| 1 | |
| 2 | |
| 3 | |
| 4 | |
| 5 | |
| 6 | |
| 7 | Network Working Group P. Deutsch |
| 8 | Request for Comments: 1952 Aladdin Enterprises |
| 9 | Category: Informational May 1996 |
| 10 | |
| 11 | |
| 12 | GZIP file format specification version 4.3 |
| 13 | |
| 14 | Status of This Memo |
| 15 | |
| 16 | This memo provides information for the Internet community. This memo |
| 17 | does not specify an Internet standard of any kind. Distribution of |
| 18 | this memo is unlimited. |
| 19 | |
| 20 | IESG Note: |
| 21 | |
| 22 | The IESG takes no position on the validity of any Intellectual |
| 23 | Property Rights statements contained in this document. |
| 24 | |
| 25 | Notices |
| 26 | |
| 27 | Copyright (c) 1996 L. Peter Deutsch |
| 28 | |
| 29 | Permission is granted to copy and distribute this document for any |
| 30 | purpose and without charge, including translations into other |
| 31 | languages and incorporation into compilations, provided that the |
| 32 | copyright notice and this notice are preserved, and that any |
| 33 | substantive changes or deletions from the original are clearly |
| 34 | marked. |
| 35 | |
| 36 | A pointer to the latest version of this and related documentation in |
| 37 | HTML format can be found at the URL |
| 38 | <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>. |
| 39 | |
| 40 | Abstract |
| 41 | |
| 42 | This specification defines a lossless compressed data format that is |
| 43 | compatible with the widely used GZIP utility. The format includes a |
| 44 | cyclic redundancy check value for detecting data corruption. The |
| 45 | format presently uses the DEFLATE method of compression but can be |
| 46 | easily extended to use other compression methods. The format can be |
| 47 | implemented readily in a manner not covered by patents. |
| 48 | |
| 49 | |
| 50 | |
| 51 | |
| 52 | |
| 53 | |
| 54 | |
| 55 | |
| 56 | |
| 57 | |
| 58 | Deutsch Informational [Page 1] |
| 59 | \f |
| 60 | RFC 1952 GZIP File Format Specification May 1996 |
| 61 | |
| 62 | |
| 63 | Table of Contents |
| 64 | |
| 65 | 1. Introduction ................................................... 2 |
| 66 | 1.1. Purpose ................................................... 2 |
| 67 | 1.2. Intended audience ......................................... 3 |
| 68 | 1.3. Scope ..................................................... 3 |
| 69 | 1.4. Compliance ................................................ 3 |
| 70 | 1.5. Definitions of terms and conventions used ................. 3 |
| 71 | 1.6. Changes from previous versions ............................ 3 |
| 72 | 2. Detailed specification ......................................... 4 |
| 73 | 2.1. Overall conventions ....................................... 4 |
| 74 | 2.2. File format ............................................... 5 |
| 75 | 2.3. Member format ............................................. 5 |
| 76 | 2.3.1. Member header and trailer ........................... 6 |
| 77 | 2.3.1.1. Extra field ................................... 8 |
| 78 | 2.3.1.2. Compliance .................................... 9 |
| 79 | 3. References .................................................. 9 |
| 80 | 4. Security Considerations .................................... 10 |
| 81 | 5. Acknowledgements ........................................... 10 |
| 82 | 6. Author's Address ........................................... 10 |
| 83 | 7. Appendix: Jean-Loup Gailly's gzip utility .................. 11 |
| 84 | 8. Appendix: Sample CRC Code .................................. 11 |
| 85 | |
| 86 | 1. Introduction |
| 87 | |
| 88 | 1.1. Purpose |
| 89 | |
| 90 | The purpose of this specification is to define a lossless |
| 91 | compressed data format that: |
| 92 | |
| 93 | * Is independent of CPU type, operating system, file system, |
