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ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4. The directory name has changed: no version number in the expanded directory now.
author Franz Glasner <fzglas.hg@dom66.de>
date Mon, 15 Sep 2025 11:43:07 +0200
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1
2 Frequently Asked Questions about zlib
3
4
5 If your question is not there, please check the zlib home page
6 http://zlib.net/ which may have more recent information.
7 The latest zlib FAQ is at http://zlib.net/zlib_faq.html
8
9
10 1. Is zlib Y2K-compliant?
11
12 Yes. zlib doesn't handle dates.
13
14 2. Where can I get a Windows DLL version?
15
16 The zlib sources can be compiled without change to produce a DLL. See the
17 file win32/DLL_FAQ.txt in the zlib distribution.
18
19 3. Where can I get a Visual Basic interface to zlib?
20
21 See
22 * http://marknelson.us/1997/01/01/zlib-engine/
23 * win32/DLL_FAQ.txt in the zlib distribution
24
25 4. compress() returns Z_BUF_ERROR.
26
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").
31
32 5. deflate() or inflate() returns Z_BUF_ERROR.
33
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.
43
44 6. Where's the zlib documentation (man pages, etc.)?
45
46 It's in zlib.h . Examples of zlib usage are in the files test/example.c
47 and test/minigzip.c, with more in examples/ .
48
49 7. Why don't you use GNU autoconf or libtool or ...?
50
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.
53
54 8. I found a bug in zlib.
55
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.
60
61 9. Why do I get "undefined reference to gzputc"?
62
63 If "make test" produces something like
64
65 example.o(.text+0x154): undefined reference to `gzputc'
66
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".
69
70 10. I need a Delphi interface to zlib.
71
72 See the contrib/delphi directory in the zlib distribution.
73
74 11. Can zlib handle .zip archives?
75
76 Not by itself, no. See the directory contrib/minizip in the zlib
77 distribution.
78
79 12. Can zlib handle .Z files?
80
81 No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt
82 the code of uncompress on your own.
83
84 13. How can I make a Unix shared library?
85
86 By default a shared (and a static) library is built for Unix. So:
87
88 make distclean
89 ./configure
90 make
91
92 14. How do I install a shared zlib library on Unix?
93
94 After the above, then:
95
96 make install
97
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
102 it. You can check the version at the top of zlib.h or with the
103 ZLIB_VERSION symbol defined in zlib.h .
104
105 15. I have a question about OttoPDF.
106
107 We are not the authors of OttoPDF. The real author is on the OttoPDF web
108 site: Joel Hainley, jhainley@myndkryme.com.
109
110 16. Can zlib decode Flate data in an Adobe PDF file?
111
112 Yes. See http://www.pdflib.com/ . To modify PDF forms, see
113 http://sourceforge.net/projects/acroformtool/ .
114
115 17. Why am I getting this "register_frame_info not found" error on Solaris?
116
117 After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
118 generates an error such as:
119
120 ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
121 symbol __register_frame_info: referenced symbol not found
122
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.
128
129 18. Why does gzip give an error on a file I make with compress/deflate?
130
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 formats
134 use the same compressed data format internally, but have different headers
135 and trailers around the compressed data.
136
137 19. Ok, so why are there two different formats?
138
139 The gzip format was designed to retain the directory information about a
140 single file, such as the name and last modification date. The zlib format
141 on the other hand was designed for in-memory and communication channel
142 applications, and has a much more compact header and trailer and uses a
143 faster integrity check than gzip.
144
145 20. Well that's nice, but how do I make a gzip file in memory?
146
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 the
149 gzip format using inflateInit2(). Read zlib.h for more details.
150
151 21. Is zlib thread-safe?
152
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
157 allow for the application to provide custom memory allocation routines.
158
159 Of course, you should only operate on any given zlib or gzip stream from a
160 single thread at a time.
161
162 22. Can I use zlib in my commercial application?
163
164 Yes. Please read the license in zlib.h.
165
166 23. Is zlib under the GNU license?
