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The text of the specification assumes a basic background in programming at the level of bits and other primitive data representations.
+---+ | | <-- 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
+---+---+---+---+---+---+---+---+---+---+ |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 | +---+---+---+---+---+---+---+---+
bit 0 FTEXT bit 1 FHCRC bit 2 FEXTRA bit 3 FNAME bit 4 FCOMMENT bit 5 reserved bit 6 reserved bit 7 reservedIf 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.
XFL = 2 - compressor used maximum compression, slowest algorithm XFL = 4 - compressor used fastest algorithm
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
+---+---+---+---+==================================+ |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:
SI1 SI2 Data
---------- ---------- ----
0x41 ('A') 0x70 ('P') Apollo file type information
LEN gives the length of the subfield data, excluding the 4 initial bytes.
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.
[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.
[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.
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.
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>
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.
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;
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);
}