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1 | |||
2 | Frequently Asked Questions about zlib | ||
3 | |||
4 | |||
5 | If your question is not there, please check the zlib home page | ||
6 | http://zlib.net/ which may have more recent information. | ||
7 | The lastest zlib FAQ is at http://zlib.net/zlib_faq.html | ||
8 | |||
9 | |||
10 | 1. Is zlib Y2K-compliant? | ||
11 | |||
12 | Yes. zlib doesn't handle dates. | ||
13 | |||
14 | 2. Where can I get a Windows DLL version? | ||
15 | |||
16 | The zlib sources can be compiled without change to produce a DLL. See the | ||
17 | file win32/DLL_FAQ.txt in the zlib distribution. Pointers to the | ||
18 | precompiled DLL are found in the zlib web site at http://zlib.net/ . | ||
19 | |||
20 | 3. Where can I get a Visual Basic interface to zlib? | ||
21 | |||
22 | See | ||
23 | * http://marknelson.us/1997/01/01/zlib-engine/ | ||
24 | * win32/DLL_FAQ.txt in the zlib distribution | ||
25 | |||
26 | 4. compress() returns Z_BUF_ERROR. | ||
27 | |||
28 | Make sure that before the call of compress(), the length of the compressed | ||
29 | buffer is equal to the available size of the compressed buffer and not | ||
30 | zero. For Visual Basic, check that this parameter is passed by reference | ||
31 | ("as any"), not by value ("as long"). | ||
32 | |||
33 | 5. deflate() or inflate() returns Z_BUF_ERROR. | ||
34 | |||
35 | Before making the call, make sure that avail_in and avail_out are not zero. | ||
36 | When setting the parameter flush equal to Z_FINISH, also make sure that | ||
37 | avail_out is big enough to allow processing all pending input. Note that a | ||
38 | Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be | ||
39 | made with more input or output space. A Z_BUF_ERROR may in fact be | ||
40 | unavoidable depending on how the functions are used, since it is not | ||
41 | possible to tell whether or not there is more output pending when | ||
42 | strm.avail_out returns with zero. See http://zlib.net/zlib_how.html for a | ||
43 | heavily annotated example. | ||
44 | |||
45 | 6. Where's the zlib documentation (man pages, etc.)? | ||
46 | |||
47 | It's in zlib.h . Examples of zlib usage are in the files test/example.c | ||
48 | and test/minigzip.c, with more in examples/ . | ||
49 | |||
50 | 7. Why don't you use GNU autoconf or libtool or ...? | ||
51 | |||
52 | Because we would like to keep zlib as a very small and simple package. | ||
53 | zlib is rather portable and doesn't need much configuration. | ||
54 | |||
55 | 8. I found a bug in zlib. | ||
56 | |||
57 | Most of the time, such problems are due to an incorrect usage of zlib. | ||
58 | Please try to reproduce the problem with a small program and send the | ||
59 | corresponding source to us at zlib@gzip.org . Do not send multi-megabyte | ||
60 | data files without prior agreement. | ||
61 | |||
62 | 9. Why do I get "undefined reference to gzputc"? | ||
63 | |||
64 | If "make test" produces something like | ||
65 | |||
66 | example.o(.text+0x154): undefined reference to `gzputc' | ||
67 | |||
68 | check that you don't have old files libz.* in /usr/lib, /usr/local/lib or | ||
69 | /usr/X11R6/lib. Remove any old versions, then do "make install". | ||
70 | |||
71 | 10. I need a Delphi interface to zlib. | ||
72 | |||
73 | See the contrib/delphi directory in the zlib distribution. | ||
74 | |||
75 | 11. Can zlib handle .zip archives? | ||
76 | |||
77 | Not by itself, no. See the directory contrib/minizip in the zlib | ||
78 | distribution. | ||
79 | |||
80 | 12. Can zlib handle .Z files? | ||
81 | |||
82 | No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt | ||
83 | the code of uncompress on your own. | ||
84 | |||
85 | 13. How can I make a Unix shared library? | ||
86 | |||
87 | By default a shared (and a static) library is built for Unix. So: | ||
88 | |||
89 | make distclean | ||
90 | ./configure | ||
91 | make | ||
92 | |||
93 | 14. How do I install a shared zlib library on Unix? | ||
94 | |||
95 | After the above, then: | ||
96 | |||
97 | make install | ||
98 | |||
99 | However, many flavors of Unix come with a shared zlib already installed. | ||
100 | Before going to the trouble of compiling a shared version of zlib and | ||
101 | trying to install it, you may want to check if it's already there! If you | ||
102 | can #include <zlib.h>, it's there. The -lz option will probably link to | ||
103 | it. You can check the version at the top of zlib.h or with the | ||
104 | ZLIB_VERSION symbol defined in zlib.h . | ||
105 | |||
106 | 15. I have a question about OttoPDF. | ||
107 | |||
108 | We are not the authors of OttoPDF. The real author is on the OttoPDF web | ||
109 | site: Joel Hainley, jhainley@myndkryme.com. | ||
110 | |||
111 | 16. Can zlib decode Flate data in an Adobe PDF file? | ||
112 | |||
