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/cmp-z-it.zip
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 append something the ZLIB_VERSION string in zlib.h. For
151 example, if the version of the base zlib you are altering is "1.2.3", then
152 you could change the string to "1.2.3-fred-mods-v3". You should not change
153 it to "1.2.4" or "1.2.3.1" since the zlib authors would like to reserve
154 those forms of the version for their own use.
156 For altered source distributions, you should also note the origin and
157 nature of the changes in zlib.h, as well as in ChangeLog and README, along
158 with the dates of the alterations. The origin should include at least your
159 name (or your company's name), and an email address to contact for help or
160 issues with the library.
162 Note that distributing a compiled zlib library along with zlib.h and
163 zconf.h is also a source distribution, and so you should change
164 ZLIB_VERSION and note the origin and nature of the changes in zlib.h as you
165 would for a full source distribution.
167 23. Will zlib work on a big-endian or little-endian architecture, and can I
168 exchange compressed data between them?
172 24. Will zlib work on a 64-bit machine?
174 It should. It has been tested on 64-bit machines, and has no dependence
175 on any data types being limited to 32-bits in length. If you have any
176 difficulties, please provide a complete problem report to zlib@gzip.org
178 25. Will zlib decompress data from the PKWare Data Compression Library?
180 No. The PKWare DCL uses a completely different compressed data format
181 than does PKZIP and zlib. However, you can look in zlib's contrib/blast
182 directory for a possible solution to your problem.
184 26. Can I access data randomly in a compressed stream?
186 No, not without some preparation. If when compressing you periodically
187 use Z_FULL_FLUSH, carefully write all the pending data at those points,
188 and keep an index of those locations, then you can start decompression
189 at those points. You have to be careful to not use Z_FULL_FLUSH too
190 often, since it can significantly degrade compression.
192 27. Does zlib work on MVS, OS/390, CICS, etc.?
194 We don't know for sure. We have heard occasional reports of success on
195 these systems. If you do use it on one of these, please provide us with
196 a report, instructions, and patches that we can reference when we get
197 these questions. Thanks.
199 28. Is there some simpler, easier to read version of inflate I can look at
200 to understand the deflate format?
202 First off, you should read RFC 1951. Second, yes. Look in zlib's
203 contrib/puff directory.
205 29. Does zlib infringe on any patents?
207 As far as we know, no. In fact, that was originally the whole point behind
208 zlib. Look here for some more information:
210 http://www.gzip.org/#faq11
212 30. Can zlib work with greater than 4 GB of data?
214 Yes. inflate() and deflate() will process any amount of data correctly.
215 However the strm.total_in and strm_total_out counters may be limited to
216 4 GB. The application can easily set up its own counters updated after each
217 call of inflate() or deflate() to count beyond 4 GB. compress() and
218 uncompress() may be limited to 4 GB, since they operate in a single call
219 using unsigned long lengths. gzseek() and gztell() may be limited to 4 GB
220 depending on how zlib is compiled.
222 31. Does zlib have any security vulnerabilities?
224 The only one that we are aware of is potentially in gzprintf(). If zlib
225 is compiled to use sprintf() or vsprintf(), then there is no protection
226 against a buffer overflow of a 4K string space, other than the caller of
227 gzprintf() assuring that the output will not exceed 4K. On the other
228 hand, if zlib is compiled to use snprintf() or vsnprintf(), which should
229 normally be the case, then there is no vulnerability. The ./configure
230 script will display warnings if an insecure variation of sprintf() will
231 be used by gzprintf().
233 If you don't have snprintf() or vsnprintf() and would like one, you can
234 find a portable implementation here:
236 http://www.ijs.si/software/snprintf/
238 Note that you should be using the most recent version of zlib. Versions
239 1.1.3 and before were subject to a double-free vulnerability.
241 32. Is there a Java version of zlib?
243 Probably what you want is to use zlib in Java. zlib is already included
244 as part of the Java SDK in the java.util.zip class. If you really want
245 a version of zlib written in the Java language, look on the zlib home
246 page for links: http://www.zlib.org/
248 33. I get this or that compiler or source-code scanner warning when I crank it
249 up to maximally-pendantic. Can't you guys write proper code?
251 Many years ago, we gave up attempting to avoid warnings on every compiler
252 in the universe. It just got to be a waste of time, and some compilers
253 were downright silly. So now, we simply make sure that the code always
256 34. Will zlib read the (insert any ancient or arcane format here) compressed
259 Probably not. Look in the comp.compression FAQ for pointers to various
260 formats and associated software.
262 35. How can I encrypt/decrypt zip files with zlib?
264 zlib doesn't support encryption. PKZIP encryption is very weak and can be
265 broken with freely available programs. To get strong encryption, use gpg
266 which already includes zlib compression.
268 36. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
270 "gzip" is the gzip format, and "deflate" is the zlib format. They should
271 probably have called the second one "zlib" instead to avoid confusion
272 with the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
273 correctly points to the zlib specification in RFC 1950 for the "deflate"
274 transfer encoding, there have been reports of servers and browsers that
275 incorrectly produce or expect raw deflate data per the deflate
276 specficiation in RFC 1951, most notably Microsoft. So even though the
277 "deflate" transfer encoding using the zlib format would be the more
278 efficient approach (and in fact exactly what the zlib format was designed
279 for), using the "gzip" transfer encoding is probably more reliable due to
280 an unfortunate choice of name on the part of the HTTP 1.1 authors.
282 37. Does zlib support the new "Deflate64" format introduced by PKWare?
284 No. PKWare has apparently decided to keep that format proprietary, since
285 they have not documented it as they have previous compression formats.
286 In any case, the compression improvements are so modest compared to other
287 more modern approaches, that it's not worth the effort to implement.
289 38. Can you please sign these lengthy legal documents and fax them back to us
290 so that we can use your software in our product?