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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.dogma.net/markn/articles/zlibtool/zlibtool.htm
25 * contrib/visual-basic.txt in the zlib distribution
26 * win32/DLL_FAQ.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 us (firstname.lastname@example.org). Examples of zlib usage are in the files
51 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 email@example.com . 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 contrib/delphi directory in the zlib distribution.
78 11. Can zlib handle .zip archives?
80 Not by itself, no. See the directory contrib/minizip in the zlib
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?
90 make clean
91 ./configure -s
94 14. How do I install a shared zlib library on Unix?
96 After the above, then:
98 make install
100 However, many flavors of Unix come with a shared zlib already installed.
101 Before going to the trouble of compiling a shared version of zlib and
102 trying to install it, you may want to check if it's already there! If you
103 can #include <zlib.h>, it's there. The -lz option will probably link to it.
105 15. I have a question about OttoPDF.
107 We are not the authors of OttoPDF. The real author is on the OttoPDF web
108 site: Joel Hainley, firstname.lastname@example.org.
110 16. Can zlib decode Flate data in an Adobe PDF file?
112 Yes. See http://www.fastio.com/ (ClibPDF), or http://www.pdflib.com/ .
113 To modify PDF forms, see http://sourceforge.net/projects/acroformtool/ .
115 17. Why am I getting this "register_frame_info not found" error on Solaris?
117 After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
118 generates an error such as:
120 ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
121 symbol __register_frame_info: referenced symbol not found
123 The symbol __register_frame_info is not part of zlib, it is generated by
124 the C compiler (cc or gcc). You must recompile applications using zlib
125 which have this problem. This problem is specific to Solaris. See
126 http://www.sunfreeware.com for Solaris versions of zlib and applications
127 using zlib.
129 18. Why does gzip give an error on a file I make with compress/deflate?
131 The compress and deflate functions produce data in the zlib format, which
132 is different and incompatible with the gzip format. The gz* functions in
133 zlib on the other hand use the gzip format. Both the zlib and gzip
134 formats use the same compressed data format internally, but have different
135 headers and trailers around the compressed data.
137 19. Ok, so why are there two different formats?
139 The gzip format was designed to retain the directory information about
140 a single file, such as the name and last modification date. The zlib
141 format on the other hand was designed for in-memory and communication
142 channel applications, and has a much more compact header and trailer and
143 uses a faster integrity check than gzip.
145 20. Well that's nice, but how do I make a gzip file in memory?
147 You can request that deflate write the gzip format instead of the zlib
148 format using deflateInit2(). You can also request that inflate decode
149 the gzip format using inflateInit2(). Read zlib.h for more details.
151 21. Is zlib thread-safe?
153 Yes. However any library routines that zlib uses and any application-
154 provided memory allocation routines must also be thread-safe. zlib's gz*
155 functions use stdio library routines, and most of zlib's functions use the
156 library memory allocation routines by default. zlib's Init functions allow
157 for the application to provide custom memory allocation routines.
159 Of course, you should only operate on any given zlib or gzip stream from a
160 single thread at a time.
162 22. Can I use zlib in my commercial application?
164 Yes. Please read the license in zlib.h.
166 23. Is zlib under the GNU license?
168 No. Please read the license in zlib.h.
170 24. The license says that altered source versions must be "plainly marked". So
171 what exactly do I need to do to meet that requirement?
173 You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In
174 particular, the final version number needs to be changed to "f", and an
175 identification string should be appended to ZLIB_VERSION. Version numbers
176 x.x.x.f are reserved for modifications to zlib by others than the zlib
177 maintainers. For example, if the version of the base zlib you are altering
178 is "188.8.131.52", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
179 ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also
180 update the version strings in deflate.c and inftrees.c.
182 For altered source distributions, you should also note the origin and
183 nature of the changes in zlib.h, as well as in ChangeLog and README, along
184 with the dates of the alterations. The origin should include at least your
185 name (or your company's name), and an email address to contact for help or
186 issues with the library.
188 Note that distributing a compiled zlib library along with zlib.h and
189 zconf.h is also a source distribution, and so you should change
190 ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes
191 in zlib.h as you would for a full source distribution.
193 25. Will zlib work on a big-endian or little-endian architecture, and can I
194 exchange compressed data between them?
196 Yes and yes.
198 26. Will zlib work on a 64-bit machine?
200 It should. It has been tested on 64-bit machines, and has no dependence
201 on any data types being limited to 32-bits in length. If you have any
202 difficulties, please provide a complete problem report to email@example.com
204 27. Will zlib decompress data from the PKWare Data Compression Library?
206 No. The PKWare DCL uses a completely different compressed data format
207 than does PKZIP and zlib. However, you can look in zlib's contrib/blast
208 directory for a possible solution to your problem.
210 28. Can I access data randomly in a compressed stream?
212 No, not without some preparation. If when compressing you periodically
213 use Z_FULL_FLUSH, carefully write all the pending data at those points,
214 and keep an index of those locations, then you can start decompression
215 at those points. You have to be careful to not use Z_FULL_FLUSH too
216 often, since it can significantly degrade compression.
218 29. Does zlib work on MVS, OS/390, CICS, etc.?