| 94 | and character set, and hence can be used for interchange; |
| 95 | * Can compress or decompress a data stream (as opposed to a |
| 96 | randomly accessible file) to produce another data stream, |
| 97 | using only an a priori bounded amount of intermediate |
| 98 | storage, and hence can be used in data communications or |
| 99 | similar structures such as Unix filters; |
| 100 | * Compresses data with efficiency comparable to the best |
| 101 | currently available general-purpose compression methods, |
| 102 | and in particular considerably better than the "compress" |
| 103 | program; |
| 104 | * Can be implemented readily in a manner not covered by |
| 105 | patents, and hence can be practiced freely; |
| 106 | * Is compatible with the file format produced by the current |
| 107 | widely used gzip utility, in that conforming decompressors |
| 108 | will be able to read data produced by the existing gzip |
| 109 | compressor. |
| 110 | |
| 111 | |
| 112 | |
| 113 | |
| 114 | Deutsch Informational [Page 2] |
| 115 | \f |
| 116 | RFC 1952 GZIP File Format Specification May 1996 |
| 117 | |
| 118 | |
| 119 | The data format defined by this specification does not attempt to: |
| 120 | |
| 121 | * Provide random access to compressed data; |
| 122 | * Compress specialized data (e.g., raster graphics) as well as |
| 123 | the best currently available specialized algorithms. |
| 124 | |
| 125 | 1.2. Intended audience |
| 126 | |
| 127 | This specification is intended for use by implementors of software |
| 128 | to compress data into gzip format and/or decompress data from gzip |
| 129 | format. |
| 130 | |
| 131 | The text of the specification assumes a basic background in |
| 132 | programming at the level of bits and other primitive data |
| 133 | representations. |
| 134 | |
| 135 | 1.3. Scope |
| 136 | |
| 137 | The specification specifies a compression method and a file format |
| 138 | (the latter assuming only that a file can store a sequence of |
| 139 | arbitrary bytes). It does not specify any particular interface to |
| 140 | a file system or anything about character sets or encodings |
| 141 | (except for file names and comments, which are optional). |
| 142 | |
| 143 | 1.4. Compliance |
| 144 | |
| 145 | Unless otherwise indicated below, a compliant decompressor must be |
| 146 | able to accept and decompress any file that conforms to all the |
| 147 | specifications presented here; a compliant compressor must produce |
| 148 | files that conform to all the specifications presented here. The |
| 149 | material in the appendices is not part of the specification per se |
| 150 | and is not relevant to compliance. |
| 151 | |
| 152 | 1.5. Definitions of terms and conventions used |
| 153 | |
| 154 | byte: 8 bits stored or transmitted as a unit (same as an octet). |
| 155 | (For this specification, a byte is exactly 8 bits, even on |
| 156 | machines which store a character on a number of bits different |
| 157 | from 8.) See below for the numbering of bits within a byte. |
| 158 | |
| 159 | 1.6. Changes from previous versions |
| 160 | |
| 161 | There have been no technical changes to the gzip format since |
| 162 | version 4.1 of this specification. In version 4.2, some |
| 163 | terminology was changed, and the sample CRC code was rewritten for |
| 164 | clarity and to eliminate the requirement for the caller to do pre- |
| 165 | and post-conditioning. Version 4.3 is a conversion of the |
| 166 | specification to RFC style. |
| 167 | |
| 168 | |
| 169 | |
| 170 | Deutsch Informational [Page 3] |
| 171 | \f |
| 172 | RFC 1952 GZIP File Format Specification May 1996 |
| 173 | |
| 174 | |
| 175 | 2. Detailed specification |
| 176 | |
| 177 | 2.1. Overall conventions |
| 178 | |
| 179 | In the diagrams below, a box like this: |
| 180 | |
| 181 | +---+ |
