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  2                Frequently Asked Questions about zlib
  5If your question is not there, please check the zlib home page
  6 which may have more recent information.
  7The lastest zlib FAQ is at
 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.  See the
 17    file win32/DLL_FAQ.txt in the zlib distribution.  Pointers to the
 18    precompiled DLL are found in the zlib web site at .
 20 3. Where can I get a Visual Basic interface to zlib?
 22    See
 23        *
 24        * win32/DLL_FAQ.txt in the zlib distribution
 26 4. compress() returns Z_BUF_ERROR.
 28    Make sure that before the call of compress(), the length of the compressed
 29    buffer is equal to the available size of the compressed buffer and not
 30    zero.  For Visual Basic, check that this parameter is passed by reference
 31    ("as any"), not by value ("as long").
 33 5. deflate() or inflate() returns Z_BUF_ERROR.
 35    Before making the call, make sure that avail_in and avail_out are not zero.
 36    When setting the parameter flush equal to Z_FINISH, also make sure that
 37    avail_out is big enough to allow processing all pending input.  Note that a
 38    Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be
 39    made with more input or output space.  A Z_BUF_ERROR may in fact be
 40    unavoidable depending on how the functions are used, since it is not
 41    possible to tell whether or not there is more output pending when
 42    strm.avail_out returns with zero.  See for a
 43    heavily annotated example.
 45 6. Where's the zlib documentation (man pages, etc.)?
 47    It's in zlib.h .  Examples of zlib usage are in the files test/example.c
 48    and test/minigzip.c, with more in examples/ .
 50 7. Why don't you use GNU autoconf or libtool or ...?
 52    Because we would like to keep zlib as a very small and simple package.
 53    zlib is rather portable and doesn't need much configuration.
 55 8. I found a bug in zlib.
 57    Most of the time, such problems are due to an incorrect usage of zlib.
 58    Please try to reproduce the problem with a small program and send the
 59    corresponding source to us at .  Do not send multi-megabyte
 60    data files without prior agreement.
 62 9. Why do I get "undefined reference to gzputc"?
 64    If "make test" produces something like
 66       example.o(.text+0x154): undefined reference to `gzputc'
 68    check that you don't have old files libz.* in /usr/lib, /usr/local/lib or
 69    /usr/X11R6/lib. Remove any old versions, then do "make install".
 7110. I need a Delphi interface to zlib.
 73    See the contrib/delphi directory in the zlib distribution.
 7511. Can zlib handle .zip archives?
 77    Not by itself, no.  See the directory contrib/minizip in the zlib
 78    distribution.
 8012. Can zlib handle .Z files?
 82    No, sorry.  You have to spawn an uncompress or gunzip subprocess, or adapt
 83    the code of uncompress on your own.
 8513. How can I make a Unix shared library?
 87    By default a shared (and a static) library is built for Unix.  So:
 89    make distclean
 90    ./configure
 91    make
 9314. How do I install a shared zlib library on Unix?
 95    After the above, then:
 97    make install
 99    However, many flavors of Unix come with a shared zlib already installed.
100    Before going to the trouble of compiling a shared version of zlib and
101    trying to install it, you may want to check if it's already there!  If you
102    can #include <zlib.h>, it's there.  The -lz option will probably link to
103    it.  You can check the version at the top of zlib.h or with the
104    ZLIB_VERSION symbol defined in zlib.h .
10615. I have a question about OttoPDF.
108    We are not the authors of OttoPDF. The real author is on the OttoPDF web
109    site: Joel Hainley,
11116. Can zlib decode Flate data in an Adobe PDF file?
113    Yes. See . To modify PDF forms, see
114 .
11617. Why am I getting this "register_frame_info not found" error on Solaris?
118    After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
119    generates an error such as:
121 rpm: fatal: relocation error: file /usr/local/lib/
122        symbol __register_frame_info: referenced symbol not found
124    The symbol __register_frame_info is not part of zlib, it is generated by
125    the C compiler (cc or gcc).  You must recompile applications using zlib
126    which have this problem.  This problem is specific to Solaris.  See
127 for Solaris versions of zlib and applications
128    using zlib.
13018. Why does gzip give an error on a file I make with compress/deflate?
132    The compress and deflate functions produce data in the zlib format, which
133    is different and incompatible with the gzip format.  The gz* functions in
134    zlib on the other hand use the gzip format.  Both the zlib and gzip formats
135    use the same compressed data format internally, but have different headers
136    and trailers around the compressed data.
13819. Ok, so why are there two different formats?
140    The gzip format was designed to retain the directory information about a
141    single file, such as the name and last modification date.  The zlib format
142    on the other hand was designed for in-memory and communication channel
143    applications, and has a much more compact header and trailer and uses a
144    faster integrity check than gzip.
