.g., raster graphics) as well as
the best currently available specialized algorithms. the best currently available specialized algorithms.
1.2. Intended audience 1.2. Intended audience
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This specification is intended for use by implementors of software This specification is intended for use by implementors of software
to compress data into gzip format and/or decompress data from gzip to compress data into gzip format and/or decompress data from gzip
format. format.
The text of the specification assumes a basic background in The text of the specification assumes a basic background in
programming at the level of bits and other primitive data programming at the level of bits and other primitive data
representations. representations.
1.3. Scope 1.3. Scope
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specifications presented here; a compliant compressor must produce specifications presented here; a compliant compressor must produce
files that conform to all the specifications presented here. The files that conform to all the specifications presented here. The
material in the appendices is not part of the specification per se material in the appendices is not part of the specification per se
and is not relevant to compliance. and is not relevant to compliance.
1.5. Definitions of terms and conventions used 1.5. Definitions of terms and conventions used
byte: 8 bits stored or transmitted as a unit (same as an octet). byte: 8 bits stored or transmitted as a unit (same as an octet).
(For this specification, a byte is exactly 8 bits, even on (For this specification, a byte is exactly 8 bits, even on
machines which store a character on a number of bits different machines which store a character on a number of bits different
from 8.) See Section 2.1, below for the numbering of bits within from 8.) See below for the numbering of bits within a byte.
a byte.
1.6. Changes from previous versions 1.6. Changes from previous versions
There have been no technical changes to the gzip format since There have been no technical changes to the gzip format since
version 4.1 of this specification. In version 4.2, some version 4.1 of this specification. In version 4.2, some
terminology was changed, and the sample CRC code was rewritten for terminology was changed, and the sample CRC code was rewritten for
clarity and to eliminate the requirement for the caller to do pre- clarity and to eliminate the requirement for the caller to do pre-
and post-conditioning. Version 4.3 is a conversion of the and post-conditioning. Version 4.3 is a conversion of the
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specification to Internet Draft style. specification to Internet Draft style.
2. Detailed specification 2. Detailed specification
2.1. Overall conventions 2.1. Overall conventions
In the diagrams below, a box like this: In the diagrams below, a box like this:
+---+ +---+
| | <-- the vertical bars might be missing | | <-- the vertical bars might be missing
+---+ +---+
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represents one byte; a box like this: represents one byte; a box like this:
+==============+ +==============+
| | | |
+==============+ +==============+
represents a variable number of bytes. represents a variable number of bytes.
Bytes stored within a computer do not have a 'bit order', since Bytes stored within a computer do not have a 'bit order', since
they are always treated as a unit. However, a byte considered as they are always treated as a unit. However, a byte considered as
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0 1 0 1
+--------+--------+ +--------+--------+
|00001000|00000010| |00001000|00000010|
+--------+--------+ +--------+--------+
^ ^ ^ ^
| | | |
| + more significant byte = 2 x 256 | + more significant byte = 2 x 256
+ less significant byte = 8 + less significant byte = 8
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2.2. File format 2.2. File format
A gzip file consists of a series of "members" (compressed data A gzip file consists of a series of "members" (compressed data
sets). The format of each member is specified in the following sets). The format of each member is specified in the following
section. The members simply appear one after another in the file, section. The members simply appear one after another in the file,
with no additional information before, between, or after them. with no additional information before, between, or after them.
2.3. Member format 2.3. Member format
Each member has the following structure: Each member has the following structure:
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+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+
|ID1|ID2|CM |FLG| MTIME |XFL|OS | (more-->) |ID1|ID2|CM |FLG| MTIME |XFL|OS | (more-->)
+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+
(if FLG.FEXTRA set) (if FLG.FEXTRA set)
+---+---+=================================+ +---+---+=================================+
| XLEN |...XLEN bytes of 'extra field'...| (more-->) | XLEN |...XLEN bytes of 'extra field'...| (more-->)
+---+---+=================================+ +---+---+=================================+
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|...compressed blocks...| (more-->) |...compressed blocks...| (more-->)
+=======================+ +=======================+
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| CRC32 | ISIZE | | CRC32 | ISIZE |
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
2.3.1. Member header and trailer 2.3.1. Member header and trailer
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ID1 (IDentification 1) ID1 (IDentification 1)
ID2 (IDentification 2) ID2 (IDentification 2)
These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139 These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139
(0x8b, \213), to identify the file as being in gzip format. (0x8b, \213), to identify the file as being in gzip format.
