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1 Frequently Asked Questions about zlib
4 If your question is not there, please check the zlib home page
5 http://zlib.net/ which may have more recent information.
6 The lastest zlib FAQ is at http://zlib.net/zlib_faq.html
9 1. Is zlib Y2K-compliant?
11 Yes. zlib doesn't handle dates.
13 2. Where can I get a Windows DLL version?
15 The zlib sources can be compiled without change to produce a DLL. See the
16 file win32/DLL_FAQ.txt in the zlib distribution. Pointers to the
17 precompiled DLL are found in the zlib web site at http://zlib.net/ .
19 3. Where can I get a Visual Basic interface to zlib?
21 See
22 * http://marknelson.us/1997/01/01/zlib-engine/
23 * win32/DLL_FAQ.txt in the zlib distribution
25 4. compress() returns Z_BUF_ERROR.
27 Make sure that before the call of compress(), the length of the compressed
28 buffer is equal to the available size of the compressed buffer and not
29 zero. For Visual Basic, check that this parameter is passed by reference
30 ("as any"), not by value ("as long").
32 5. deflate() or inflate() returns Z_BUF_ERROR.
34 Before making the call, make sure that avail_in and avail_out are not zero.
35 When setting the parameter flush equal to Z_FINISH, also make sure that
36 avail_out is big enough to allow processing all pending input. Note that a
37 Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be
38 made with more input or output space. A Z_BUF_ERROR may in fact be
39 unavoidable depending on how the functions are used, since it is not
40 possible to tell whether or not there is more output pending when
41 strm.avail_out returns with zero. See http://zlib.net/zlib_how.html for a
42 heavily annotated example.
44 6. Where's the zlib documentation (man pages, etc.)?
46 It's in zlib.h . Examples of zlib usage are in the files example.c and
47 minigzip.c, with more in examples/ .
49 7. Why don't you use GNU autoconf or libtool or ...?
51 Because we would like to keep zlib as a very small and simple package.
52 zlib is rather portable and doesn't need much configuration.
54 8. I found a bug in zlib.
56 Most of the time, such problems are due to an incorrect usage of zlib.
57 Please try to reproduce the problem with a small program and send the
58 corresponding source to us at zlib@gzip.org . Do not send multi-megabyte
59 data files without prior agreement.
61 9. Why do I get "undefined reference to gzputc"?
63 If "make test" produces something like
65 example.o(.text+0x154): undefined reference to `gzputc'
67 check that you don't have old files libz.* in /usr/lib, /usr/local/lib or
68 /usr/X11R6/lib. Remove any old versions, then do "make install".
70 10. I need a Delphi interface to zlib.
72 See the contrib/delphi directory in the zlib distribution.
74 11. Can zlib handle .zip archives?
76 Not by itself, no. See the directory contrib/minizip in the zlib
77 distribution.
79 12. Can zlib handle .Z files?
81 No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt
82 the code of uncompress on your own.
84 13. How can I make a Unix shared library?
86 make clean
87 ./configure -s
88 make
90 14. How do I install a shared zlib library on Unix?
92 After the above, then:
94 make install
96 However, many flavors of Unix come with a shared zlib already installed.
97 Before going to the trouble of compiling a shared version of zlib and
98 trying to install it, you may want to check if it's already there! If you
99 can #include <zlib.h>, it's there. The -lz option will probably link to
100 it. You can check the version at the top of zlib.h or with the
101 ZLIB_VERSION symbol defined in zlib.h .
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. Can zlib decode Flate data in an Adobe PDF file?
110 Yes. See http://www.pdflib.com/ . To modify PDF forms, see
111 http://sourceforge.net/projects/acroformtool/ .
113 17. Why am I getting this "register_frame_info not found" error on Solaris?
115 After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
116 generates an error such as:
118 ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
119 symbol __register_frame_info: referenced symbol not found
121 The symbol __register_frame_info is not part of zlib, it is generated by
122 the C compiler (cc or gcc). You must recompile applications using zlib
123 which have this problem. This problem is specific to Solaris. See
124 http://www.sunfreeware.com for Solaris versions of zlib and applications
125 using zlib.
127 18. Why does gzip give an error on a file I make with compress/deflate?
