rfc1952.txt

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RFC 1952             GZIP File Format Specification             May 1996


            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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RFC 1952             GZIP File Format Specification             May 1996


                 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
         <[e-mail]> 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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RFC 1952             GZIP File Format Specification             May 1996


   [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: [phone] (AM only)
   FAX:   [phone]
   EMail: <[e-mail]>

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

   Jean-Loup Gailly <[e-mail]> and
   Mark Adler <[e-mail]>

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

   L. Peter Deutsch <[e-mail]> and
   Glenn Randers-Pehrson <[e-mail]>



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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 <[e-mail]>.  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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