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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 example.c and
   48     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     make clean
   88     ./configure -s
   89     make
   91 14. How do I install a shared zlib library on Unix?
   93     After the above, then:
   95     make install
   97     However, many flavors of Unix come with a shared zlib already installed.
   98     Before going to the trouble of compiling a shared version of zlib and
   99     trying to install it, you may want to check if it's already there!  If you
  100     can #include <zlib.h>, it's there.  The -lz option will probably link to
  101     it.  You can check the version at the top of zlib.h or with the
  102     ZLIB_VERSION symbol defined in zlib.h .
  104 15. I have a question about OttoPDF.
  106     We are not the authors of OttoPDF. The real author is on the OttoPDF web
  107     site: Joel Hainley, jhainley@myndkryme.com.
  109 16. Can zlib decode Flate data in an Adobe PDF file?
  111     Yes. See http://www.pdflib.com/ . To modify PDF forms, see
  112     http://sourceforge.net/projects/acroformtool/ .
  114 17. Why am I getting this "register_frame_info not found" error on Solaris?
  116     After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
  117     generates an error such as:
  119         ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
  120         symbol __register_frame_info: referenced symbol not found
  122     The symbol __register_frame_info is not part of zlib, it is generated by
  123     the C compiler (cc or gcc).  You must recompile applications using zlib
  124     which have this problem.  This problem is specific to Solaris.  See
  125     http://www.sunfreeware.com for Solaris versions of zlib and applications
  126     using zlib.
  128 18. Why does gzip give an error on a file I make with compress/deflate?
  130     The compress and deflate functions produce data in the zlib format, which
  131     is different and incompatible with the gzip format.  The gz* functions in
  132     zlib on the other hand use the gzip format.  Both the zlib and gzip formats
  133     use the same compressed data format internally, but have different headers
  134     and trailers around the compressed data.
  136 19. Ok, so why are there two different formats?
  138     The gzip format was designed to retain the directory information about a
  139     single file, such as the name and last modification date.  The zlib format
  140     on the other hand was designed for in-memory and communication channel
  141     applications, and has a much more compact header and trailer and uses a
  142     faster integrity check than gzip.
  144 20. Well that's nice, but how do I make a gzip file in memory?
  146     You can request that deflate write the gzip format instead of the zlib
  147     format using deflateInit2().  You can also request that inflate decode the
  148     gzip format using inflateInit2().  Read zlib.h for more details.
  150 21. Is zlib thread-safe?
  152     Yes.  However any library routines that zlib uses and any application-
  153     provided memory allocation routines must also be thread-safe.  zlib's gz*
  154     functions use stdio library routines, and most of zlib's functions use the
  155     library memory allocation routines by default.  zlib's *Init* functions
  156     allow for the application to provide custom memory allocation routines.
  158     Of course, you should only operate on any given zlib or gzip stream from a
  159     single thread at a time.
  161 22. Can I use zlib in my commercial application?
  163     Yes.  Please read the license in zlib.h.
  165 23. Is zlib under the GNU license?
  167     No.  Please read the license in zlib.h.
  169 24. The license says that altered source versions must be "plainly marked". So
  170     what exactly do I need to do to meet that requirement?
  172     You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h.  In
  173     particular, the final version number needs to be changed to "f", and an
  174     identification string should be appended to ZLIB_VERSION.  Version numbers
  175     x.x.x.f are reserved for modifications to zlib by others than the zlib
  176     maintainers.  For example, if the version of the base zlib you are altering
  177     is "", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
  178     ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3".  You can also
  179     update the version strings in deflate.c and inftrees.c.
  181     For altered source distributions, you should also note the origin and
  182     nature of the changes in zlib.h, as well as in ChangeLog and README, along
  183     with the dates of the alterations.  The origin should include at least your
  184     name (or your company's name), and an email address to contact for help or
  185     issues with the library.
