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.
17 See the file win32/DLL_FAQ.txt in the zlib distribution.
18 Pointers to the precompiled DLL are found in the zlib web site at
21 3. Where can I get a Visual Basic interface to zlib?
24 * http://www.winimage.com/zLibDll/
25 * http://www.dogma.net/markn/articles/zlibtool/zlibtool.htm
26 * contrib/visual-basic.txt in the zlib distribution
28 4. compress() returns Z_BUF_ERROR
30 Make sure that before the call of compress, the length of the compressed
31 buffer is equal to the total size of the compressed buffer and not
32 zero. For Visual Basic, check that this parameter is passed by reference
33 ("as any"), not by value ("as long").
35 5. deflate() or inflate() returns Z_BUF_ERROR
37 Before making the call, make sure that avail_in and avail_out are not
38 zero. When setting the parameter flush equal to Z_FINISH, also make sure
39 that avail_out is big enough to allow processing all pending input.
40 Note that a Z_BUF_ERROR is not fatal--another call to deflate() or
41 inflate() can be made with more input or output space. A Z_BUF_ERROR
42 may in fact be unavoidable depending on how the functions are used, since
43 it is not possible to tell whether or not there is more output pending
44 when strm.avail_out returns with zero.
46 6. Where's the zlib documentation (man pages, etc.)?
48 It's in zlib.h for the moment, and Francis S. Lin has converted it to a
49 web page zlib.html. Volunteers to transform this to Unix-style man pages,
50 please contact Jean-loup Gailly (jloup@gzip.org). Examples of zlib usage
51 are in the files example.c and minigzip.c.
53 7. Why don't you use GNU autoconf or libtool or ...?
55 Because we would like to keep zlib as a very small and simple
56 package. zlib is rather portable and doesn't need much configuration.
58 8. I found a bug in zlib.
60 Most of the time, such problems are due to an incorrect usage of
61 zlib. Please try to reproduce the problem with a small program and send
62 the corresponding source to us at zlib@gzip.org . Do not send
63 multi-megabyte data files without prior agreement.
65 9. Why do I get "undefined reference to gzputc"?
67 If "make test" produces something like
69 example.o(.text+0x154): undefined reference to `gzputc'
71 check that you don't have old files libz.* in /usr/lib, /usr/local/lib or
72 /usr/X11R6/lib. Remove any old versions, then do "make install".
74 10. I need a Delphi interface to zlib.
76 See the directories contrib/delphi and contrib/delphi2 in the zlib
79 11. Can zlib handle .zip archives?
81 See the directory contrib/minizip in the zlib distribution.
83 12. Can zlib handle .Z files?
85 No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt
86 the code of uncompress on your own.
88 13. How can I make a Unix shared library?
94 14. How do I install a shared zlib library on Unix?
98 However, many flavors of Unix come with a shared zlib already installed.
99 Before going to the trouble of compiling a shared version of zlib and
100 trying to install it, you may want to check if it's already there! If you
101 can #include <zlib.h>, it's there. The -lz option will probably link to it.
103 15. I have a question about OttoPDF
105 We are not the authors of OttoPDF. The real author is on the OttoPDF web
106 site Joel Hainley jhainley@myndkryme.com.
108 16. Why does gzip give an error on a file I make with compress/deflate?
110 The compress and deflate functions produce data in the zlib format, which
111 is different and incompatible with the gzip format. The gz* functions in
112 zlib on the other hand use the gzip format. Both the zlib and gzip
113 formats use the same compressed data format internally, but have different
114 headers and trailers around the compressed data.
116 17. Ok, so why are there two different formats?
118 The gzip format was designed to retain the directory information about
119 a single file, such as the name and last modification date. The zlib
120 format on the other hand was designed for in-memory and communication
121 channel applications, and has a much more compact header and trailer and
122 uses a faster integrity check than gzip.
124 18. Well that's nice, but how do I make a gzip file in memory?
