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1 william 31 Frequently Asked Questions about zlib
2    
3    
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
7    
8    
9     1. Is zlib Y2K-compliant?
10    
11     Yes. zlib doesn't handle dates.
12    
13     2. Where can I get a Windows DLL version?
14    
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/ .
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 example.c and
47     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     make clean
87     ./configure -s
88     make
89    
90     14. How do I install a shared zlib library on Unix?
91    
92     After the above, then:
93    
94     make install
95    
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 .
102    
103     15. I have a question about OttoPDF.
104    
105     We are not the authors of OttoPDF. The real author is on the OttoPDF web
106     site: Joel Hainley, jhainley@myndkryme.com.
107    
108     16. Can zlib decode Flate data in an Adobe PDF file?
109    
110     Yes. See http://www.pdflib.com/ . To modify PDF forms, see
111     http://sourceforge.net/projects/acroformtool/ .
112    
113     17. Why am I getting this "register_frame_info not found" error on Solaris?
114    
115     After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
116     generates an error such as:
117    
118     ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
119     symbol __register_frame_info: referenced symbol not found
120    
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.
126    
127     18. Why does gzip give an error on a file I make with compress/deflate?
128    
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.
134    
135     19. Ok, so why are there two different formats?
136    
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.
142    
143     20. Well that's nice, but how do I make a gzip file in memory?
144    
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.
148    
149     21. Is zlib thread-safe?
150    
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.
156    
157     Of course, you should only operate on any given zlib or gzip stream from a
158     single thread at a time.
159    
160     22. Can I use zlib in my commercial application?
161    
162     Yes. Please read the license in zlib.h.
163    
164     23. Is zlib under the GNU license?
165    
166     No. Please read the license in zlib.h.
167    
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?
170    
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 "1.2.3.4", 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.
179    
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.
185    
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.
190    
191     25. Will zlib work on a big-endian or little-endian architecture, and can I
192     exchange compressed data between them?
193    
194     Yes and yes.
195    
196     26. Will zlib work on a 64-bit machine?
197    
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
201    
202     27. Will zlib decompress data from the PKWare Data Compression Library?
203    
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.
207    
208     28. Can I access data randomly in a compressed stream?
209    
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 .
217    
218     29. Does zlib work on MVS, OS/390, CICS, etc.?
219    
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.
224    
225     30. Is there some simpler, easier to read version of inflate I can look at to
226     understand the deflate format?
227    
228     First off, you should read RFC 1951. Second, yes. Look in zlib's
229     contrib/puff directory.
230    
231     31. Does zlib infringe on any patents?
232    
233     As far as we know, no. In fact, that was originally the whole point behind
234     zlib. Look here for some more information:
235    
236     http://www.gzip.org/#faq11
237    
238     32. Can zlib work with greater than 4 GB of data?
239    
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.
251    
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.
255    
256     33. Does zlib have any security vulnerabilities?
257    
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().
268    
269     If you don't have snprintf() or vsnprintf() and would like one, you can
270     find a portable implementation here:
271    
272     http://www.ijs.si/software/snprintf/
273    
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.
278    
279     34. Is there a Java version of zlib?
280    
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/ .
285    
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?
288    
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.
293    
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?
297    
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.
304    
305     37. Will zlib read the (insert any ancient or arcane format here) compressed
306     data format?
307    
308     Probably not. Look in the comp.compression FAQ for pointers to various
309     formats and associated software.
310    
311     38. How can I encrypt/decrypt zip files with zlib?
312    
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/
318    
319     39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
320    
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.
332    
333     Bottom line: use the gzip format for HTTP 1.1 encoding.
334    
335     40. Does zlib support the new "Deflate64" format introduced by PKWare?
336    
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.
341    
342     41. I'm having a problem with the zip functions in zlib, can you help?
343    
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.
349    
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?
353    
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.
357    
358     43. Is zlib subject to export controls? What is its ECCN?
359    
360     zlib is not subject to export controls, and so is classified as EAR99.
361    
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?
364    
365     No. Go away. Shoo.
366    

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