| 182 | | | <-- the vertical bars might be missing |
| 183 | +---+ |
| 184 | |
| 185 | represents one byte; a box like this: |
| 186 | |
| 187 | +==============+ |
| 188 | | | |
| 189 | +==============+ |
| 190 | |
| 191 | represents a variable number of bytes. |
| 192 | |
| 193 | Bytes stored within a computer do not have a "bit order", since |
| 194 | they are always treated as a unit. However, a byte considered as |
| 195 | an integer between 0 and 255 does have a most- and least- |
| 196 | significant bit, and since we write numbers with the most- |
| 197 | significant digit on the left, we also write bytes with the most- |
| 198 | significant bit on the left. In the diagrams below, we number the |
| 199 | bits of a byte so that bit 0 is the least-significant bit, i.e., |
| 200 | the bits are numbered: |
| 201 | |
| 202 | +--------+ |
| 203 | |76543210| |
| 204 | +--------+ |
| 205 | |
| 206 | This document does not address the issue of the order in which |
| 207 | bits of a byte are transmitted on a bit-sequential medium, since |
| 208 | the data format described here is byte- rather than bit-oriented. |
| 209 | |
| 210 | Within a computer, a number may occupy multiple bytes. All |
| 211 | multi-byte numbers in the format described here are stored with |
| 212 | the least-significant byte first (at the lower memory address). |
| 213 | For example, the decimal number 520 is stored as: |
| 214 | |
| 215 | 0 1 |
| 216 | +--------+--------+ |
| 217 | |00001000|00000010| |
| 218 | +--------+--------+ |
| 219 | ^ ^ |
| 220 | | | |
| 221 | | + more significant byte = 2 x 256 |
| 222 | + less significant byte = 8 |
| 223 | |
| 224 | |
| 225 | |
| 226 | Deutsch Informational [Page 4] |
| 227 | \f |
| 228 | RFC 1952 GZIP File Format Specification May 1996 |
| 229 | |
| 230 | |
| 231 | 2.2. File format |
| 232 | |
| 233 | A gzip file consists of a series of "members" (compressed data |
| 234 | sets). The format of each member is specified in the following |
| 235 | section. The members simply appear one after another in the file, |
| 236 | with no additional information before, between, or after them. |
| 237 | |
| 238 | 2.3. Member format |
| 239 | |
| 240 | Each member has the following structure: |
| 241 | |
| 242 | +---+---+---+---+---+---+---+---+---+---+ |
| 243 | |ID1|ID2|CM |FLG| MTIME |XFL|OS | (more-->) |
| 244 | +---+---+---+---+---+---+---+---+---+---+ |
| 245 | |
| 246 | (if FLG.FEXTRA set) |
| 247 | |
| 248 | +---+---+=================================+ |
| 249 | | XLEN |...XLEN bytes of "extra field"...| (more-->) |
| 250 | +---+---+=================================+ |
| 251 | |
| 252 | (if FLG.FNAME set) |
| 253 | |
| 254 | +=========================================+ |
| 255 | |...original file name, zero-terminated...| (more-->) |
| 256 | +=========================================+ |
| 257 | |
| 258 | (if FLG.FCOMMENT set) |
| 259 | |
| 260 | +===================================+ |
| 261 | |...file comment, zero-terminated...| (more-->) |
| 262 | +===================================+ |
| 263 | |
| 264 | (if FLG.FHCRC set) |
| 265 | |
| 266 | +---+---+ |
| 267 | | CRC16 | |
| 268 | +---+---+ |
| 269 | |
| 270 | +=======================+ |
| 271 | |...compressed blocks...| (more-->) |
| 272 | +=======================+ |
| 273 | |
| 274 | 0 1 2 3 4 5 6 7 |
| 275 | +---+---+---+---+---+---+---+---+ |
| 276 | | CRC32 | ISIZE | |
| 277 | +---+---+---+---+---+---+---+---+ |
| 278 | |
| 279 | |
| 280 | |
| 281 | |
| 282 | Deutsch Informational [Page 5] |
| 283 | \f |
| 284 | RFC 1952 GZIP File Format Specification May 1996 |
| 285 | |
| 286 | |
| 287 | 2.3.1. Member header and trailer |
| 288 | |
| 289 | ID1 (IDentification 1) |
| 290 | ID2 (IDentification 2) |
| 291 | These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139 |
