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. Why does "make test" fail on Mac OS X?
95 Mac OS X already includes zlib as a shared library, and so -lz links the
96 shared library instead of the one that the "make" compiled. The two are
97 incompatible due to different compile-time options. Simply change the -lz
98 in the Makefile to libz.a, and it will use the compiled library instead
99 of the shared one and the "make test" will succeed.
101 15. I have a question about OttoPDF
103 We are not the authors of OttoPDF. The real author is on the OttoPDF web
104 site Joel Hainley jhainley@myndkryme.com.
106 16. Why does gzip give an error on a file I make with compress/deflate?
108 The compress and deflate functions produce data in the zlib format, which
109 is different and incompatible with the gzip format. The gz* functions in
110 zlib on the other hand use the gzip format. Both the zlib and gzip
111 formats use the same compressed data format, but have different headers
114 17. Ok, so why are there two different formats?
116 The gzip format was designed to retain the directory information about
117 a single file, such as the name and last modification date. The zlib
118 format on the other hand was designed for in-memory and communication
119 channel applications, and has a much more compact header and trailer and
120 uses a faster integrity check than gzip.
122 18. Well that's nice, but how do I make a gzip file in memory?
124 Read RFC 1952 for the gzip header and trailer format, and roll your own
125 gzip formatted data using raw deflate and crc32().
127 19. Is zlib thread-safe?
129 Yes. However any library routines that zlib uses and any application-
130 provided memory allocation routines must also be thread-safe. Of course,
131 you should only operate on any given zlib or gzip stream from a single
132 thread. zlib's gz* functions use stdio library routines, and most of
133 zlib's functions use the library memory allocation routines by default.
134 zlib's Init functions allow for the application to provide custom memory
137 20. Can I use zlib in my commercial application?
139 Yes. Please read the license in zlib.h.
141 21. Is zlib under the GNU license?
143 No. Please read the license in zlib.h.
145 22. Will zlib work on a big-endian or little-endian architecture, and can I
146 exchange compressed data between them?
150 23. Will zlib work on a 64-bit machine?
152 It should. It has been tested on 64-bit machines, and has no dependence
153 on any data types being limited to 32-bits in length. If you have any
154 difficulties, please provide a complete problem report to zlib@gzip.org
156 24. Will zlib decompress data from the PKWare Data Compression Library?
158 No. The PKWare DCL uses a completely different compressed data format
159 than does PKZIP and zlib. However, you can look in zlib's contrib/blast
160 directory for a possible solution to your problem.
162 25. Can I access data randomly in a compressed stream?
164 No, not without some preparation. If when compressing you periodically
165 use Z_FULL_FLUSH, carefully write all the pending data at those points,
166 and keep an index of those locations, then you can start decompression
167 at those points. You have to be careful to not use Z_FULL_FLUSH too
168 often, since it can significantly degrade compression.
170 26. Does zlib work on MVS, OS/390, CICS, etc.?
172 We don't know for sure. We have heard occasional reports of success on
173 these systems. If you do use it on one of these, please provide us with
174 a report, instructions, and patches that we can reference when we get
175 these questions. Thanks.
177 27. Is there some simpler, easier to read version of inflate I can look at
178 to understand the deflate format?
180 First off, you should read RFC 1951. Second, yes. Look in zlib's
181 contrib/puff directory.
183 28. Does zlib infringe on any patents?
185 As far as we know, no. In fact, that was originally the whole point behind
186 zlib. Look here for some more information:
188 http://www.gzip.org/#faq11
190 29. Can zlib work with greater than 4 GB of data?
192 Yes. inflate() and deflate() will process any amount of data correctly.
193 However the strm.total_in and strm_total_out counters may be limited to
194 4 GB. The user can easily set up their own counters updated after each
195 call of inflate() or deflate() to count beyond 4 GB. compress() and
196 uncompress() may be limited to 4 GB, since they operate in a single
197 call using unsigned long lengths. gzseek() may be limited to 4 GB
198 depending on how zlib is compiled.
200 30. Does zlib have any security vulnerabilities?
202 The only one that we are aware of is potentially in gzprintf(). If zlib
203 is compiled to use sprintf() or vsprintf(), then there is no protection
204 against a buffer overflow of a 4K string space, other than the caller of
205 gzprintf() assuring that the output will not exceed 4K. On the other
206 hand, if zlib is compiled to use snprintf() or vsnprintf(), then there is
209 Note that you should be using the most recent version of zlib. Versions
210 1.1.3 and before were subject to a double-free vulnerability.
212 31. Is there a Java version of zlib?
214 Probably what you want is to use zlib in Java. zlib is already included
215 as part of the Java SDK in the java.util.zip class. If you really want
216 a version of zlib written in the Java language, look on the zlib home
217 page for links: http://www.zlib.org/
219 32. I get this or that compiler or source-code scanner warning. Can't you guys
222 Many years ago, we gave up attempting to avoid warnings on every compiler
223 in the universe. It just got to be a waste of time, and some compilers
224 were downright silly. So now, we simply make sure that the code always
227 33. Will zlib read the (insert any ancient or arcane format here) compressed
230 Probably not. Look in the comp.compression FAQ for pointers to various
231 formats and associated software.
233 34. How can I encrypt/decrypt zip files with zlib?
235 zlib doesn't support encryption. PKZIP encryption is very weak and can be
236 broken with freely available programs. To get strong encryption, use gpg
237 which already includes zlib compression.
239 35. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
241 "gzip" is the gzip format, and "deflate" is the zlib format. They should
242 probably have called the second one "zlib" instead to avoid confusion
243 with the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
244 correctly points to the zlib specification in RFC 1950 for the "deflate"
245 transfer encoding, there have been reports of servers and browsers that
246 incorrectly produce or expect raw deflate data per the deflate
247 specficiation in RFC 1951, most notably Microsoft. So even though the
248 "deflate" transfer encoding using the zlib format would be the more
249 efficient approach (and in fact exactly what the zlib format was designed
250 for), using the "gzip" transfer encoding is probably more reliable due to
251 an unfortunate choice of name on the part of the HTTP 1.1 authors.
253 36. Does zlib support the new "Deflate64" format introduced by PKWare?
255 No. PKWare has apparently decided to keep that format proprietary, since
256 they have not documented it as they have previous compression formats.
257 In any case, the compression improvements are so modest compared to other
258 more modern approaches, that it's not worth the effort to implement.
260 37. Can you please sign these lengthy legal documents and fax them back to us
261 so that we can use your software in our product?