126 You can request that deflate write the gzip format instead of the zlib
127 format using deflateInit2(). You can also request that inflate decode
128 the gzip format using inflateInit2(). Read zlib.h for more details.
130 Note that you cannot specify special gzip header contents (e.g. a file
131 name or modification date), nor will inflate tell you what was in the
132 gzip header. If you need to customize the header or see what's in it,
133 you can use the raw deflate and inflate operations and the crc32()
134 function and roll your own gzip encoding and decoding. Read the gzip
135 RFC 1952 for details of the header and trailer format.
137 19. Is zlib thread-safe?
139 Yes. However any library routines that zlib uses and any application-
140 provided memory allocation routines must also be thread-safe. zlib's gz*
141 functions use stdio library routines, and most of zlib's functions use the
142 library memory allocation routines by default. zlib's Init functions allow
143 for the application to provide custom memory allocation routines.
145 Of course, you should only operate on any given zlib or gzip stream from a
146 single thread at a time.
148 20. Can I use zlib in my commercial application?
150 Yes. Please read the license in zlib.h.
152 21. Is zlib under the GNU license?
154 No. Please read the license in zlib.h.
156 22. The license says that altered source versions must be "plainly marked". So
157 what exactly do I need to do to meet that requirement?
159 You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In
160 particular, the final version number needs to be changed to "f", and an
161 identification string should be appended to ZLIB_VERSION. Version numbers
162 x.x.x.f are reserved for modifications to zlib by others than the zlib
163 maintainers. For example, if the version of the base zlib you are altering
164 is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
165 ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also
166 update the version strings in deflate.c and inftrees.c.
168 For altered source distributions, you should also note the origin and
169 nature of the changes in zlib.h, as well as in ChangeLog and README, along
170 with the dates of the alterations. The origin should include at least your
171 name (or your company's name), and an email address to contact for help or
172 issues with the library.
174 Note that distributing a compiled zlib library along with zlib.h and
175 zconf.h is also a source distribution, and so you should change
176 ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes
177 in zlib.h as you would for a full source distribution.
179 23. Will zlib work on a big-endian or little-endian architecture, and can I
180 exchange compressed data between them?
184 24. Will zlib work on a 64-bit machine?
186 It should. It has been tested on 64-bit machines, and has no dependence
187 on any data types being limited to 32-bits in length. If you have any
188 difficulties, please provide a complete problem report to zlib@gzip.org
190 25. Will zlib decompress data from the PKWare Data Compression Library?
192 No. The PKWare DCL uses a completely different compressed data format
193 than does PKZIP and zlib. However, you can look in zlib's contrib/blast
194 directory for a possible solution to your problem.
196 26. Can I access data randomly in a compressed stream?
198 No, not without some preparation. If when compressing you periodically
199 use Z_FULL_FLUSH, carefully write all the pending data at those points,
200 and keep an index of those locations, then you can start decompression
201 at those points. You have to be careful to not use Z_FULL_FLUSH too
202 often, since it can significantly degrade compression.
204 27. Does zlib work on MVS, OS/390, CICS, etc.?
206 We don't know for sure. We have heard occasional reports of success on
207 these systems. If you do use it on one of these, please provide us with
208 a report, instructions, and patches that we can reference when we get
209 these questions. Thanks.
211 28. Is there some simpler, easier to read version of inflate I can look at
212 to understand the deflate format?
214 First off, you should read RFC 1951. Second, yes. Look in zlib's
215 contrib/puff directory.
217 29. Does zlib infringe on any patents?
219 As far as we know, no. In fact, that was originally the whole point behind
220 zlib. Look here for some more information:
222 http://www.gzip.org/#faq11
224 30. Can zlib work with greater than 4 GB of data?