129 The compress and deflate functions produce data in the zlib format, which
130 is different and incompatible with the gzip format. The gz* functions in
131 zlib on the other hand use the gzip format. Both the zlib and gzip formats
132 use the same compressed data format internally, but have different headers
133 and trailers around the compressed data.
135 19. Ok, so why are there two different formats?
137 The gzip format was designed to retain the directory information about a
138 single file, such as the name and last modification date. The zlib format
139 on the other hand was designed for in-memory and communication channel
140 applications, and has a much more compact header and trailer and uses a
141 faster integrity check than gzip.
143 20. Well that's nice, but how do I make a gzip file in memory?
145 You can request that deflate write the gzip format instead of the zlib
146 format using deflateInit2(). You can also request that inflate decode the
147 gzip format using inflateInit2(). Read zlib.h for more details.
149 21. Is zlib thread-safe?
151 Yes. However any library routines that zlib uses and any application-
152 provided memory allocation routines must also be thread-safe. zlib's gz*
153 functions use stdio library routines, and most of zlib's functions use the
154 library memory allocation routines by default. zlib's *Init* functions
155 allow for the application to provide custom memory allocation routines.
157 Of course, you should only operate on any given zlib or gzip stream from a
158 single thread at a time.
160 22. Can I use zlib in my commercial application?
162 Yes. Please read the license in zlib.h.
164 23. Is zlib under the GNU license?
166 No. Please read the license in zlib.h.
168 24. The license says that altered source versions must be "plainly marked". So
169 what exactly do I need to do to meet that requirement?
171 You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In
172 particular, the final version number needs to be changed to "f", and an
173 identification string should be appended to ZLIB_VERSION. Version numbers
174 x.x.x.f are reserved for modifications to zlib by others than the zlib
175 maintainers. For example, if the version of the base zlib you are altering
176 is "", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
177 ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also
178 update the version strings in deflate.c and inftrees.c.
180 For altered source distributions, you should also note the origin and
181 nature of the changes in zlib.h, as well as in ChangeLog and README, along
182 with the dates of the alterations. The origin should include at least your
183 name (or your company's name), and an email address to contact for help or
184 issues with the library.
186 Note that distributing a compiled zlib library along with zlib.h and
187 zconf.h is also a source distribution, and so you should change
188 ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes
189 in zlib.h as you would for a full source distribution.
191 25. Will zlib work on a big-endian or little-endian architecture, and can I
192 exchange compressed data between them?
194 Yes and yes.
196 26. Will zlib work on a 64-bit machine?
198 Yes. It has been tested on 64-bit machines, and has no dependence on any
199 data types being limited to 32-bits in length. If you have any
200 difficulties, please provide a complete problem report to zlib@gzip.org
202 27. Will zlib decompress data from the PKWare Data Compression Library?
204 No. The PKWare DCL uses a completely different compressed data format than
205 does PKZIP and zlib. However, you can look in zlib's contrib/blast
206 directory for a possible solution to your problem.
208 28. Can I access data randomly in a compressed stream?
210 No, not without some preparation. If when compressing you periodically use
211 Z_FULL_FLUSH, carefully write all the pending data at those points, and
212 keep an index of those locations, then you can start decompression at those
213 points. You have to be careful to not use Z_FULL_FLUSH too often, since it
214 can significantly degrade compression. Alternatively, you can scan a
215 deflate stream once to generate an index, and then use that index for
216 random access. See examples/zran.c .
218 29. Does zlib work on MVS, OS/390, CICS, etc.?
220 It has in the past, but we have not heard of any recent evidence. There
221 were working ports of zlib 1.1.4 to MVS, but those links no longer work.
222 If you know of recent, successful applications of zlib on these operating
223 systems, please let us know. Thanks.
225 30. Is there some simpler, easier to read version of inflate I can look at to
226 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:
236 http://www.gzip.org/#faq11
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 only
253 if the compiler's "long" type is 32 bits. If the compiler's "long" type is
254 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 is
259 compiled to use sprintf() or vsprintf(), then there is no protection
260 against a buffer overflow of an 8K string space (or other value as set by
261 gzbuffer()), other than the caller of gzprintf() assuring that the output
262 will not exceed 8K. On the other hand, if zlib is compiled to use
263 snprintf() or vsnprintf(), which should normally be the case, then there is
264 no vulnerability. The ./configure script will display warnings if an
265 insecure variation of sprintf() will be used by gzprintf(). Also the
266 zlibCompileFlags() function will return information on what variant of
267 sprintf() is used by gzprintf().