113 | Yes. See http://www.pdflib.com/ . To modify PDF forms, see | ||
114 | http://sourceforge.net/projects/acroformtool/ . | ||
115 | |||
116 | 17. Why am I getting this "register_frame_info not found" error on Solaris? | ||
117 | |||
118 | After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib | ||
119 | generates an error such as: | ||
120 | |||
121 | ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so: | ||
122 | symbol __register_frame_info: referenced symbol not found | ||
123 | |||
124 | The symbol __register_frame_info is not part of zlib, it is generated by | ||
125 | the C compiler (cc or gcc). You must recompile applications using zlib | ||
126 | which have this problem. This problem is specific to Solaris. See | ||
127 | http://www.sunfreeware.com for Solaris versions of zlib and applications | ||
128 | using zlib. | ||
129 | |||
130 | 18. Why does gzip give an error on a file I make with compress/deflate? | ||
131 | |||
132 | The compress and deflate functions produce data in the zlib format, which | ||
133 | is different and incompatible with the gzip format. The gz* functions in | ||
134 | zlib on the other hand use the gzip format. Both the zlib and gzip formats | ||
135 | use the same compressed data format internally, but have different headers | ||
136 | and trailers around the compressed data. | ||
137 | |||
138 | 19. Ok, so why are there two different formats? | ||
139 | |||
140 | The gzip format was designed to retain the directory information about a | ||
141 | single file, such as the name and last modification date. The zlib format | ||
142 | on the other hand was designed for in-memory and communication channel | ||
143 | applications, and has a much more compact header and trailer and uses a | ||
144 | faster integrity check than gzip. | ||
145 | |||
146 | 20. Well that's nice, but how do I make a gzip file in memory? | ||
147 | |||
148 | You can request that deflate write the gzip format instead of the zlib | ||
149 | format using deflateInit2(). You can also request that inflate decode the | ||
150 | gzip format using inflateInit2(). Read zlib.h for more details. | ||
151 | |||
152 | 21. Is zlib thread-safe? | ||
153 | |||
154 | Yes. However any library routines that zlib uses and any application- | ||
155 | provided memory allocation routines must also be thread-safe. zlib's gz* | ||
156 | functions use stdio library routines, and most of zlib's functions use the | ||
157 | library memory allocation routines by default. zlib's *Init* functions | ||
158 | allow for the application to provide custom memory allocation routines. | ||
159 | |||
160 | Of course, you should only operate on any given zlib or gzip stream from a | ||
161 | single thread at a time. | ||
162 | |||
163 | 22. Can I use zlib in my commercial application? | ||
164 | |||
165 | Yes. Please read the license in zlib.h. | ||
166 | |||
167 | 23. Is zlib under the GNU license? | ||
168 | |||
169 | No. Please read the license in zlib.h. | ||
170 | |||
171 | 24. The license says that altered source versions must be "plainly marked". So | ||
172 | what exactly do I need to do to meet that requirement? | ||
173 | |||
174 | You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In | ||
175 | particular, the final version number needs to be changed to "f", and an | ||
176 | identification string should be appended to ZLIB_VERSION. Version numbers | ||
177 | x.x.x.f are reserved for modifications to zlib by others than the zlib | ||
178 | maintainers. For example, if the version of the base zlib you are altering | ||
179 | is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and | ||
180 | ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also | ||
181 | update the version strings in deflate.c and inftrees.c. | ||
182 | |||
183 | For altered source distributions, you should also note the origin and | ||
184 | nature of the changes in zlib.h, as well as in ChangeLog and README, along | ||
185 | with the dates of the alterations. The origin should include at least your | ||
186 | name (or your company's name), and an email address to contact for help or | ||
187 | issues with the library. | ||
188 | |||
189 | Note that distributing a compiled zlib library along with zlib.h and | ||
190 | zconf.h is also a source distribution, and so you should change | ||
191 | ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes | ||
192 | in zlib.h as you would for a full source distribution. | ||
193 | |||
194 | 25. Will zlib work on a big-endian or little-endian architecture, and can I | ||
195 | exchange compressed data between them? | ||
196 | |||
197 | Yes and yes. | ||
198 | |||
199 | 26. Will zlib work on a 64-bit machine? | ||
200 | |||
201 | Yes. It has been tested on 64-bit machines, and has no dependence on any | ||