226 Yes. inflate() and deflate() will process any amount of data correctly.
227 Each call of inflate() or deflate() is limited to input and output chunks
228 of the maximum value that can be stored in the compiler's "unsigned int"
229 type, but there is no limit to the number of chunks. Note however that the
230 strm.total_in and strm_total_out counters may be limited to 4 GB. These
231 counters are provided as a convenience and are not used internally by
232 inflate() or deflate(). The application can easily set up its own counters
233 updated after each call of inflate() or deflate() to count beyond 4 GB.
234 compress() and uncompress() may be limited to 4 GB, since they operate in a
235 single call. gzseek() and gztell() may be limited to 4 GB depending on how
236 zlib is compiled. See the zlibCompileFlags() function in zlib.h.
238 The word "may" appears several times above since there is a 4 GB limit
239 only if the compiler's "long" type is 32 bits. If the compiler's "long"
240 type is 64 bits, then the limit is 16 exabytes.
242 31. Does zlib have any security vulnerabilities?
244 The only one that we are aware of is potentially in gzprintf(). If zlib
245 is compiled to use sprintf() or vsprintf(), then there is no protection
246 against a buffer overflow of a 4K string space, other than the caller of
247 gzprintf() assuring that the output will not exceed 4K. On the other
248 hand, if zlib is compiled to use snprintf() or vsnprintf(), which should
249 normally be the case, then there is no vulnerability. The ./configure
250 script will display warnings if an insecure variation of sprintf() will
251 be used by gzprintf(). Also the zlibCompileFlags() function will return
252 information on what variant of sprintf() is used by gzprintf().
254 If you don't have snprintf() or vsnprintf() and would like one, you can
255 find a portable implementation here:
257 http://www.ijs.si/software/snprintf/
259 Note that you should be using the most recent version of zlib. Versions
260 1.1.3 and before were subject to a double-free vulnerability.
262 32. Is there a Java version of zlib?
264 Probably what you want is to use zlib in Java. zlib is already included
265 as part of the Java SDK in the java.util.zip package. If you really want
266 a version of zlib written in the Java language, look on the zlib home
267 page for links: http://www.zlib.org/
269 33. I get this or that compiler or source-code scanner warning when I crank it
270 up to maximally-pendantic. Can't you guys write proper code?
272 Many years ago, we gave up attempting to avoid warnings on every compiler
273 in the universe. It just got to be a waste of time, and some compilers
274 were downright silly. So now, we simply make sure that the code always
277 34. Will zlib read the (insert any ancient or arcane format here) compressed
280 Probably not. Look in the comp.compression FAQ for pointers to various
281 formats and associated software.
283 35. How can I encrypt/decrypt zip files with zlib?
285 zlib doesn't support encryption. The original PKZIP encryption is very weak
286 and can be broken with freely available programs. To get strong encryption,
287 use gpg ( http://www.gnupg.org/ ) which already includes zlib compression.
288 For PKZIP compatible "encryption", look at http://www.info-zip.org/
290 36. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
292 "gzip" is the gzip format, and "deflate" is the zlib format. They should
293 probably have called the second one "zlib" instead to avoid confusion
294 with the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
295 correctly points to the zlib specification in RFC 1950 for the "deflate"
296 transfer encoding, there have been reports of servers and browsers that
297 incorrectly produce or expect raw deflate data per the deflate
298 specficiation in RFC 1951, most notably Microsoft. So even though the
299 "deflate" transfer encoding using the zlib format would be the more
300 efficient approach (and in fact exactly what the zlib format was designed
301 for), using the "gzip" transfer encoding is probably more reliable due to
302 an unfortunate choice of name on the part of the HTTP 1.1 authors.
304 Bottom line: use the gzip format for HTTP 1.1 encoding.
306 37. Does zlib support the new "Deflate64" format introduced by PKWare?
308 No. PKWare has apparently decided to keep that format proprietary, since
309 they have not documented it as they have previous compression formats.
310 In any case, the compression improvements are so modest compared to other
311 more modern approaches, that it's not worth the effort to implement.
313 38. Can you please sign these lengthy legal documents and fax them back to us
314 so that we can use your software in our product?