| 292 | (0x8b, \213), to identify the file as being in gzip format. |
| 293 | |
| 294 | CM (Compression Method) |
| 295 | This identifies the compression method used in the file. CM |
| 296 | = 0-7 are reserved. CM = 8 denotes the "deflate" |
| 297 | compression method, which is the one customarily used by |
| 298 | gzip and which is documented elsewhere. |
| 299 | |
| 300 | FLG (FLaGs) |
| 301 | This flag byte is divided into individual bits as follows: |
| 302 | |
| 303 | bit 0 FTEXT |
| 304 | bit 1 FHCRC |
| 305 | bit 2 FEXTRA |
| 306 | bit 3 FNAME |
| 307 | bit 4 FCOMMENT |
| 308 | bit 5 reserved |
| 309 | bit 6 reserved |
| 310 | bit 7 reserved |
| 311 | |
| 312 | If FTEXT is set, the file is probably ASCII text. This is |
| 313 | an optional indication, which the compressor may set by |
| 314 | checking a small amount of the input data to see whether any |
| 315 | non-ASCII characters are present. In case of doubt, FTEXT |
| 316 | is cleared, indicating binary data. For systems which have |
| 317 | different file formats for ascii text and binary data, the |
| 318 | decompressor can use FTEXT to choose the appropriate format. |
| 319 | We deliberately do not specify the algorithm used to set |
| 320 | this bit, since a compressor always has the option of |
| 321 | leaving it cleared and a decompressor always has the option |
| 322 | of ignoring it and letting some other program handle issues |
| 323 | of data conversion. |
| 324 | |
| 325 | If FHCRC is set, a CRC16 for the gzip header is present, |
| 326 | immediately before the compressed data. The CRC16 consists |
| 327 | of the two least significant bytes of the CRC32 for all |
| 328 | bytes of the gzip header up to and not including the CRC16. |
| 329 | [The FHCRC bit was never set by versions of gzip up to |
| 330 | 1.2.4, even though it was documented with a different |
| 331 | meaning in gzip 1.2.4.] |
| 332 | |
| 333 | If FEXTRA is set, optional extra fields are present, as |
| 334 | described in a following section. |
| 335 | |
| 336 | |
| 337 | |
| 338 | Deutsch Informational [Page 6] |
| 339 | \f |
| 340 | RFC 1952 GZIP File Format Specification May 1996 |
| 341 | |
| 342 | |
| 343 | If FNAME is set, an original file name is present, |
| 344 | terminated by a zero byte. The name must consist of ISO |
| 345 | 8859-1 (LATIN-1) characters; on operating systems using |
| 346 | EBCDIC or any other character set for file names, the name |
| 347 | must be translated to the ISO LATIN-1 character set. This |
| 348 | is the original name of the file being compressed, with any |
| 349 | directory components removed, and, if the file being |
| 350 | compressed is on a file system with case insensitive names, |
| 351 | forced to lower case. There is no original file name if the |
| 352 | data was compressed from a source other than a named file; |
| 353 | for example, if the source was stdin on a Unix system, there |
| 354 | is no file name. |
| 355 | |
| 356 | If FCOMMENT is set, a zero-terminated file comment is |
| 357 | present. This comment is not interpreted; it is only |
| 358 | intended for human consumption. The comment must consist of |
| 359 | ISO 8859-1 (LATIN-1) characters. Line breaks should be |
| 360 | denoted by a single line feed character (10 decimal). |
| 361 | |
| 362 | Reserved FLG bits must be zero. |
| 363 | |
| 364 | MTIME (Modification TIME) |
| 365 | This gives the most recent modification time of the original |
| 366 | file being compressed. The time is in Unix format, i.e., |
| 367 | seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this |
| 368 | may cause problems for MS-DOS and other systems that use |
| 369 | local rather than Universal time.) If the compressed data |
| 370 | did not come from a file, MTIME is set to the time at which |
| 371 | compression started. MTIME = 0 means no time stamp is |
