2 Frequently Asked Questions about zlib
5 If your question is not there, please check the zlib home page
6 http://www.zlib.org which may have more recent information.
7 The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html
10 1. Is zlib Y2K-compliant?
12 Yes. zlib doesn't handle dates.
14 2. Where can I get a Windows DLL version?
16 The zlib sources can be compiled without change to produce a DLL. If you
17 want a precompiled DLL, see http://www.winimage.com/zLibDll/ . Questions
18 about the zlib DLL should be sent to Gilles Vollant (info@winimage.com).
20 3. Where can I get a Visual Basic interface to zlib?
23 * http://www.winimage.com/zLibDll/
24 * http://www.dogma.net/markn/articles/zlibtool/zlibtool.htm
25 * contrib/visual-basic.txt in the zlib distribution
27 4. compress() returns Z_BUF_ERROR
29 Make sure that before the call of compress, the length of the compressed
30 buffer is equal to the total size of the compressed buffer and not
31 zero. For Visual Basic, check that this parameter is passed by reference
32 ("as any"), not by value ("as long").
34 5. deflate() or inflate() returns Z_BUF_ERROR
36 Before making the call, make sure that avail_in and avail_out are not
37 zero. When setting the parameter flush equal to Z_FINISH, also make sure
38 that avail_out is big enough to allow processing all pending input.
39 Note that a Z_BUF_ERROR is not fatal--another call to deflate() or
40 inflate() can be made with more input or output space. A Z_BUF_ERROR
41 may in fact be unavoidable depending on how the functions are used, since
42 it is not possible to tell whether or not there is more output pending
43 when strm.avail_out returns with zero.
45 6. Where's the zlib documentation (man pages, etc.)?
47 It's in zlib.h for the moment, and Francis S. Lin has converted it to a
48 web page zlib.html. Volunteers to transform this to Unix-style man pages,
49 please contact Jean-loup Gailly (jloup@gzip.org). Examples of zlib usage
50 are in the files example.c and minigzip.c.
52 7. Why don't you use GNU autoconf or libtool or ...?
54 Because we would like to keep zlib as a very small and simple
55 package. zlib is rather portable and doesn't need much configuration.
57 8. I found a bug in zlib.
59 Most of the time, such problems are due to an incorrect usage of
60 zlib. Please try to reproduce the problem with a small program and send
61 the corresponding source to us at zlib@gzip.org . Do not send
62 multi-megabyte data files without prior agreement.
64 9. Why do I get "undefined reference to gzputc"?
66 If "make test" produces something like
68 example.o(.text+0x154): undefined reference to `gzputc'
70 check that you don't have old files libz.* in /usr/lib, /usr/local/lib or
71 /usr/X11R6/lib. Remove any old versions, then do "make install".
73 10. I need a Delphi interface to zlib.
75 See the directories contrib/delphi and contrib/delphi2 in the zlib
78 11. Can zlib handle .zip archives?
80 See the directory contrib/minizip in the zlib distribution.
82 12. Can zlib handle .Z files?
84 No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt
85 the code of uncompress on your own.
87 13. How can I make a Unix shared library?
93 14. How do I install a shared zlib library on Unix?
97 However, many flavors of Unix come with a shared zlib already installed.
98 Before going to the trouble of compiling a shared version of zlib and
99 trying to install it, you may want to check if it's already there! If you
100 can #include <zlib.h>, it's there. The -lz option will probably link to it.
102 15. I have a question about OttoPDF
104 We are not the authors of OttoPDF. The real author is on the OttoPDF web
105 site Joel Hainley jhainley@myndkryme.com.
107 16. Why does gzip give an error on a file I make with compress/deflate?
109 The compress and deflate functions produce data in the zlib format, which
110 is different and incompatible with the gzip format. The gz* functions in
111 zlib on the other hand use the gzip format. Both the zlib and gzip
112 formats use the same compressed data format internally, but have different
113 headers and trailers around the compressed data.