  187     Note that distributing a compiled zlib library along with zlib.h and
  188     zconf.h is also a source distribution, and so you should change
  189     ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes
  190     in zlib.h as you would for a full source distribution.
  192 25. Will zlib work on a big-endian or little-endian architecture, and can I
  193     exchange compressed data between them?
  195     Yes and yes.
  197 26. Will zlib work on a 64-bit machine?
  199     Yes.  It has been tested on 64-bit machines, and has no dependence on any
  200     data types being limited to 32-bits in length.  If you have any
  201     difficulties, please provide a complete problem report to zlib@gzip.org
  203 27. Will zlib decompress data from the PKWare Data Compression Library?
  205     No.  The PKWare DCL uses a completely different compressed data format than
  206     does PKZIP and zlib.  However, you can look in zlib's contrib/blast
  207     directory for a possible solution to your problem.
  209 28. Can I access data randomly in a compressed stream?
  211     No, not without some preparation.  If when compressing you periodically use
  212     Z_FULL_FLUSH, carefully write all the pending data at those points, and
  213     keep an index of those locations, then you can start decompression at those
  214     points.  You have to be careful to not use Z_FULL_FLUSH too often, since it
  215     can significantly degrade compression.  Alternatively, you can scan a
  216     deflate stream once to generate an index, and then use that index for
  217     random access.  See examples/zran.c .
  219 29. Does zlib work on MVS, OS/390, CICS, etc.?
  221     It has in the past, but we have not heard of any recent evidence.  There
  222     were working ports of zlib 1.1.4 to MVS, but those links no longer work.
  223     If you know of recent, successful applications of zlib on these operating
  224     systems, please let us know.  Thanks.
  226 30. Is there some simpler, easier to read version of inflate I can look at to
  227     understand the deflate format?
  229     First off, you should read RFC 1951.  Second, yes.  Look in zlib's
  230     contrib/puff directory.
  232 31. Does zlib infringe on any patents?
  234     As far as we know, no.  In fact, that was originally the whole point behind
  235     zlib.  Look here for some more information:
  237     http://www.gzip.org/#faq11
  239 32. Can zlib work with greater than 4 GB of data?
  241     Yes.  inflate() and deflate() will process any amount of data correctly.
  242     Each call of inflate() or deflate() is limited to input and output chunks
  243     of the maximum value that can be stored in the compiler's "unsigned int"
  244     type, but there is no limit to the number of chunks.  Note however that the
  245     strm.total_in and strm_total_out counters may be limited to 4 GB.  These
  246     counters are provided as a convenience and are not used internally by
  247     inflate() or deflate().  The application can easily set up its own counters
  248     updated after each call of inflate() or deflate() to count beyond 4 GB.
  249     compress() and uncompress() may be limited to 4 GB, since they operate in a
  250     single call.  gzseek() and gztell() may be limited to 4 GB depending on how
  251     zlib is compiled.  See the zlibCompileFlags() function in zlib.h.
  253     The word "may" appears several times above since there is a 4 GB limit only
  254     if the compiler's "long" type is 32 bits.  If the compiler's "long" type is
  255     64 bits, then the limit is 16 exabytes.
  257 33. Does zlib have any security vulnerabilities?
  259     The only one that we are aware of is potentially in gzprintf().  If zlib is
  260     compiled to use sprintf() or vsprintf(), then there is no protection
  261     against a buffer overflow of an 8K string space (or other value as set by
  262     gzbuffer()), other than the caller of gzprintf() assuring that the output
  263     will not exceed 8K.  On the other hand, if zlib is compiled to use
  264     snprintf() or vsnprintf(), which should normally be the case, then there is
  265     no vulnerability.  The ./configure script will display warnings if an
  266     insecure variation of sprintf() will be used by gzprintf().  Also the
  267     zlibCompileFlags() function will return information on what variant of
  268     sprintf() is used by gzprintf().
