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  1
  2                Frequently Asked Questions about zlib
  3
  4
  5If your question is not there, please check the zlib home page
  6http://www.zlib.org which may have more recent information.
  7The lastest zlib FAQ is at http://www.gzip.org/zlib/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.
 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
 19    http://www.zlib.org.
 20
 21 3. Where can I get a Visual Basic interface to zlib?
 22
 23    See
 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
 27
 28 4. compress() returns Z_BUF_ERROR.
 29
 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").
 34
 35 5. deflate() or inflate() returns Z_BUF_ERROR.
 36
 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.
 45
 46 6. Where's the zlib documentation (man pages, etc.)?
 47
 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 (zlib@gzip.org). Examples of zlib usage are in the files
 51    example.c and minigzip.c.
 52
 53 7. Why don't you use GNU autoconf or libtool or ...?
 54
 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.
 57
 58 8. I found a bug in zlib.
 59
 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.
 64
 65 9. Why do I get "undefined reference to gzputc"?
 66
 67    If "make test" produces something like
 68
 69       example.o(.text+0x154): undefined reference to `gzputc'
 70
 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".
 73
 7410. I need a Delphi interface to zlib.
 75
 76    See the contrib/delphi directory in the zlib distribution.
 77
 7811. Can zlib handle .zip archives?
 79
 80    Not by itself, no.  See the directory contrib/minizip in the zlib
 81    distribution.
 82
 8312. Can zlib handle .Z files?
 84
 85    No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt
 86    the code of uncompress on your own.
 87
 8813. How can I make a Unix shared library?
 89
 90    make clean
 91    ./configure -s
 92    make
 93
 9414. How do I install a shared zlib library on Unix?
 95
 96    After the above, then:
 97
 98    make install
 99
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.
104
10515. 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
11016. Can zlib decode Flate data in an Adobe PDF file?
111
112    Yes. See http://www.fastio.com/ (ClibPDF), or http://www.pdflib.com/ .
113    To modify PDF forms, see http://sourceforge.net/projects/acroformtool/ .
114
11517. 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
12918. 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
134    formats use the same compressed data format internally, but have different
135    headers and trailers around the compressed data.
136
13719. Ok, so why are there two different formats?
138
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.
144
14520. 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
149    the gzip format using inflateInit2(). Read zlib.h for more details.
150
15121. 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 allow
157    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
16222. Can I use zlib in my commercial application?
163
164    Yes. Please read the license in zlib.h.
165
16623. Is zlib under the GNU license?
167
168    No. Please read the license in zlib.h.
169
17024. 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
19325. 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
19826. Will zlib work on a 64-bit machine?
199
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 zlib@gzip.org
203
20427. Will zlib decompress data from the PKWare Data Compression Library?
205
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.
209
21028. Can I access data randomly in a compressed stream?
211
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.
217
21829. Does zlib work on MVS, OS/390, CICS, etc.?
219
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.
224
22530. Is there some simpler, easier to read version of inflate I can look at
226    to understand the deflate format?
227
228    First off, you should read RFC 1951. Second, yes. Look in zlib's
229    contrib/puff directory.
230
23131. 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
23832. 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
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.
255
25633. Does zlib have any security vulnerabilities?
257
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().
267
268    If you don't have snprintf() or vsnprintf() and would like one, you can
269    find a portable implementation here:
270
271        http://www.ijs.si/software/snprintf/
272
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.
275
27634. Is there a Java version of zlib?
277
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/
282
28335. 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?
285
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
289    works.
290
29136. 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?
294
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.
299
30037. Will zlib read the (insert any ancient or arcane format here) compressed
301    data format?
302
303    Probably not. Look in the comp.compression FAQ for pointers to various
304    formats and associated software.
305
30638. How can I encrypt/decrypt zip files with zlib?
307
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/
312
31339. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
314
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.
326
327    Bottom line: use the gzip format for HTTP 1.1 encoding.
328
32940. Does zlib support the new "Deflate64" format introduced by PKWare?
330
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.
335
33641. 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?
338
339    No. Go away. Shoo.