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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 |