CM (Compression Method) CM (Compression Method)
This identifies the compression method used in the file. CM This identifies the compression method used in the file. CM
= 0-7 are reserved. CM = 8 denotes the 'deflate' = 0-7 are reserved. CM = 8 denotes the 'deflate'
compression method, which is the one customarily used by compression method, which is the one customarily used by
gzip and which is documented elsewhere. gzip and which is documented elsewhere.
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FLG (FLaGs) FLG (FLaGs)
This flag byte is divided into individual bits as follows: This flag byte is divided into individual bits as follows:
bit 0 FTEXT bit 0 FTEXT
bit 1 FHCRC bit 1 FHCRC
bit 2 FEXTRA bit 2 FEXTRA
bit 3 FNAME bit 3 FNAME
bit 4 FCOMMENT bit 4 FCOMMENT
bit 5 reserved bit 5 reserved
bit 6 reserved bit 6 reserved
bit 7 reserved bit 7 reserved
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meaning in gzip 1.2.4.] meaning in gzip 1.2.4.]
If FEXTRA is set, optional extra fields are present, as If FEXTRA is set, optional extra fields are present, as
described in a following section. described in a following section.
If FNAME is set, an original file name is present, If FNAME is set, an original file name is present,
terminated by a zero byte. The name must consist of ISO terminated by a zero byte. The name must consist of ISO
8859-1 (LATIN-1) characters; on operating systems using 8859-1 (LATIN-1) characters; on operating systems using
EBCDIC or any other character set for file names, the name EBCDIC or any other character set for file names, the name
must be translated to the ISO LATIN-1 character set. This 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 is the original name of the file being compressed, with any
directory components removed, and, if the file being directory components removed, and, if the file being
compressed is on a file system with case insensitive names, compressed is on a file system with case insensitive names,
forced to lower case. There is no original file name if the forced to lower case. There is no original file name if the
data was compressed from a source other than a named file; data was compressed from a source other than a named file;
for example, if the source was stdin on a Unix system, there for example, if the source was stdin on a Unix system, there
is no file name. is no file name.
If FCOMMENT is set, a zero-terminated file comment is If FCOMMENT is set, a zero-terminated file comment is
present. This comment is not interpreted; it is only present. This comment is not interpreted; it is only
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intended for human consumption. The comment must consist of intended for human consumption. The comment must consist of
ISO 8859-1 (LATIN-1) characters. Line breaks should be ISO 8859-1 (LATIN-1) characters. Line breaks should be
denoted by a single line feed character (10 decimal). denoted by a single line feed character (10 decimal).
Reserved FLG bits must be zero. Reserved FLG bits must be zero.
MTIME (Modification TIME) MTIME (Modification TIME)
This gives the most recent modification time of the original This gives the most recent modification time of the original
file being compressed. The time is in Unix format, i.e., file being compressed. The time is in Unix format, i.e.,
seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this
may cause problems for MS-DOS and other systems that use may cause problems for MS-DOS and other systems that use
local rather than Universal time.) If the compressed data local rather than Universal time.) If the compressed data
did not come from a file, MTIME is set to the time at which did not come from a file, MTIME is set to the time at which
compression started. MTIME = 0 means no time stamp is compression started. MTIME = 0 means no time stamp is
available. available.
XFL (eXtra FLags) XFL (eXtra FLags)
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This gives the most recent modification time of the original This gives the most recent modification time of the original
file being compressed. The time is in Unix format, i.e., file being compressed. The time is in Unix format, i.e.,
seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this
may cause problems for MS-DOS and other systems that use may cause problems for MS-DOS and other systems that use
local rather than Universal time.) If the compressed data local rather than Universal time.) If the compressed data
did not come from a file, MTIME is set to the time at which did not come from a file, MTIME is set to the time at which
compression started. MTIME = 0 means no time stamp is compression started. MTIME = 0 means no time stamp is
available. available.