167
168 No. Please read the license in zlib.h.
169
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?
172
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 "1.2.3.4", 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.
181
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.
187
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.
192
193 25. Will zlib work on a big-endian or little-endian architecture, and can I
194 exchange compressed data between them?
195
196 Yes and yes.
197
198 26. Will zlib work on a 64-bit machine?
199
200 Yes. It has been tested on 64-bit machines, and has no dependence on any
201 data types being limited to 32-bits in length. If you have any
202 difficulties, please provide a complete problem report to zlib@gzip.org
203
204 27. Will zlib decompress data from the PKWare Data Compression Library?
205
206 No. The PKWare DCL uses a completely different compressed data format than
207 does PKZIP and zlib. However, you can look in zlib's contrib/blast
208 directory for a possible solution to your problem.
209
210 28. Can I access data randomly in a compressed stream?
211
212 No, not without some preparation. If when compressing you periodically use
213 Z_FULL_FLUSH, carefully write all the pending data at those points, and
214 keep an index of those locations, then you can start decompression at those
215 points. You have to be careful to not use Z_FULL_FLUSH too often, since it
216 can significantly degrade compression. Alternatively, you can scan a
217 deflate stream once to generate an index, and then use that index for
218 random access. See examples/zran.c .
219
220 29. Does zlib work on MVS, OS/390, CICS, etc.?
221
222 It has in the past, but we have not heard of any recent evidence. There
223 were working ports of zlib 1.1.4 to MVS, but those links no longer work.
224 If you know of recent, successful applications of zlib on these operating
225 systems, please let us know. Thanks.
226
227 30. Is there some simpler, easier to read version of inflate I can look at to
228 understand the deflate format?
229
230 First off, you should read RFC 1951. Second, yes. Look in zlib's
231 contrib/puff directory.
232
233 31. Does zlib infringe on any patents?
234
235 As far as we know, no. In fact, that was originally the whole point behind
236 zlib. Look here for some more information:
237
238 http://www.gzip.org/#faq11
239
240 32. Can zlib work with greater than 4 GB of data?
241
242 Yes. inflate() and deflate() will process any amount of data correctly.
243 Each call of inflate() or deflate() is limited to input and output chunks
244 of the maximum value that can be stored in the compiler's "unsigned int"
245 type, but there is no limit to the number of chunks. Note however that the
246 strm.total_in and strm_total_out counters may be limited to 4 GB. These
247 counters are provided as a convenience and are not used internally by
248 inflate() or deflate(). The application can easily set up its own counters
249 updated after each call of inflate() or deflate() to count beyond 4 GB.
250 compress() and uncompress() may be limited to 4 GB, since they operate in a
251 single call. gzseek() and gztell() may be limited to 4 GB depending on how
252 zlib is compiled. See the zlibCompileFlags() function in zlib.h.
253
254 The word "may" appears several times above since there is a 4 GB limit only
255 if the compiler's "long" type is 32 bits. If the compiler's "long" type is
256 64 bits, then the limit is 16 exabytes.
257
258 33. Does zlib have any security vulnerabilities?
259
260 The only one that we are aware of is potentially in gzprintf(). If zlib is
261 compiled to use sprintf() or vsprintf(), then there is no protection
262 against a buffer overflow of an 8K string space (or other value as set by
263 gzbuffer()), other than the caller of gzprintf() assuring that the output
264 will not exceed 8K. On the other hand, if zlib is compiled to use
265 snprintf() or vsnprintf(), which should normally be the case, then there is
266 no vulnerability. The ./configure script will display warnings if an
267 insecure variation of sprintf() will be used by gzprintf(). Also the
268 zlibCompileFlags() function will return information on what variant of
269 sprintf() is used by gzprintf().