| 372 | available. |
| 373 | |
| 374 | XFL (eXtra FLags) |
| 375 | These flags are available for use by specific compression |
| 376 | methods. The "deflate" method (CM = 8) sets these flags as |
| 377 | follows: |
| 378 | |
| 379 | XFL = 2 - compressor used maximum compression, |
| 380 | slowest algorithm |
| 381 | XFL = 4 - compressor used fastest algorithm |
| 382 | |
| 383 | OS (Operating System) |
| 384 | This identifies the type of file system on which compression |
| 385 | took place. This may be useful in determining end-of-line |
| 386 | convention for text files. The currently defined values are |
| 387 | as follows: |
| 388 | |
| 389 | |
| 390 | |
| 391 | |
| 392 | |
| 393 | |
| 394 | Deutsch Informational [Page 7] |
| 395 | \f |
| 396 | RFC 1952 GZIP File Format Specification May 1996 |
| 397 | |
| 398 | |
| 399 | 0 - FAT filesystem (MS-DOS, OS/2, NT/Win32) |
| 400 | 1 - Amiga |
| 401 | 2 - VMS (or OpenVMS) |
| 402 | 3 - Unix |
| 403 | 4 - VM/CMS |
| 404 | 5 - Atari TOS |
| 405 | 6 - HPFS filesystem (OS/2, NT) |
| 406 | 7 - Macintosh |
| 407 | 8 - Z-System |
| 408 | 9 - CP/M |
| 409 | 10 - TOPS-20 |
| 410 | 11 - NTFS filesystem (NT) |
| 411 | 12 - QDOS |
| 412 | 13 - Acorn RISCOS |
| 413 | 255 - unknown |
| 414 | |
| 415 | XLEN (eXtra LENgth) |
| 416 | If FLG.FEXTRA is set, this gives the length of the optional |
| 417 | extra field. See below for details. |
| 418 | |
| 419 | CRC32 (CRC-32) |
| 420 | This contains a Cyclic Redundancy Check value of the |
| 421 | uncompressed data computed according to CRC-32 algorithm |
| 422 | used in the ISO 3309 standard and in section 8.1.1.6.2 of |
| 423 | ITU-T recommendation V.42. (See http://www.iso.ch for |
| 424 | ordering ISO documents. See gopher://info.itu.ch for an |
| 425 | online version of ITU-T V.42.) |
| 426 | |
| 427 | ISIZE (Input SIZE) |
| 428 | This contains the size of the original (uncompressed) input |
| 429 | data modulo 2^32. |
| 430 | |
| 431 | 2.3.1.1. Extra field |
| 432 | |
| 433 | If the FLG.FEXTRA bit is set, an "extra field" is present in |
| 434 | the header, with total length XLEN bytes. It consists of a |
| 435 | series of subfields, each of the form: |
| 436 | |
| 437 | +---+---+---+---+==================================+ |
| 438 | |SI1|SI2| LEN |... LEN bytes of subfield data ...| |
| 439 | +---+---+---+---+==================================+ |
| 440 | |
| 441 | SI1 and SI2 provide a subfield ID, typically two ASCII letters |
| 442 | with some mnemonic value. Jean-Loup Gailly |
| 443 | <gzip@prep.ai.mit.edu> is maintaining a registry of subfield |
| 444 | IDs; please send him any subfield ID you wish to use. Subfield |
| 445 | IDs with SI2 = 0 are reserved for future use. The following |
| 446 | IDs are currently defined: |
| 447 | |
| 448 | |
| 449 | |
| 450 | Deutsch Informational [Page 8] |
| 451 | \f |
| 452 | RFC 1952 GZIP File Format Specification May 1996 |
| 453 | |
| 454 | |
| 455 | SI1 SI2 Data |
| 456 | ---------- ---------- ---- |
| 457 | 0x41 ('A') 0x70 ('P') Apollo file type information |
| 458 | |
| 459 | LEN gives the length of the subfield data, excluding the 4 |
| 460 | initial bytes. |
| 461 | |
| 462 | 2.3.1.2. Compliance |
| 463 | |
| 464 | A compliant compressor must produce files with correct ID1, |
| 465 | ID2, CM, CRC32, and ISIZE, but may set all the other fields in |
| 466 | the fixed-length part of the header to default values (255 for |
| 467 | OS, 0 for all others). The compressor must set all reserved |
| 468 | bits to zero. |
| 469 | |
| 470 | A compliant decompressor must check ID1, ID2, and CM, and |
| 471 | provide an error indication if any of these have incorrect |
| 472 | values. It must examine FEXTRA/XLEN, FNAME, FCOMMENT and FHCRC |
| 473 | at least so it can skip over the optional fields if they are |