202 | data types being limited to 32-bits in length. If you have any | ||
203 | difficulties, please provide a complete problem report to zlib@gzip.org | ||
204 | |||
205 | 27. Will zlib decompress data from the PKWare Data Compression Library? | ||
206 | |||
207 | No. The PKWare DCL uses a completely different compressed data format than | ||
208 | does PKZIP and zlib. However, you can look in zlib's contrib/blast | ||
209 | directory for a possible solution to your problem. | ||
210 | |||
211 | 28. Can I access data randomly in a compressed stream? | ||
212 | |||
213 | No, not without some preparation. If when compressing you periodically use | ||
214 | Z_FULL_FLUSH, carefully write all the pending data at those points, and | ||
215 | keep an index of those locations, then you can start decompression at those | ||
216 | points. You have to be careful to not use Z_FULL_FLUSH too often, since it | ||
217 | can significantly degrade compression. Alternatively, you can scan a | ||
218 | deflate stream once to generate an index, and then use that index for | ||
219 | random access. See examples/zran.c . | ||
220 | |||
221 | 29. Does zlib work on MVS, OS/390, CICS, etc.? | ||
222 | |||
223 | It has in the past, but we have not heard of any recent evidence. There | ||
224 | were working ports of zlib 1.1.4 to MVS, but those links no longer work. | ||
225 | If you know of recent, successful applications of zlib on these operating | ||
226 | systems, please let us know. Thanks. | ||
227 | |||
228 | 30. Is there some simpler, easier to read version of inflate I can look at to | ||
229 | understand the deflate format? | ||
230 | |||
231 | First off, you should read RFC 1951. Second, yes. Look in zlib's | ||
232 | contrib/puff directory. | ||
233 | |||
234 | 31. Does zlib infringe on any patents? | ||
235 | |||
236 | As far as we know, no. In fact, that was originally the whole point behind | ||
237 | zlib. Look here for some more information: | ||
238 | |||
239 | http://www.gzip.org/#faq11 | ||
240 | |||
241 | 32. Can zlib work with greater than 4 GB of data? | ||
242 | |||
243 | Yes. inflate() and deflate() will process any amount of data correctly. | ||
244 | Each call of inflate() or deflate() is limited to input and output chunks | ||
245 | of the maximum value that can be stored in the compiler's "unsigned int" | ||
246 | type, but there is no limit to the number of chunks. Note however that the | ||
247 | strm.total_in and strm_total_out counters may be limited to 4 GB. These | ||
248 | counters are provided as a convenience and are not used internally by | ||
249 | inflate() or deflate(). The application can easily set up its own counters | ||
250 | updated after each call of inflate() or deflate() to count beyond 4 GB. | ||
251 | compress() and uncompress() may be limited to 4 GB, since they operate in a | ||
252 | single call. gzseek() and gztell() may be limited to 4 GB depending on how | ||
253 | zlib is compiled. See the zlibCompileFlags() function in zlib.h. | ||
254 | |||
255 | The word "may" appears several times above since there is a 4 GB limit only | ||
256 | if the compiler's "long" type is 32 bits. If the compiler's "long" type is | ||
257 | 64 bits, then the limit is 16 exabytes. | ||
258 | |||
259 | 33. Does zlib have any security vulnerabilities? | ||
260 | |||
261 | The only one that we are aware of is potentially in gzprintf(). If zlib is | ||
262 | compiled to use sprintf() or vsprintf(), then there is no protection | ||
263 | against a buffer overflow of an 8K string space (or other value as set by | ||
264 | gzbuffer()), other than the caller of gzprintf() assuring that the output | ||
265 | will not exceed 8K. On the other hand, if zlib is compiled to use | ||
266 | snprintf() or vsnprintf(), which should normally be the case, then there is | ||
267 | no vulnerability. The ./configure script will display warnings if an | ||
268 | insecure variation of sprintf() will be used by gzprintf(). Also the | ||
269 | zlibCompileFlags() function will return information on what variant of | ||
270 | sprintf() is used by gzprintf(). | ||
271 | |||
272 | If you don't have snprintf() or vsnprintf() and would like one, you can | ||
273 | find a portable implementation here: | ||
274 | |||
275 | http://www.ijs.si/software/snprintf/ | ||
276 | |||
277 | Note that you should be using the most recent version of zlib. Versions | ||
278 | 1.1.3 and before were subject to a double-free vulnerability, and versions | ||
279 | 1.2.1 and 1.2.2 were subject to an access exception when decompressing | ||
280 | invalid compressed data. | ||
281 | |||
282 | 34. Is there a Java version of zlib? | ||
283 | |||
284 | Probably what you want is to use zlib in Java. zlib is already included | ||