14620. Well that's nice, but how do I make a gzip file in memory?
148    You can request that deflate write the gzip format instead of the zlib
149    format using deflateInit2().  You can also request that inflate decode the
150    gzip format using inflateInit2().  Read zlib.h for more details.
15221. Is zlib thread-safe?
154    Yes.  However any library routines that zlib uses and any application-
155    provided memory allocation routines must also be thread-safe.  zlib's gz*
156    functions use stdio library routines, and most of zlib's functions use the
157    library memory allocation routines by default.  zlib's *Init* functions
158    allow for the application to provide custom memory allocation routines.
160    Of course, you should only operate on any given zlib or gzip stream from a
161    single thread at a time.
16322. Can I use zlib in my commercial application?
165    Yes.  Please read the license in zlib.h.
16723. Is zlib under the GNU license?
169    No.  Please read the license in zlib.h.
17124. The license says that altered source versions must be "plainly marked". So
172    what exactly do I need to do to meet that requirement?
174    You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h.  In
175    particular, the final version number needs to be changed to "f", and an
176    identification string should be appended to ZLIB_VERSION.  Version numbers
177    x.x.x.f are reserved for modifications to zlib by others than the zlib
178    maintainers.  For example, if the version of the base zlib you are altering
179    is "", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
180    ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3".  You can also
181    update the version strings in deflate.c and inftrees.c.
183    For altered source distributions, you should also note the origin and
184    nature of the changes in zlib.h, as well as in ChangeLog and README, along
185    with the dates of the alterations.  The origin should include at least your
186    name (or your company's name), and an email address to contact for help or
187    issues with the library.
189    Note that distributing a compiled zlib library along with zlib.h and
190    zconf.h is also a source distribution, and so you should change
191    ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes
192    in zlib.h as you would for a full source distribution.
19425. Will zlib work on a big-endian or little-endian architecture, and can I
195    exchange compressed data between them?
197    Yes and yes.
19926. Will zlib work on a 64-bit machine?
201    Yes.  It has been tested on 64-bit machines, and has no dependence on any
202    data types being limited to 32-bits in length.  If you have any
203    difficulties, please provide a complete problem report to
20527. Will zlib decompress data from the PKWare Data Compression Library?
207    No.  The PKWare DCL uses a completely different compressed data format than
208    does PKZIP and zlib.  However, you can look in zlib's contrib/blast
209    directory for a possible solution to your problem.
21128. Can I access data randomly in a compressed stream?
213    No, not without some preparation.  If when compressing you periodically use
214    Z_FULL_FLUSH, carefully write all the pending data at those points, and
215    keep an index of those locations, then you can start decompression at those
216    points.  You have to be careful to not use Z_FULL_FLUSH too often, since it
217    can significantly degrade compression.  Alternatively, you can scan a
218    deflate stream once to generate an index, and then use that index for
219    random access.  See examples/zran.c .
22129. Does zlib work on MVS, OS/390, CICS, etc.?
223    It has in the past, but we have not heard of any recent evidence.  There
224    were working ports of zlib 1.1.4 to MVS, but those links no longer work.
225    If you know of recent, successful applications of zlib on these operating
226    systems, please let us know.  Thanks.
22830. Is there some simpler, easier to read version of inflate I can look at to
229    understand the deflate format?
231    First off, you should read RFC 1951.  Second, yes.  Look in zlib's
232    contrib/puff directory.
23431. Does zlib infringe on any patents?
236    As far as we know, no.  In fact, that was originally the whole point behind
237    zlib.  Look here for some more information:
24132. Can zlib work with greater than 4 GB of data?
243    Yes.  inflate() and deflate() will process any amount of data correctly.
244    Each call of inflate() or deflate() is limited to input and output chunks
245    of the maximum value that can be stored in the compiler's "unsigned int"
246    type, but there is no limit to the number of chunks.  Note however that the
247    strm.total_in and strm_total_out counters may be limited to 4 GB.  These
248    counters are provided as a convenience and are not used internally by
249    inflate() or deflate().  The application can easily set up its own counters
250    updated after each call of inflate() or deflate() to count beyond 4 GB.
251    compress() and uncompress() may be limited to 4 GB, since they operate in a
252    single call.  gzseek() and gztell() may be limited to 4 GB depending on how
253    zlib is compiled.  See the zlibCompileFlags() function in zlib.h.
255    The word "may" appears several times above since there is a 4 GB limit only
256    if the compiler's "long" type is 32 bits.  If the compiler's "long" type is
257    64 bits, then the limit is 16 exabytes.