269 If you don't have snprintf() or vsnprintf() and would like one, you can
270 find a portable implementation here:
272 http://www.ijs.si/software/snprintf/
274 Note that you should be using the most recent version of zlib. Versions
275 1.1.3 and before were subject to a double-free vulnerability, and versions
276 1.2.1 and 1.2.2 were subject to an access exception when decompressing
277 invalid compressed data.
279 34. Is there a Java version of zlib?
281 Probably what you want is to use zlib in Java. zlib is already included
282 as part of the Java SDK in the java.util.zip package. If you really want
283 a version of zlib written in the Java language, look on the zlib home
284 page for links: http://zlib.net/ .
286 35. I get this or that compiler or source-code scanner warning when I crank it
287 up to maximally-pedantic. Can't you guys write proper code?
289 Many years ago, we gave up attempting to avoid warnings on every compiler
290 in the universe. It just got to be a waste of time, and some compilers
291 were downright silly as well as contradicted each other. So now, we simply
292 make sure that the code always works.
294 36. Valgrind (or some similar memory access checker) says that deflate is
295 performing a conditional jump that depends on an uninitialized value.
296 Isn't that a bug?
298 No. That is intentional for performance reasons, and the output of deflate
299 is not affected. This only started showing up recently since zlib 1.2.x
300 uses malloc() by default for allocations, whereas earlier versions used
301 calloc(), which zeros out the allocated memory. Even though the code was
302 correct, versions 1.2.4 and later was changed to not stimulate these
303 checkers.
305 37. Will zlib read the (insert any ancient or arcane format here) compressed
306 data format?
308 Probably not. Look in the comp.compression FAQ for pointers to various
309 formats and associated software.
311 38. How can I encrypt/decrypt zip files with zlib?
313 zlib doesn't support encryption. The original PKZIP encryption is very
314 weak and can be broken with freely available programs. To get strong
315 encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib
316 compression. For PKZIP compatible "encryption", look at
317 http://www.info-zip.org/
319 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
321 "gzip" is the gzip format, and "deflate" is the zlib format. They should
322 probably have called the second one "zlib" instead to avoid confusion with
323 the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
324 correctly points to the zlib specification in RFC 1950 for the "deflate"
325 transfer encoding, there have been reports of servers and browsers that
326 incorrectly produce or expect raw deflate data per the deflate
327 specficiation in RFC 1951, most notably Microsoft. So even though the
328 "deflate" transfer encoding using the zlib format would be the more
329 efficient approach (and in fact exactly what the zlib format was designed
330 for), using the "gzip" transfer encoding is probably more reliable due to
331 an unfortunate choice of name on the part of the HTTP 1.1 authors.
333 Bottom line: use the gzip format for HTTP 1.1 encoding.
335 40. Does zlib support the new "Deflate64" format introduced by PKWare?
337 No. PKWare has apparently decided to keep that format proprietary, since
338 they have not documented it as they have previous compression formats. In
339 any case, the compression improvements are so modest compared to other more
340 modern approaches, that it's not worth the effort to implement.
342 41. I'm having a problem with the zip functions in zlib, can you help?
344 There are no zip functions in zlib. You are probably using minizip by
345 Giles Vollant, which is found in the contrib directory of zlib. It is not
346 part of zlib. In fact none of the stuff in contrib is part of zlib. The
347 files in there are not supported by the zlib authors. You need to contact
348 the authors of the respective contribution for help.
350 42. The match.asm code in contrib is under the GNU General Public License.
351 Since it's part of zlib, doesn't that mean that all of zlib falls under the
352 GNU GPL?
354 No. The files in contrib are not part of zlib. They were contributed by
355 other authors and are provided as a convenience to the user within the zlib
356 distribution. Each item in contrib has its own license.
358 43. Is zlib subject to export controls? What is its ECCN?
360 zlib is not subject to export controls, and so is classified as EAR99.
362 44. Can you please sign these lengthy legal documents and fax them back to us
363 so that we can use your software in our product?
365 No. Go away. Shoo.

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