285 | as part of the Java SDK in the java.util.zip package. If you really want | ||
286 | a version of zlib written in the Java language, look on the zlib home | ||
287 | page for links: http://zlib.net/ . | ||
288 | |||
289 | 35. I get this or that compiler or source-code scanner warning when I crank it | ||
290 | up to maximally-pedantic. Can't you guys write proper code? | ||
291 | |||
292 | Many years ago, we gave up attempting to avoid warnings on every compiler | ||
293 | in the universe. It just got to be a waste of time, and some compilers | ||
294 | were downright silly as well as contradicted each other. So now, we simply | ||
295 | make sure that the code always works. | ||
296 | |||
297 | 36. Valgrind (or some similar memory access checker) says that deflate is | ||
298 | performing a conditional jump that depends on an uninitialized value. | ||
299 | Isn't that a bug? | ||
300 | |||
301 | No. That is intentional for performance reasons, and the output of deflate | ||
302 | is not affected. This only started showing up recently since zlib 1.2.x | ||
303 | uses malloc() by default for allocations, whereas earlier versions used | ||
304 | calloc(), which zeros out the allocated memory. Even though the code was | ||
305 | correct, versions 1.2.4 and later was changed to not stimulate these | ||
306 | checkers. | ||
307 | |||
308 | 37. Will zlib read the (insert any ancient or arcane format here) compressed | ||
309 | data format? | ||
310 | |||
311 | Probably not. Look in the comp.compression FAQ for pointers to various | ||
312 | formats and associated software. | ||
313 | |||
314 | 38. How can I encrypt/decrypt zip files with zlib? | ||
315 | |||
316 | zlib doesn't support encryption. The original PKZIP encryption is very | ||
317 | weak and can be broken with freely available programs. To get strong | ||
318 | encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib | ||
319 | compression. For PKZIP compatible "encryption", look at | ||
320 | http://www.info-zip.org/ | ||
321 | |||
322 | 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings? | ||
323 | |||
324 | "gzip" is the gzip format, and "deflate" is the zlib format. They should | ||
325 | probably have called the second one "zlib" instead to avoid confusion with | ||
326 | the raw deflate compressed data format. While the HTTP 1.1 RFC 2616 | ||
327 | correctly points to the zlib specification in RFC 1950 for the "deflate" | ||
328 | transfer encoding, there have been reports of servers and browsers that | ||
329 | incorrectly produce or expect raw deflate data per the deflate | ||
330 | specification in RFC 1951, most notably Microsoft. So even though the | ||
331 | "deflate" transfer encoding using the zlib format would be the more | ||
332 | efficient approach (and in fact exactly what the zlib format was designed | ||
333 | for), using the "gzip" transfer encoding is probably more reliable due to | ||
334 | an unfortunate choice of name on the part of the HTTP 1.1 authors. | ||
335 | |||
336 | Bottom line: use the gzip format for HTTP 1.1 encoding. | ||
337 | |||
338 | 40. Does zlib support the new "Deflate64" format introduced by PKWare? | ||
339 | |||
340 | No. PKWare has apparently decided to keep that format proprietary, since | ||
341 | they have not documented it as they have previous compression formats. In | ||
342 | any case, the compression improvements are so modest compared to other more | ||
343 | modern approaches, that it's not worth the effort to implement. | ||
344 | |||
345 | 41. I'm having a problem with the zip functions in zlib, can you help? | ||
346 | |||
347 | There are no zip functions in zlib. You are probably using minizip by | ||
348 | Giles Vollant, which is found in the contrib directory of zlib. It is not | ||
349 | part of zlib. In fact none of the stuff in contrib is part of zlib. The | ||
350 | files in there are not supported by the zlib authors. You need to contact | ||
351 | the authors of the respective contribution for help. | ||
352 | |||
353 | 42. The match.asm code in contrib is under the GNU General Public License. | ||
354 | Since it's part of zlib, doesn't that mean that all of zlib falls under the | ||
355 | GNU GPL? | ||
356 | |||
357 | No. The files in contrib are not part of zlib. They were contributed by | ||
358 | other authors and are provided as a convenience to the user within the zlib | ||
359 | distribution. Each item in contrib has its own license. | ||
360 | |||
361 | 43. Is zlib subject to export controls? What is its ECCN? | ||
362 | |||
363 | zlib is not subject to export controls, and so is classified as EAR99. | ||
364 | |||
365 | 44. Can you please sign these lengthy legal documents and fax them back to us | ||
366 | so that we can use your software in our product? | ||
367 | |||
368 | No. Go away. Shoo. | ||