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GZIP file format specification version 4.3 :: RFC1952

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.


   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

   A pointer to the latest version of this and related documentation in
   HTML format can be found at the URL


   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
     Extra field ................................... 8
     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"
          * 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

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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

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

   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:


      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
          ^        ^
          |        |
          |        + more significant byte = 2 x 256
          + less significant byte = 8

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RFC 1952             GZIP File Format Specification             May 1996

   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

         XFL (eXtra FLags)
            These flags are available for use by specific compression
            methods.  The "deflate" method (CM = 8) sets these flags as

               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 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. 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
          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. 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

   [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,

   [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

   Questions about the technical content of this specification can be
   sent by email to:

   Jean-Loup Gailly  and
   Mark Adler 

   Editorial comments on this specification can be sent by email to:

   L. Peter Deutsch  and
   Glenn Randers-Pehrson 

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RFC 1952             GZIP File Format Specification             May 1996

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 .  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)
        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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