XFL (eXtra FLags) XFL (eXtra FLags)
These flags are available for use by specific compression These flags are available for use by specific compression
methods. The 'deflate' method (CM = 8) sets these flags as methods. The 'deflate' method (CM = 8) sets these flags as
follows: follows:
XFL = 2 - compressor used maximum compression, XFL = 2 - compressor used maximum compression,
slowest algorithm slowest algorithm
XFL = 4 - compressor used fastest algorithm XFL = 4 - compressor used fastest algorithm
OS (Operating System) OS (Operating System)
This identifies the type of file system on which compression This identifies the type of file system on which compression
took place. This may be useful in determining end-of-line took place. This may be useful in determining end-of-line
convention for text files. The currently defined values are convention for text files. The currently defined values are
as follows: as follows:
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0 - FAT filesystem (MS-DOS, OS/2, NT/Win32) 0 - FAT filesystem (MS-DOS, OS/2, NT/Win32)
1 - Amiga 1 - Amiga
2 - VMS (or OpenVMS) 2 - VMS (or OpenVMS)
3 - Unix 3 - Unix
4 - VM/CMS 4 - VM/CMS
5 - Atari TOS 5 - Atari TOS
6 - HPFS filesystem (OS/2, NT) 6 - HPFS filesystem (OS/2, NT)
7 - Macintosh 7 - Macintosh
8 - Z-System 8 - Z-System
9 - CP/M 9 - CP/M
10 - TOPS-20 10 - TOPS-20
11 - NTFS filesystem (NT) 11 - NTFS filesystem (NT)
12 - QDOS 12 - QDOS
13 - Acorn RISCOS 13 - Acorn RISCOS
255 - unknown 255 - unknown
XLEN (eXtra LENgth) XLEN (eXtra LENgth)
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If FLG.FEXTRA is set, this gives the length of the optional If FLG.FEXTRA is set, this gives the length of the optional
extra field. See below for details. extra field. See below for details.
CRC32 (CRC-32) CRC32 (CRC-32)
This contains a Cyclic Redundancy Check value of the This contains a Cyclic Redundancy Check value of the
uncompressed data computed according to CRC-32 algorithm uncompressed data computed according to CRC-32 algorithm
used in the ISO 3309 standard and in section 8.1.1.6.2 of used in the ISO 3309 standard and in section 8.1.1.6.2 of
ITU-T recommendation V.42. (See http://www.iso.ch for ITU-T recommendation V.42. (See http://www.iso.ch for
ordering ISO documents. See gopher://info.itu.ch for an ordering ISO documents. See gopher://info.itu.ch for an
online version of ITU-T V.42.) online version of ITU-T V.42.)
ISIZE (Input SIZE) ISIZE (Input SIZE)
This contains the size of the original (uncompressed) input This contains the size of the original (uncompressed) input
data modulo 2^32. data modulo 2^32.
2.3.1.1. Extra field 2.3.1.1. Extra field
If the FLG.FEXTRA bit is set, an "extra field" is present in If the FLG.FEXTRA bit is set, an "extra field" is present in
the header, with total length XLEN bytes. It consists of a the header, with total length XLEN bytes. It consists of a
series of subfields, each of the form: series of subfields, each of the form:
+---+---+---+---+==================================+ +---+---+---+---+==================================+
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with some mnemonic value. Jean-loup Gailly with some mnemonic value. Jean-loup Gailly
<gzip@prep.ai.mit.edu> is maintaining a registry of subfield <gzip@prep.ai.mit.edu> is maintaining a registry of subfield
IDs; please send him any subfield ID you wish to use. Subfield IDs; please send him any subfield ID you wish to use. Subfield
IDs with SI2 = 0 are reserved for future use. The following IDs with SI2 = 0 are reserved for future use. The following
IDs are currently defined: IDs are currently defined:
SI1 SI2 Data SI1 SI2 Data
---------- ---------- ---- ---------- ---------- ----
0x41 ('A') 0x70 ('P') Apollo file type information 0x41 ('A') 0x70 ('P') Apollo file type information
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LEN gives the length of the subfield data, excluding the 4 LEN gives the length of the subfield data, excluding the 4
initial bytes. initial bytes.
