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  2.                 Frequently Asked Questions about zlib
  5. If your question is not there, please check the zlib home page
  6. which may have more recent information.
  7. The 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".
  71. 10. I need a Delphi interface to zlib.
  73.     See the contrib/delphi directory in the zlib distribution.
  75. 11. Can zlib handle .zip archives?
  77.     Not by itself, no.  See the directory contrib/minizip in the zlib
  78.     distribution.
  80. 12. 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.
  85. 13. 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
  93. 14. 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 .
  106. 15. 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,
  111. 16. Can zlib decode Flate data in an Adobe PDF file?
  113.     Yes. See . To modify PDF forms, see
  114. .
  116. 17. 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.
  130. 18. 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.
  138. 19. 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.
  146. 20. 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.
  152. 21. 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.
  163. 22. Can I use zlib in my commercial application?
  165.     Yes.  Please read the license in zlib.h.
  167. 23. Is zlib under the GNU license?
  169.     No.  Please read the license in zlib.h.
  171. 24. 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.
  194. 25. Will zlib work on a big-endian or little-endian architecture, and can I
  195.     exchange compressed data between them?
  197.     Yes and yes.
  199. 26. 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
  205. 27. 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.
  211. 28. 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 .
  221. 29. 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.
  228. 30. 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.
  234. 31. 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:
  241. 32. 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.
  259. 33. 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.
  282. 34. 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: .
  289. 35. 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.
  297. 36. 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.
  308. 37. 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.
  314. 38. 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
  322. 39. 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.
  338. 40. 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.
  345. 41. 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.
  353. 42. 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.
  361. 43. Is zlib subject to export controls?  What is its ECCN?
  363.     zlib is not subject to export controls, and so is classified as EAR99.
  365. 44. 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.