| 474 | present. It need not examine any other part of the header or |
| 475 | trailer; in particular, a decompressor may ignore FTEXT and OS |
| 476 | and always produce binary output, and still be compliant. A |
| 477 | compliant decompressor must give an error indication if any |
| 478 | reserved bit is non-zero, since such a bit could indicate the |
| 479 | presence of a new field that would cause subsequent data to be |
| 480 | interpreted incorrectly. |
| 481 | |
| 482 | 3. References |
| 483 | |
| 484 | [1] "Information Processing - 8-bit single-byte coded graphic |
| 485 | character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987). |
| 486 | The ISO 8859-1 (Latin-1) character set is a superset of 7-bit |
| 487 | ASCII. Files defining this character set are available as |
| 488 | iso_8859-1.* in ftp://ftp.uu.net/graphics/png/documents/ |
| 489 | |
| 490 | [2] ISO 3309 |
| 491 | |
| 492 | [3] ITU-T recommendation V.42 |
| 493 | |
| 494 | [4] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification", |
| 495 | available in ftp://ftp.uu.net/pub/archiving/zip/doc/ |
| 496 | |
| 497 | [5] Gailly, J.-L., GZIP documentation, available as gzip-*.tar in |
| 498 | ftp://prep.ai.mit.edu/pub/gnu/ |
| 499 | |
| 500 | [6] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table |
| 501 | Look-Up", Communications of the ACM, 31(8), pp.1008-1013. |
| 502 | |
| 503 | |
| 504 | |
| 505 | |
| 506 | Deutsch Informational [Page 9] |
| 507 | \f |
| 508 | RFC 1952 GZIP File Format Specification May 1996 |
| 509 | |
| 510 | |
| 511 | [7] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal, |
| 512 | pp.118-133. |
| 513 | |
| 514 | [8] ftp://ftp.adelaide.edu.au/pub/rocksoft/papers/crc_v3.txt, |
| 515 | describing the CRC concept. |
| 516 | |
| 517 | 4. Security Considerations |
| 518 | |
| 519 | Any data compression method involves the reduction of redundancy in |
| 520 | the data. Consequently, any corruption of the data is likely to have |
| 521 | severe effects and be difficult to correct. Uncompressed text, on |
| 522 | the other hand, will probably still be readable despite the presence |
| 523 | of some corrupted bytes. |
| 524 | |
| 525 | It is recommended that systems using this data format provide some |
| 526 | means of validating the integrity of the compressed data, such as by |
| 527 | setting and checking the CRC-32 check value. |
| 528 | |
| 529 | 5. Acknowledgements |
| 530 | |
| 531 | Trademarks cited in this document are the property of their |
| 532 | respective owners. |
| 533 | |
| 534 | Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler, |
| 535 | the related software described in this specification. Glenn |
| 536 | Randers-Pehrson converted this document to RFC and HTML format. |
| 537 | |
| 538 | 6. Author's Address |
| 539 | |
| 540 | L. Peter Deutsch |
| 541 | Aladdin Enterprises |
| 542 | 203 Santa Margarita Ave. |
| 543 | Menlo Park, CA 94025 |
| 544 | |
| 545 | Phone: (415) 322-0103 (AM only) |
| 546 | FAX: (415) 322-1734 |
| 547 | EMail: <ghost@aladdin.com> |
| 548 | |
| 549 | Questions about the technical content of this specification can be |
| 550 | sent by email to: |
| 551 | |
| 552 | Jean-Loup Gailly <gzip@prep.ai.mit.edu> and |
| 553 | Mark Adler <madler@alumni.caltech.edu> |
| 554 | |
| 555 | Editorial comments on this specification can be sent by email to: |
| 556 | |
| 557 | L. Peter Deutsch <ghost@aladdin.com> and |
| 558 | Glenn Randers-Pehrson <randeg@alumni.rpi.edu> |
| 559 | |
| 560 | |
| 561 | |
| 562 | Deutsch Informational [Page 10] |
| 563 | \f |
| 564 | RFC 1952 GZIP File Format Specification May 1996 |
| 565 | |
| 566 | |
| 567 | 7. Appendix: Jean-Loup Gailly's gzip utility |
| 568 | |
| 569 | The most widely used implementation of gzip compression, and the |
| 570 | original documentation on which this specification is based, were |