2.3.1.2. Compliance 2.3.1.2. Compliance
A compliant compressor must produce files with correct ID1, A compliant compressor must produce files with correct ID1,
ID2, CM, CRC32, and ISIZE, but may set all the other fields in ID2, CM, CRC32, and ISIZE, but may set all the other fields in
the fixed-length part of the header to default values (255 for the fixed-length part of the header to default values (255 for
OS, 0 for all others). The compressor must set all reserved OS, 0 for all others). The compressor must set all reserved
bits to zero. bits to zero.
A compliant decompressor must check ID1, ID2, and CM, and A compliant decompressor must check ID1, ID2, and CM, and
provide an error indication if any of these have incorrect provide an error indication if any of these have incorrect
values. It must examine FEXTRA/XLEN, FNAME, FCOMMENT and FHCRC 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 at least so it can skip over the optional fields if they are
present. It need not examine any other part of the header or present. It need not examine any other part of the header or
trailer; in particular, a decompressor may ignore FTEXT and OS trailer; in particular, a decompressor may ignore FTEXT and OS
and always produce binary output, and still be compliant. A and always produce binary output, and still be compliant. A
compliant decompressor must give an error indication if any compliant decompressor must give an error indication if any
reserved bit is non-zero, since such a bit could indicate the reserved bit is non-zero, since such a bit could indicate the
presence of a new field that would cause subsequent data to be presence of a new field that would cause subsequent data to be
interpreted incorrectly. interpreted incorrectly.
3. References 3. References
3.1. Related standards [1] "Information Processing - 8-bit single-byte coded graphic
character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987).
"Information Processing - 8-bit single-byte coded graphic The ISO 8859-1 (Latin-1) character set is a superset of 7-bit
character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987). ASCII. Files defining this character set may be obtained from
The ISO 8859-1 (Latin-1) character set is a superset of 7-bit ftp.uu.net:/graphics/png/documents/iso_8859-1.*
ASCII. Files defining this character set may be obtained from
ftp.uu.net:/graphics/png/documents/iso_8859-1.*
ISO 3309
ITU-T recommendation V.42 [2] ISO 3309
3.2. Other related publications [3] ITU-T recommendation V.42
[1] Deutsch, L.P.,"'Deflate' Compressed Data Format [4] Deutsch, L.P.,"'Deflate' Compressed Data Format Specification".
Specification". available in available in ftp.uu.net:/pub/archiving/zip/doc/deflate-*.doc
ftp.uu.net:/pub/archiving/zip/doc/deflate-*.doc
[2] Gailly, J.-L., gzip documentation, available in [5] Gailly, J.-L., gzip documentation, available in
prep.ai.mit.edu:/pub/gnu/gzip-*.tar prep.ai.mit.edu:/pub/gnu/gzip-*.tar
[3] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via [6] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table
Table Look-Up", Communications of the ACM, 31(8), pp.1008-1013. Look-Up", Communications of the ACM, 31(8), pp.1008-1013.
[4] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal, [7] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal,
pp.118-133. pp.118-133.
[5] ftp.adelaide.edu.au:/pub/rocksoft/papers/crc_v3.txt, [8] ftp.adelaide.edu.au:/pub/rocksoft/papers/crc_v3.txt, describing
describing the CRC concept. the CRC concept.
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4. Security considerations 4. Security considerations
Any data compression method involves the reduction of redundancy in Any data compression method involves the reduction of redundancy in
the data. Consequently, any corruption of the data is likely to have the data. Consequently, any corruption of the data is likely to have
severe effects and be difficult to correct. Uncompressed text, on severe effects and be difficult to correct. Uncompressed text, on
the other hand, will probably still be readable despite the presence the other hand, will probably still be readable despite the presence
of some corrupted bytes. of some corrupted bytes.
It is recommended that systems using this data format provide some It is recommended that systems using this data format provide some
means of validating the integrity of the compressed data, such as by means of validating the integrity of the compressed data, such as by
setting and checking the CRC-32 check value. setting and checking the CRC-32 check value.
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5. Acknowledgements 5. Acknowledgements
Trademarks cited in this document are the property of their Trademarks cited in this document are the property of their
respective owners. respective owners.
Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler, Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler,
the related software described in this specification. Glenn the related software described in this specification. Glenn
Randers-Pehrson converted this document to Internet Draft and HTML Randers-Pehrson converted this document to Internet Draft and HTML
format. format.
skipping to change at line 512 skipping to change at line 498
sent by email to sent by email to
Jean-loup Gailly <gzip@prep.ai.mit.edu> and Jean-loup Gailly <gzip@prep.ai.mit.edu> and
Mark Adler <madler@alumni.caltech.edu> Mark Adler <madler@alumni.caltech.edu>
Editorial comments on this specification can be sent by email to Editorial comments on this specification can be sent by email to
L. Peter Deutsch <ghost@aladdin.com> and L. Peter Deutsch <ghost@aladdin.com> and
Glenn Randers-Pehrson <randeg@alumni.rpi.edu> Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
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7. Appendix: Jean-loup Gailly's gzip utility 7. Appendix: Jean-loup Gailly's gzip utility
The most widely used implementation of gzip compression, and the The most widely used implementation of gzip compression, and the
original documentation on which this specification is based, were original documentation on which this specification is based, were
created by Jean-loup Gailly <gzip@prep.ai.mit.edu>. Since this created by Jean-loup Gailly <gzip@prep.ai.mit.edu>. Since this
implementation is a de facto standard, we mention some more of its implementation is a de facto standard, we mention some more of its
features here. Again, the material in this section is not part of features here. Again, the material in this section is not part of
the specification per se, and implementations need not follow it to the specification per se, and implementations need not follow it to
be compliant. be compliant.
When compressing or decompressing a file, gzip preserves the When compressing or decompressing a file, gzip preserves the
protection, ownership, and modification time attributes on the local protection, ownership, and modification time attributes on the local
file system, since there is no provision for representing protection file system, since there is no provision for representing protection
attributes in the gzip file format itself. Since the file format attributes in the gzip file format itself. Since the file format
includes a modification time, the gzip decompressor provides a includes a modification time, the gzip decompressor provides a
command line switch that assigns the modification time from the file, command line switch that assigns the modification time from the file,
rather than the local modification time of the compressed input, to rather than the local modification time of the compressed input, to
the decompressed output. the decompressed output.
Deutsch [Page 10]
8. Appendix: Sample CRC Code 8. Appendix: Sample CRC Code
The following sample code represents a practical implementation of The following sample code represents a practical implementation of
the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42 the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42
for a formal specification.) for a formal specification.)
The sample code is in the ANSI C programming language. Non C users The sample code is in the ANSI C programming language. Non C users
may find it easier to read with these hints: may find it easier to read with these hints:
& Bitwise AND operator. & Bitwise AND operator.
skipping to change at line 563 skipping to change at line 549
/* Flag: has the table been computed? Initially false. */ /* Flag: has the table been computed? Initially false. */
int crc_table_computed = 0; int crc_table_computed = 0;
/* Make the table for a fast CRC. */ /* Make the table for a fast CRC. */
void make_crc_table(void) void make_crc_table(void)
{ {
unsigned long c; unsigned long c;
int n, k; int n, k;
Deutsch [Page 11]
for (n = 0; n < 256; n++) { for (n = 0; n < 256; n++) {
c = (unsigned long) n; c = (unsigned long) n;
for (k = 0; k < 8; k++) { for (k = 0; k < 8; k++) {
if (c & 1) { if (c & 1) {
c = 0xedb88320L ^ (c >> 1); c = 0xedb88320L ^ (c >> 1);
} else { } else {
c = c >> 1; c = c >> 1;
} }
} }
crc_table[n] = c; crc_table[n] = c;
} }
crc_table_computed = 1; crc_table_computed = 1;
} }
Deutsch [Page 11]
/* /*
Update a running crc with the bytes buf[0..len-1] and return Update a running crc with the bytes buf[0..len-1] and return
the updated crc. The crc should be initialized to zero. Pre- and the updated crc. The crc should be initialized to zero. Pre- and
post-conditioning (one's complement) is performed within this post-conditioning (one's complement) is performed within this
function so it shouldn't be done by the caller. Usage example: function so it shouldn't be done by the caller. Usage example:
unsigned long crc = 0L; unsigned long crc = 0L;
while (read_buffer(buffer, length) != EOF) { while (read_buffer(buffer, length) != EOF) {
crc = update_crc(crc, buffer, length); crc = update_crc(crc, buffer, length);
 End of changes. 52 change blocks. 
78 lines changed or deleted 64 lines changed or added

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