115 17. Ok, so why are there two different formats?
117 The gzip format was designed to retain the directory information about
118 a single file, such as the name and last modification date. The zlib
119 format on the other hand was designed for in-memory and communication
120 channel applications, and has a much more compact header and trailer and
121 uses a faster integrity check than gzip.
123 18. Well that's nice, but how do I make a gzip file in memory?
125 Read RFC 1952 for the gzip header and trailer format, and roll your own
126 gzip formatted data using raw deflate and crc32().
128 19. Is zlib thread-safe?
130 Yes. However any library routines that zlib uses and any application-
131 provided memory allocation routines must also be thread-safe. zlib's gz*
132 functions use stdio library routines, and most of zlib's functions use the
133 library memory allocation routines by default. zlib's Init functions allow
134 for the application to provide custom memory allocation routines.
136 Of course, you should only operate on any given zlib or gzip stream from a
137 single thread at a time.
139 20. Can I use zlib in my commercial application?
141 Yes. Please read the license in zlib.h.
143 21. Is zlib under the GNU license?
145 No. Please read the license in zlib.h.
147 22. The license says that altered source versions must be "plainly marked". So
148 what exactly do I need to do to meet that requirement?
150 You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In
151 particular, the final version number needs to be changed to "f", and an
152 identification string should be appended to ZLIB_VERSION. Version numbers
153 x.x.x.f are reserved for modifications to zlib by others than the zlib
154 maintainers. For example, if the version of the base zlib you are altering
155 is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
156 ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also
157 update the version strings in deflate.c and inftrees.c.
159 For altered source distributions, you should also note the origin and
160 nature of the changes in zlib.h, as well as in ChangeLog and README, along
161 with the dates of the alterations. The origin should include at least your
162 name (or your company's name), and an email address to contact for help or
163 issues with the library.
165 Note that distributing a compiled zlib library along with zlib.h and
166 zconf.h is also a source distribution, and so you should change
167 ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes
168 in zlib.h as you would for a full source distribution.
170 23. Will zlib work on a big-endian or little-endian architecture, and can I
171 exchange compressed data between them?
175 24. Will zlib work on a 64-bit machine?
177 It should. It has been tested on 64-bit machines, and has no dependence
178 on any data types being limited to 32-bits in length. If you have any
179 difficulties, please provide a complete problem report to zlib@gzip.org
181 25. Will zlib decompress data from the PKWare Data Compression Library?
183 No. The PKWare DCL uses a completely different compressed data format
184 than does PKZIP and zlib. However, you can look in zlib's contrib/blast
185 directory for a possible solution to your problem.
187 26. Can I access data randomly in a compressed stream?
189 No, not without some preparation. If when compressing you periodically
190 use Z_FULL_FLUSH, carefully write all the pending data at those points,
191 and keep an index of those locations, then you can start decompression
192 at those points. You have to be careful to not use Z_FULL_FLUSH too
193 often, since it can significantly degrade compression.
195 27. Does zlib work on MVS, OS/390, CICS, etc.?
197 We don't know for sure. We have heard occasional reports of success on
198 these systems. If you do use it on one of these, please provide us with
199 a report, instructions, and patches that we can reference when we get
200 these questions. Thanks.
202 28. Is there some simpler, easier to read version of inflate I can look at
203 to understand the deflate format?
205 First off, you should read RFC 1951. Second, yes. Look in zlib's
206 contrib/puff directory.
208 29. Does zlib infringe on any patents?
210 As far as we know, no. In fact, that was originally the whole point behind
211 zlib. Look here for some more information:
213 http://www.gzip.org/#faq11
215 30. Can zlib work with greater than 4 GB of data?