25933. Does zlib have any security vulnerabilities?
261    The only one that we are aware of is potentially in gzprintf().  If zlib is
262    compiled to use sprintf() or vsprintf(), then there is no protection
263    against a buffer overflow of an 8K string space (or other value as set by
264    gzbuffer()), other than the caller of gzprintf() assuring that the output
265    will not exceed 8K.  On the other hand, if zlib is compiled to use
266    snprintf() or vsnprintf(), which should normally be the case, then there is
267    no vulnerability.  The ./configure script will display warnings if an
268    insecure variation of sprintf() will be used by gzprintf().  Also the
269    zlibCompileFlags() function will return information on what variant of
270    sprintf() is used by gzprintf().
272    If you don't have snprintf() or vsnprintf() and would like one, you can
273    find a portable implementation here:
277    Note that you should be using the most recent version of zlib.  Versions
278    1.1.3 and before were subject to a double-free vulnerability, and versions
279    1.2.1 and 1.2.2 were subject to an access exception when decompressing
280    invalid compressed data.
28234. Is there a Java version of zlib?
284    Probably what you want is to use zlib in Java. zlib is already included
285    as part of the Java SDK in the package. If you really want
286    a version of zlib written in the Java language, look on the zlib home
287    page for links: .
28935. I get this or that compiler or source-code scanner warning when I crank it
290    up to maximally-pedantic. Can't you guys write proper code?
292    Many years ago, we gave up attempting to avoid warnings on every compiler
293    in the universe.  It just got to be a waste of time, and some compilers
294    were downright silly as well as contradicted each other.  So now, we simply
295    make sure that the code always works.
29736. Valgrind (or some similar memory access checker) says that deflate is
298    performing a conditional jump that depends on an uninitialized value.
299    Isn't that a bug?
301    No.  That is intentional for performance reasons, and the output of deflate
302    is not affected.  This only started showing up recently since zlib 1.2.x
303    uses malloc() by default for allocations, whereas earlier versions used
304    calloc(), which zeros out the allocated memory.  Even though the code was
305    correct, versions 1.2.4 and later was changed to not stimulate these
306    checkers.
30837. Will zlib read the (insert any ancient or arcane format here) compressed
309    data format?
311    Probably not. Look in the comp.compression FAQ for pointers to various
312    formats and associated software.
31438. How can I encrypt/decrypt zip files with zlib?
316    zlib doesn't support encryption.  The original PKZIP encryption is very
317    weak and can be broken with freely available programs.  To get strong
318    encryption, use GnuPG, , which already includes zlib
319    compression.  For PKZIP compatible "encryption", look at
32239. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
324    "gzip" is the gzip format, and "deflate" is the zlib format.  They should
325    probably have called the second one "zlib" instead to avoid confusion with
326    the raw deflate compressed data format.  While the HTTP 1.1 RFC 2616
327    correctly points to the zlib specification in RFC 1950 for the "deflate"
328    transfer encoding, there have been reports of servers and browsers that
329    incorrectly produce or expect raw deflate data per the deflate
330    specification in RFC 1951, most notably Microsoft.  So even though the
331    "deflate" transfer encoding using the zlib format would be the more
332    efficient approach (and in fact exactly what the zlib format was designed
333    for), using the "gzip" transfer encoding is probably more reliable due to
334    an unfortunate choice of name on the part of the HTTP 1.1 authors.
336    Bottom line: use the gzip format for HTTP 1.1 encoding.
33840. Does zlib support the new "Deflate64" format introduced by PKWare?
340    No.  PKWare has apparently decided to keep that format proprietary, since
341    they have not documented it as they have previous compression formats.  In
342    any case, the compression improvements are so modest compared to other more
343    modern approaches, that it's not worth the effort to implement.
34541. I'm having a problem with the zip functions in zlib, can you help?
347    There are no zip functions in zlib.  You are probably using minizip by
348    Giles Vollant, which is found in the contrib directory of zlib.  It is not
349    part of zlib.  In fact none of the stuff in contrib is part of zlib.  The
350    files in there are not supported by the zlib authors.  You need to contact
351    the authors of the respective contribution for help.
35342. The match.asm code in contrib is under the GNU General Public License.
354    Since it's part of zlib, doesn't that mean that all of zlib falls under the
355    GNU GPL?
357    No.  The files in contrib are not part of zlib.  They were contributed by
358    other authors and are provided as a convenience to the user within the zlib
359    distribution.  Each item in contrib has its own license.
36143. Is zlib subject to export controls?  What is its ECCN?
363    zlib is not subject to export controls, and so is classified as EAR99.
36544. Can you please sign these lengthy legal documents and fax them back to us
366    so that we can use your software in our product?
368    No. Go away. Shoo.