270
271 If you don't have snprintf() or vsnprintf() and would like one, you can
272 find a portable implementation here:
273
274 http://www.ijs.si/software/snprintf/
275
276 Note that you should be using the most recent version of zlib. Versions
277 1.1.3 and before were subject to a double-free vulnerability, and versions
278 1.2.1 and 1.2.2 were subject to an access exception when decompressing
279 invalid compressed data.
280
281 34. Is there a Java version of zlib?
282
283 Probably what you want is to use zlib in Java. zlib is already included
284 as part of the Java SDK in the java.util.zip package. If you really want
285 a version of zlib written in the Java language, look on the zlib home
286 page for links: http://zlib.net/ .
287
288 35. I get this or that compiler or source-code scanner warning when I crank it
289 up to maximally-pedantic. Can't you guys write proper code?
290
291 Many years ago, we gave up attempting to avoid warnings on every compiler
292 in the universe. It just got to be a waste of time, and some compilers
293 were downright silly as well as contradicted each other. So now, we simply
294 make sure that the code always works.
295
296 36. Valgrind (or some similar memory access checker) says that deflate is
297 performing a conditional jump that depends on an uninitialized value.
298 Isn't that a bug?
299
300 No. That is intentional for performance reasons, and the output of deflate
301 is not affected. This only started showing up recently since zlib 1.2.x
302 uses malloc() by default for allocations, whereas earlier versions used
303 calloc(), which zeros out the allocated memory. Even though the code was
304 correct, versions 1.2.4 and later was changed to not stimulate these
305 checkers.
306
307 37. Will zlib read the (insert any ancient or arcane format here) compressed
308 data format?
309
310 Probably not. Look in the comp.compression FAQ for pointers to various
311 formats and associated software.
312
313 38. How can I encrypt/decrypt zip files with zlib?
314
315 zlib doesn't support encryption. The original PKZIP encryption is very
316 weak and can be broken with freely available programs. To get strong
317 encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib
318 compression. For PKZIP compatible "encryption", look at
319 http://www.info-zip.org/
320
321 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
322
323 "gzip" is the gzip format, and "deflate" is the zlib format. They should
324 probably have called the second one "zlib" instead to avoid confusion with
325 the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
326 correctly points to the zlib specification in RFC 1950 for the "deflate"
327 transfer encoding, there have been reports of servers and browsers that
328 incorrectly produce or expect raw deflate data per the deflate
329 specification in RFC 1951, most notably Microsoft. So even though the
330 "deflate" transfer encoding using the zlib format would be the more
331 efficient approach (and in fact exactly what the zlib format was designed
332 for), using the "gzip" transfer encoding is probably more reliable due to
333 an unfortunate choice of name on the part of the HTTP 1.1 authors.
334
335 Bottom line: use the gzip format for HTTP 1.1 encoding.
336
337 40. Does zlib support the new "Deflate64" format introduced by PKWare?
338
339 No. PKWare has apparently decided to keep that format proprietary, since
340 they have not documented it as they have previous compression formats. In
341 any case, the compression improvements are so modest compared to other more
342 modern approaches, that it's not worth the effort to implement.
343
344 41. I'm having a problem with the zip functions in zlib, can you help?
345
346 There are no zip functions in zlib. You are probably using minizip by
347 Giles Vollant, which is found in the contrib directory of zlib. It is not
348 part of zlib. In fact none of the stuff in contrib is part of zlib. The
349 files in there are not supported by the zlib authors. You need to contact
350 the authors of the respective contribution for help.
351
352 42. The match.asm code in contrib is under the GNU General Public License.
353 Since it's part of zlib, doesn't that mean that all of zlib falls under the
354 GNU GPL?
355
356 No. The files in contrib are not part of zlib. They were contributed by
357 other authors and are provided as a convenience to the user within the zlib
358 distribution. Each item in contrib has its own license.
359
360 43. Is zlib subject to export controls? What is its ECCN?
361
362 zlib is not subject to export controls, and so is classified as EAR99.
363
364 44. Can you please sign these lengthy legal documents and fax them back to us
365 so that we can use your software in our product?
366
367 No. Go away. Shoo.