217 Yes. inflate() and deflate() will process any amount of data correctly.
218 Each call of inflate() or deflate() is limited to input and output chunks
219 of the maximum value that can be stored in the compiler's "unsigned int"
220 type, but there is no limit to the number of chunks. Note however that the
221 strm.total_in and strm_total_out counters may be limited to 4 GB. These
222 counters are provided as a convenience and are not used internally by
223 inflate() or deflate(). The application can easily set up its own counters
224 updated after each call of inflate() or deflate() to count beyond 4 GB.
225 compress() and uncompress() may be limited to 4 GB, since they operate in a
226 single call. gzseek() and gztell() may be limited to 4 GB depending on how
227 zlib is compiled. See the zlibCompileFlags() function in zlib.h.
229 The word "may" appears several times above since there is a 4 GB limit
230 only if the compiler's "long" type is 32 bits. If the compiler's "long"
231 type is 64 bits, then the limit is 16 exabytes.
233 31. Does zlib have any security vulnerabilities?
235 The only one that we are aware of is potentially in gzprintf(). If zlib
236 is compiled to use sprintf() or vsprintf(), then there is no protection
237 against a buffer overflow of a 4K string space, other than the caller of
238 gzprintf() assuring that the output will not exceed 4K. On the other
239 hand, if zlib is compiled to use snprintf() or vsnprintf(), which should
240 normally be the case, then there is no vulnerability. The ./configure
241 script will display warnings if an insecure variation of sprintf() will
242 be used by gzprintf(). Also the zlibCompileFlags() function will return
243 information on what variant of sprintf() is used by gzprintf().
245 If you don't have snprintf() or vsnprintf() and would like one, you can
246 find a portable implementation here:
248 http://www.ijs.si/software/snprintf/
250 Note that you should be using the most recent version of zlib. Versions
251 1.1.3 and before were subject to a double-free vulnerability.
253 32. Is there a Java version of zlib?
255 Probably what you want is to use zlib in Java. zlib is already included
256 as part of the Java SDK in the java.util.zip class. If you really want
257 a version of zlib written in the Java language, look on the zlib home
258 page for links: http://www.zlib.org/
260 33. I get this or that compiler or source-code scanner warning when I crank it
261 up to maximally-pendantic. Can't you guys write proper code?
263 Many years ago, we gave up attempting to avoid warnings on every compiler
264 in the universe. It just got to be a waste of time, and some compilers
265 were downright silly. So now, we simply make sure that the code always
268 34. Will zlib read the (insert any ancient or arcane format here) compressed
271 Probably not. Look in the comp.compression FAQ for pointers to various
272 formats and associated software.
274 35. How can I encrypt/decrypt zip files with zlib?
276 zlib doesn't support encryption. The original PKZIP encryption is very weak
277 and can be broken with freely available programs. To get strong encryption,
278 use gpg ( http://www.gnupg.org/ ) which already includes zlib compression.
279 For PKZIP compatible "encryption", look at http://www.info-zip.org/
281 36. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
283 "gzip" is the gzip format, and "deflate" is the zlib format. They should
284 probably have called the second one "zlib" instead to avoid confusion
285 with the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
286 correctly points to the zlib specification in RFC 1950 for the "deflate"
287 transfer encoding, there have been reports of servers and browsers that
288 incorrectly produce or expect raw deflate data per the deflate
289 specficiation in RFC 1951, most notably Microsoft. So even though the
290 "deflate" transfer encoding using the zlib format would be the more
291 efficient approach (and in fact exactly what the zlib format was designed
292 for), using the "gzip" transfer encoding is probably more reliable due to
293 an unfortunate choice of name on the part of the HTTP 1.1 authors.
295 Bottom line: use the gzip format for HTTP 1.1 encoding.
297 37. Does zlib support the new "Deflate64" format introduced by PKWare?
299 No. PKWare has apparently decided to keep that format proprietary, since
300 they have not documented it as they have previous compression formats.
301 In any case, the compression improvements are so modest compared to other
302 more modern approaches, that it's not worth the effort to implement.
304 38. Can you please sign these lengthy legal documents and fax them back to us
305 so that we can use your software in our product?