GZIP FILE FORMAT SPECIFICATION draft #4.1 February 26, 1995 Copyright (C) 1995 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 it is copied as a whole (including the copyright notice and this notice) and with no changes. 1. Introduction. 1.1 Purpose. The purpose of this specification is to define a lossless compressed data format that: (a) Is independent of CPU type, operating system, file system, and character set, and hence can be used for interchange; (b) 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; (c) Compresses data with efficiency comparable to the best currently available general-purpose compression methods, and in particular considerably better than the 'compress' program; (d) Can be implemented readily in a manner not covered by patents, and hence can be practiced freely; (e) 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. The data format defined by this specification does not attempt to: (a) Provide random access to compressed data; (b) 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 octets). 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 sections 3 and 4 is not part of the specification per se and is not relevant to compliance. 1.4 Related standards. "Information Processing - 8-bit single-byte coded graphic character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987). ISO 3309 ITU-T recommandation V.42 1.5 Other related publications. Deutsch, L.P.,"'Deflate' Compressed Data Format Specification". Drafts being circulated. Gailly, J.-L., gzip documentation. [Sarwate88] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table Look-Up", Communications of the ACM, 31(8), pp.1008-1013. Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal, pp.118-133. ftp.adelaide.edu.au:/pub/rocksoft/papers/crc_v3.txt, describing the CRC concept. 1.6 Definitions of terms and conventions used. octet: 8 bits stored or transmitted as a unit. See section 3.1 below for the numbering of bits within an octet. 2. Detailed specification. 2.1 Overall conventions. In the diagrams below, a box like this: +---+ | | <-- the vertical bars might be missing +---+ represents one octet; a box like this: +==============+ | | +==============+ represents a variable number of octets. Octets stored within a computer do not have a 'bit order', since they are always treated as a unit. However, an octet 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 octets with the most-significant bit on the left. In the diagrams below, we number the bits of an octet 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 an octet are transmitted on a bit-sequential medium, since the data format described here is octet- rather than bit-oriented. Within a computer, a number may occupy multiple octets. All multi-octet numbers in the format described here are stored with the least-significant octet first (at the lower memory address). For example, the decimal number 520 is stored as: 0 1 +--------+--------+ |00001000|00000010| +--------+--------+ ^ ^ | | | + more significant octet = 2 x 256 + less significant octet = 8 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 | +---+---+---+---+---+---+---+---+ 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. 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 12:00 PM on December 31, 1969 Universal time (GMT). (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: 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 recommandation 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. gzip@prep.ai.mit.edu (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: SI1 SI2 Data ---------- ---------- ---- 'A' (0x41) 'P' (0x70) 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. 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 (bug-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. 4. Appendix: Sample CRC Code The following sample code represents a practical implementation of the CRC (Cyclic Redundancy Check). The sample code provided is in the C programming language. (See also ISO 3309 and ITU-T V.42 for a formal specification.) /* table of crc's of all 8-bit messages */ unsigned long crc_table[256]; /* make the table for a fast crc */ void make_crc_table(void) { unsigned long c; int n, k; for (n = 0; n < 256; n++) { c = (unsigned long)n; for (k = 0; k < 8; k++) c = c & 1 ? 0xedb88320L ^ (c >> 1) : c >> 1; crc_table[n] = c; } } /* update a running crc with the bytes buf[0..len-1]--the crc should be initialized to all 1's, and the transmitted value is the 1's complement of the final running crc (see the crc() routine below)) */ unsigned long update_crc(unsigned long crc, unsigned char *buf, int len) { unsigned long c = crc; unsigned char *p = buf; int n = len; if (n > 0) do { c = crc_table[(c ^ (*p++)) & 0xff] ^ (c >> 8); } while (--n); return c; } /* return the crc of the bytes buf[0..len-1] */ unsigned long crc(unsigned char *buf, int len) { if (crc_table[1] != 0x77073096L) { make_crc_table(); } return ~update_crc(0xffffffffL, buf, len); }