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    2                 Frequently Asked Questions about zlib
    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 lastest zlib FAQ is at http://zlib.net/zlib_faq.html
   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 http://zlib.net/ .
   20  3. Where can I get a Visual Basic interface to zlib?
   22     See
   23         * http://marknelson.us/1997/01/01/zlib-engine/
   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 http://zlib.net/zlib_how.html 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 zlib@gzip.org .  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, jhainley@myndkryme.com.
  111 16. Can zlib decode Flate data in an Adobe PDF file?
  113     Yes. See http://www.pdflib.com/ . To modify PDF forms, see
  114     http://sourceforge.net/projects/acroformtool/ .
  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         ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
  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     http://www.sunfreeware.com 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 zlib@gzip.org
  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:
  239     http://www.gzip.org/#faq11
  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:
  275         http://www.ijs.si/software/snprintf/
  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 java.util.zip package. If you really want
  286     a version of zlib written in the Java language, look on the zlib home
  287     page for links: http://zlib.net/ .
  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, http://www.gnupg.org/ , which already includes zlib
  319     compression.  For PKZIP compatible "encryption", look at
  320     http://www.info-zip.org/
  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.