| 571 | created by Jean-Loup Gailly <gzip@prep.ai.mit.edu>. Since this |
| 572 | implementation is a de facto standard, we mention some more of its |
| 573 | features here. Again, the material in this section is not part of |
| 574 | the specification per se, and implementations need not follow it to |
| 575 | be compliant. |
| 576 | |
| 577 | When compressing or decompressing a file, gzip preserves the |
| 578 | protection, ownership, and modification time attributes on the local |
| 579 | file system, since there is no provision for representing protection |
| 580 | attributes in the gzip file format itself. Since the file format |
| 581 | includes a modification time, the gzip decompressor provides a |
| 582 | command line switch that assigns the modification time from the file, |
| 583 | rather than the local modification time of the compressed input, to |
| 584 | the decompressed output. |
| 585 | |
| 586 | 8. Appendix: Sample CRC Code |
| 587 | |
| 588 | The following sample code represents a practical implementation of |
| 589 | the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42 |
| 590 | for a formal specification.) |
| 591 | |
| 592 | The sample code is in the ANSI C programming language. Non C users |
| 593 | may find it easier to read with these hints: |
| 594 | |
| 595 | & Bitwise AND operator. |
| 596 | ^ Bitwise exclusive-OR operator. |
| 597 | >> Bitwise right shift operator. When applied to an |
| 598 | unsigned quantity, as here, right shift inserts zero |
| 599 | bit(s) at the left. |
| 600 | ! Logical NOT operator. |
| 601 | ++ "n++" increments the variable n. |
| 602 | 0xNNN 0x introduces a hexadecimal (base 16) constant. |
| 603 | Suffix L indicates a long value (at least 32 bits). |
| 604 | |
| 605 | /* Table of CRCs of all 8-bit messages. */ |
| 606 | unsigned long crc_table[256]; |
| 607 | |
| 608 | /* Flag: has the table been computed? Initially false. */ |
| 609 | int crc_table_computed = 0; |
| 610 | |
| 611 | /* Make the table for a fast CRC. */ |
| 612 | void make_crc_table(void) |
| 613 | { |
| 614 | unsigned long c; |
| 615 | |
| 616 | |
| 617 | |
| 618 | Deutsch Informational [Page 11] |
| 619 | \f |
| 620 | RFC 1952 GZIP File Format Specification May 1996 |
| 621 | |
| 622 | |
| 623 | int n, k; |
| 624 | for (n = 0; n < 256; n++) { |
| 625 | c = (unsigned long) n; |
| 626 | for (k = 0; k < 8; k++) { |
| 627 | if (c & 1) { |
| 628 | c = 0xedb88320L ^ (c >> 1); |
| 629 | } else { |
| 630 | c = c >> 1; |
| 631 | } |
| 632 | } |
| 633 | crc_table[n] = c; |
| 634 | } |
| 635 | crc_table_computed = 1; |
| 636 | } |
| 637 | |
| 638 | /* |
| 639 | Update a running crc with the bytes buf[0..len-1] and return |
| 640 | the updated crc. The crc should be initialized to zero. Pre- and |
| 641 | post-conditioning (one's complement) is performed within this |
| 642 | function so it shouldn't be done by the caller. Usage example: |
| 643 | |
| 644 | unsigned long crc = 0L; |
| 645 | |
| 646 | while (read_buffer(buffer, length) != EOF) { |
| 647 | crc = update_crc(crc, buffer, length); |
| 648 | } |
| 649 | if (crc != original_crc) error(); |
| 650 | */ |
| 651 | unsigned long update_crc(unsigned long crc, |
| 652 | unsigned char *buf, int len) |
| 653 | { |
| 654 | unsigned long c = crc ^ 0xffffffffL; |
| 655 | int n; |
| 656 | |
| 657 | if (!crc_table_computed) |
| 658 | make_crc_table(); |
| 659 | for (n = 0; n < len; n++) { |
| 660 | c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8); |
| 661 | } |
| 662 | return c ^ 0xffffffffL; |
| 663 | } |
| 664 | |
| 665 | /* Return the CRC of the bytes buf[0..len-1]. */ |
| 666 | unsigned long crc(unsigned char *buf, int len) |
| 667 | { |
| 668 | return update_crc(0L, buf, len); |
| 669 | } |
| 670 | |
| 671 | |
| 672 | |
| 673 | |
| 674 | Deutsch Informational [Page 12] |
| 675 | \f |