220 We don't know for sure. We have heard occasional reports of success on
221 these systems. If you do use it on one of these, please provide us with
222 a report, instructions, and patches that we can reference when we get
223 these questions. Thanks.
225 30. Is there some simpler, easier to read version of inflate I can look at
226 to understand the deflate format?
228 First off, you should read RFC 1951. Second, yes. Look in zlib's
229 contrib/puff directory.
231 31. Does zlib infringe on any patents?
233 As far as we know, no. In fact, that was originally the whole point behind
234 zlib. Look here for some more information:
238 32. Can zlib work with greater than 4 GB of data?
240 Yes. inflate() and deflate() will process any amount of data correctly.
241 Each call of inflate() or deflate() is limited to input and output chunks
242 of the maximum value that can be stored in the compiler's "unsigned int"
243 type, but there is no limit to the number of chunks. Note however that the
244 strm.total_in and strm_total_out counters may be limited to 4 GB. These
245 counters are provided as a convenience and are not used internally by
246 inflate() or deflate(). The application can easily set up its own counters
247 updated after each call of inflate() or deflate() to count beyond 4 GB.
248 compress() and uncompress() may be limited to 4 GB, since they operate in a
249 single call. gzseek() and gztell() may be limited to 4 GB depending on how
250 zlib is compiled. See the zlibCompileFlags() function in zlib.h.
252 The word "may" appears several times above since there is a 4 GB limit
253 only if the compiler's "long" type is 32 bits. If the compiler's "long"
254 type is 64 bits, then the limit is 16 exabytes.
256 33. Does zlib have any security vulnerabilities?
258 The only one that we are aware of is potentially in gzprintf(). If zlib
259 is compiled to use sprintf() or vsprintf(), then there is no protection
260 against a buffer overflow of a 4K string space, other than the caller of
261 gzprintf() assuring that the output will not exceed 4K. On the other
262 hand, if zlib is compiled to use snprintf() or vsnprintf(), which should
263 normally be the case, then there is no vulnerability. The ./configure
264 script will display warnings if an insecure variation of sprintf() will
265 be used by gzprintf(). Also the zlibCompileFlags() function will return
266 information on what variant of sprintf() is used by gzprintf().
268 If you don't have snprintf() or vsnprintf() and would like one, you can
269 find a portable implementation here:
273 Note that you should be using the most recent version of zlib. Versions
274 1.1.3 and before were subject to a double-free vulnerability.
276 34. Is there a Java version of zlib?
278 Probably what you want is to use zlib in Java. zlib is already included
279 as part of the Java SDK in the java.util.zip package. If you really want
280 a version of zlib written in the Java language, look on the zlib home
281 page for links: http://www.zlib.org/
283 35. I get this or that compiler or source-code scanner warning when I crank it
284 up to maximally-pedantic. Can't you guys write proper code?
286 Many years ago, we gave up attempting to avoid warnings on every compiler
287 in the universe. It just got to be a waste of time, and some compilers
288 were downright silly. So now, we simply make sure that the code always
291 36. Valgrind (or some similar memory access checker) says that deflate is
292 performing a conditional jump that depends on an uninitialized value.
293 Isn't that a bug?
295 No. That is intentional for performance reasons, and the output of
296 deflate is not affected. This only started showing up recently since
297 zlib 1.2.x uses malloc() by default for allocations, whereas earlier
298 versions used calloc(), which zeros out the allocated memory.
300 37. Will zlib read the (insert any ancient or arcane format here) compressed
301 data format?
303 Probably not. Look in the comp.compression FAQ for pointers to various
304 formats and associated software.
306 38. How can I encrypt/decrypt zip files with zlib?
308 zlib doesn't support encryption. The original PKZIP encryption is very weak
309 and can be broken with freely available programs. To get strong encryption,
310 use GnuPG, http://www.gnupg.org/ , which already includes zlib compression.
311 For PKZIP compatible "encryption", look at http://www.info-zip.org/
313 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
315 "gzip" is the gzip format, and "deflate" is the zlib format. They should
316 probably have called the second one "zlib" instead to avoid confusion
317 with the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
318 correctly points to the zlib specification in RFC 1950 for the "deflate"
319 transfer encoding, there have been reports of servers and browsers that
320 incorrectly produce or expect raw deflate data per the deflate
321 specficiation in RFC 1951, most notably Microsoft. So even though the
322 "deflate" transfer encoding using the zlib format would be the more
323 efficient approach (and in fact exactly what the zlib format was designed
324 for), using the "gzip" transfer encoding is probably more reliable due to
325 an unfortunate choice of name on the part of the HTTP 1.1 authors.
327 Bottom line: use the gzip format for HTTP 1.1 encoding.
329 40. Does zlib support the new "Deflate64" format introduced by PKWare?
331 No. PKWare has apparently decided to keep that format proprietary, since
332 they have not documented it as they have previous compression formats.
333 In any case, the compression improvements are so modest compared to other
334 more modern approaches, that it's not worth the effort to implement.
336 41. Can you please sign these lengthy legal documents and fax them back to us
337 so that we can use your software in our product?
339 No. Go away. Shoo.