FreeBSD/Linux Kernel Cross Reference
sys/contrib/zlib/doc/rfc1952.txt

     
     
     
     
     
     
     Network Working Group                                         P. Deutsch
     Request for Comments: 1952                           Aladdin Enterprises
     Category: Informational                                         May 1996
    
    
                   GZIP file format specification version 4.3
    
    Status of This Memo
    
       This memo provides information for the Internet community.  This memo
       does not specify an Internet standard of any kind.  Distribution of
       this memo is unlimited.
    
    IESG Note:
    
       The IESG takes no position on the validity of any Intellectual
       Property Rights statements contained in this document.
    
    Notices
    
       Copyright (c) 1996 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 the
       copyright notice and this notice are preserved, and that any
       substantive changes or deletions from the original are clearly
       marked.
    
       A pointer to the latest version of this and related documentation in
       HTML format can be found at the URL
       <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>.
    
    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
       implemented readily in a manner not covered by patents.
    
    
    
    
    
    
    
    
    
    
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    RFC 1952             GZIP File Format Specification             May 1996
    
    
    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
          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:
    
              * Is independent of CPU type, operating system, file system,
                and character set, and hence can be used for interchange;
              * 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;
             * Compresses data with efficiency comparable to the best
               currently available general-purpose compression methods,
               and in particular considerably better than the "compress"
               program;
             * Can be implemented readily in a manner not covered by
               patents, and hence can be practiced freely;
             * 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:
   
             * Provide random access to compressed data;
             * 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 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 RFC style.
   
   
   
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   2. Detailed specification
   
      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
   
   
   
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      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,
         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     |
            +---+---+---+---+---+---+---+---+
   
   
   
   
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         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.
   
            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.
   
   
   
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               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,
               forced to lower case. There is no original file name if the
               data was compressed from a source other than a named file;
               for example, if the source was stdin on a Unix system, there
               is no file name.
   
               If FCOMMENT is set, a zero-terminated file comment is
               present.  This comment is not interpreted; it is only
               intended for human consumption.  The comment must consist of
               ISO 8859-1 (LATIN-1) characters.  Line breaks should be
               denoted by a single line feed character (10 decimal).
   
               Reserved FLG bits must be zero.
   
            MTIME (Modification TIME)
               This gives the most recent modification time of the original
               file being compressed.  The time is in Unix format, i.e.,
               seconds since 00:00:00 GMT, Jan.  1, 1970.  (Note that this
               may cause problems for MS-DOS and other systems that use
               local rather than Universal time.)  If the compressed data
               did not come from a file, MTIME is set to the time at which
               compression started.  MTIME = 0 means no time stamp is
               available.
   
            XFL (eXtra FLags)
               These flags are available for use by specific compression
               methods.  The "deflate" method (CM = 8) sets these flags as
               follows:
   
                  XFL = 2 - compressor used maximum compression,
                            slowest algorithm
                  XFL = 4 - compressor used fastest algorithm
   
            OS (Operating System)
               This identifies the type of file system on which compression
               took place.  This may be useful in determining end-of-line
               convention for text files.  The currently defined values are
               as follows:
   
   
   
   
   
   
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                    0 - FAT filesystem (MS-DOS, OS/2, NT/Win32)
                    1 - Amiga
                    2 - VMS (or OpenVMS)
                    3 - Unix
                    4 - VM/CMS
                    5 - Atari TOS
                    6 - HPFS filesystem (OS/2, NT)
                    7 - Macintosh
                    8 - Z-System
                    9 - CP/M
                   10 - TOPS-20
                   11 - NTFS filesystem (NT)
                   12 - QDOS
                   13 - Acorn RISCOS
                  255 - unknown
   
            XLEN (eXtra LENgth)
               If FLG.FEXTRA is set, this gives the length of the optional
               extra field.  See below for details.
   
            CRC32 (CRC-32)
               This contains a Cyclic Redundancy Check value of the
               uncompressed data computed according to CRC-32 algorithm
               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
               ordering ISO documents. See gopher://info.itu.ch for an
               online version of ITU-T V.42.)
   
            ISIZE (Input SIZE)
               This contains the size of the original (uncompressed) input
               data modulo 2^32.
   
         2.3.1.1. Extra field
   
            If the FLG.FEXTRA bit is set, an "extra field" is present in
            the header, with total length XLEN bytes.  It consists of a
            series of subfields, each of the form:
   
               +---+---+---+---+==================================+
               |SI1|SI2|  LEN  |... LEN bytes of subfield data ...|
               +---+---+---+---+==================================+
   
            SI1 and SI2 provide a subfield ID, typically two ASCII letters
            with some mnemonic value.  Jean-Loup Gailly
            <gzip@prep.ai.mit.edu> is maintaining a registry of 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 are currently defined:
   
   
   
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               SI1         SI2         Data
               ----------  ----------  ----
               0x41 ('A')  0x70 ('P')  Apollo file type information
   
            LEN gives the length of the subfield data, excluding the 4
            initial bytes.
   
         2.3.1.2. Compliance
   
            A compliant compressor must produce files with correct ID1,
            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
            OS, 0 for all others).  The compressor must set all reserved
            bits to zero.
   
            A compliant decompressor must check ID1, ID2, and CM, and
            provide an error indication if any of these have incorrect
            values.  It must examine FEXTRA/XLEN, FNAME, FCOMMENT and FHCRC
            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
            trailer; in particular, a decompressor may ignore FTEXT and OS
            and always produce binary output, and still be compliant.  A
            compliant decompressor must give an error indication if any
            reserved bit is non-zero, since such a bit could indicate the
            presence of a new field that would cause subsequent data to be
            interpreted incorrectly.
   
   3. References
   
      [1] "Information Processing - 8-bit single-byte coded graphic
          character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987).
          The ISO 8859-1 (Latin-1) character set is a superset of 7-bit
          ASCII. Files defining this character set are available as
          iso_8859-1.* in ftp://ftp.uu.net/graphics/png/documents/
   
      [2] ISO 3309
   
      [3] ITU-T recommendation V.42
   
      [4] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",
          available in ftp://ftp.uu.net/pub/archiving/zip/doc/
   
      [5] Gailly, J.-L., GZIP documentation, available as gzip-*.tar in
          ftp://prep.ai.mit.edu/pub/gnu/
   
      [6] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table
          Look-Up", Communications of the ACM, 31(8), pp.1008-1013.
   
   
   
   
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      [7] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal,
          pp.118-133.
   
      [8] ftp://ftp.adelaide.edu.au/pub/rocksoft/papers/crc_v3.txt,
          describing the CRC concept.
   
   4. Security Considerations
   
      Any data compression method involves the reduction of redundancy in
      the data.  Consequently, any corruption of the data is likely to have
      severe effects and be difficult to correct.  Uncompressed text, on
      the other hand, will probably still be readable despite the presence
      of some corrupted bytes.
   
      It is recommended that systems using this data format provide some
      means of validating the integrity of the compressed data, such as by
      setting and checking the CRC-32 check value.
   
   5. Acknowledgements
   
      Trademarks cited in this document are the property of their
      respective owners.
   
      Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler,
      the related software described in this specification.  Glenn
      Randers-Pehrson converted this document to RFC and HTML format.
   
   6. Author's Address
   
      L. Peter Deutsch
      Aladdin Enterprises
      203 Santa Margarita Ave.
      Menlo Park, CA 94025
   
      Phone: (415) 322-0103 (AM only)
      FAX:   (415) 322-1734
      EMail: <ghost@aladdin.com>
   
      Questions about the technical content of this specification can be
      sent by email to:
   
      Jean-Loup Gailly <gzip@prep.ai.mit.edu> and
      Mark Adler <madler@alumni.caltech.edu>
   
      Editorial comments on this specification can be sent by email to:
   
      L. Peter Deutsch <ghost@aladdin.com> and
      Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
   
   
   
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   7. Appendix: Jean-Loup Gailly's gzip utility
   
      The most widely used implementation of gzip compression, and the
      original documentation on which this specification is based, were
      created by Jean-Loup Gailly <gzip@prep.ai.mit.edu>.  Since this
      implementation is a de facto standard, we mention some more of its
      features here.  Again, the material in this section is not part of
      the specification per se, and implementations need not follow it to
      be compliant.
   
      When compressing or decompressing a file, gzip preserves the
      protection, ownership, and modification time attributes on the local
      file system, since there is no provision for representing protection
      attributes in the gzip file format itself.  Since the file format
      includes a modification time, the gzip decompressor provides a
      command line switch that assigns the modification time from the file,
      rather than the local modification time of the compressed input, to
      the decompressed output.
   
   8. Appendix: Sample CRC Code
   
      The following sample code represents a practical implementation of
      the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42
      for a formal specification.)
   
      The sample code is in the ANSI C programming language. Non C users
      may find it easier to read with these hints:
   
         &      Bitwise AND operator.
         ^      Bitwise exclusive-OR operator.
         >>     Bitwise right shift operator. When applied to an
                unsigned quantity, as here, right shift inserts zero
                bit(s) at the left.
         !      Logical NOT operator.
         ++     "n++" increments the variable n.
         0xNNN  0x introduces a hexadecimal (base 16) constant.
                Suffix L indicates a long value (at least 32 bits).
   
         /* Table of CRCs of all 8-bit messages. */
         unsigned long crc_table[256];
   
         /* Flag: has the table been computed? Initially false. */
         int crc_table_computed = 0;
   
         /* Make the table for a fast CRC. */
         void make_crc_table(void)
         {
           unsigned long c;
   
   
   
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           int n, k;
           for (n = 0; n < 256; n++) {
             c = (unsigned long) n;
             for (k = 0; k < 8; k++) {
               if (c & 1) {
                 c = 0xedb88320L ^ (c >> 1);
               } else {
                 c = c >> 1;
               }
             }
             crc_table[n] = c;
           }
           crc_table_computed = 1;
         }
   
         /*
            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
          post-conditioning (one's complement) is performed within this
          function so it shouldn't be done by the caller. Usage example:
   
            unsigned long crc = 0L;
   
            while (read_buffer(buffer, length) != EOF) {
              crc = update_crc(crc, buffer, length);
            }
            if (crc != original_crc) error();
         */
         unsigned long update_crc(unsigned long crc,
                         unsigned char *buf, int len)
         {
           unsigned long c = crc ^ 0xffffffffL;
           int n;
   
           if (!crc_table_computed)
             make_crc_table();
           for (n = 0; n < len; n++) {
             c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
           }
           return c ^ 0xffffffffL;
         }
   
         /* Return the CRC of the bytes buf[0..len-1]. */
         unsigned long crc(unsigned char *buf, int len)
         {
           return update_crc(0L, buf, len);
         }
   
   
   
   
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