  270     If you don't have snprintf() or vsnprintf() and would like one, you can
  271     find a portable implementation here:
  273         http://www.ijs.si/software/snprintf/
  275     Note that you should be using the most recent version of zlib.  Versions
  276     1.1.3 and before were subject to a double-free vulnerability, and versions
  277     1.2.1 and 1.2.2 were subject to an access exception when decompressing
  278     invalid compressed data.
  280 34. Is there a Java version of zlib?
  282     Probably what you want is to use zlib in Java. zlib is already included
  283     as part of the Java SDK in the java.util.zip package. If you really want
  284     a version of zlib written in the Java language, look on the zlib home
  285     page for links: http://zlib.net/ .
  287 35. I get this or that compiler or source-code scanner warning when I crank it
  288     up to maximally-pedantic. Can't you guys write proper code?
  290     Many years ago, we gave up attempting to avoid warnings on every compiler
  291     in the universe.  It just got to be a waste of time, and some compilers
  292     were downright silly as well as contradicted each other.  So now, we simply
  293     make sure that the code always works.
  295 36. Valgrind (or some similar memory access checker) says that deflate is
  296     performing a conditional jump that depends on an uninitialized value.
  297     Isn't that a bug?
  299     No.  That is intentional for performance reasons, and the output of deflate
  300     is not affected.  This only started showing up recently since zlib 1.2.x
  301     uses malloc() by default for allocations, whereas earlier versions used
  302     calloc(), which zeros out the allocated memory.  Even though the code was
  303     correct, versions 1.2.4 and later was changed to not stimulate these
  304     checkers.
  306 37. Will zlib read the (insert any ancient or arcane format here) compressed
  307     data format?
  309     Probably not. Look in the comp.compression FAQ for pointers to various
  310     formats and associated software.
  312 38. How can I encrypt/decrypt zip files with zlib?
  314     zlib doesn't support encryption.  The original PKZIP encryption is very
  315     weak and can be broken with freely available programs.  To get strong
  316     encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib
  317     compression.  For PKZIP compatible "encryption", look at
  318     http://www.info-zip.org/
  320 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
  322     "gzip" is the gzip format, and "deflate" is the zlib format.  They should
  323     probably have called the second one "zlib" instead to avoid confusion with
  324     the raw deflate compressed data format.  While the HTTP 1.1 RFC 2616
  325     correctly points to the zlib specification in RFC 1950 for the "deflate"
  326     transfer encoding, there have been reports of servers and browsers that
  327     incorrectly produce or expect raw deflate data per the deflate
  328     specficiation in RFC 1951, most notably Microsoft.  So even though the
  329     "deflate" transfer encoding using the zlib format would be the more
  330     efficient approach (and in fact exactly what the zlib format was designed
  331     for), using the "gzip" transfer encoding is probably more reliable due to
  332     an unfortunate choice of name on the part of the HTTP 1.1 authors.
  334     Bottom line: use the gzip format for HTTP 1.1 encoding.
  336 40. Does zlib support the new "Deflate64" format introduced by PKWare?
  338     No.  PKWare has apparently decided to keep that format proprietary, since
  339     they have not documented it as they have previous compression formats.  In
  340     any case, the compression improvements are so modest compared to other more
  341     modern approaches, that it's not worth the effort to implement.
  343 41. I'm having a problem with the zip functions in zlib, can you help?
  345     There are no zip functions in zlib.  You are probably using minizip by
  346     Giles Vollant, which is found in the contrib directory of zlib.  It is not
  347     part of zlib.  In fact none of the stuff in contrib is part of zlib.  The
  348     files in there are not supported by the zlib authors.  You need to contact
  349     the authors of the respective contribution for help.
  351 42. The match.asm code in contrib is under the GNU General Public License.
  352     Since it's part of zlib, doesn't that mean that all of zlib falls under the
  353     GNU GPL?
  355     No.  The files in contrib are not part of zlib.  They were contributed by
  356     other authors and are provided as a convenience to the user within the zlib
  357     distribution.  Each item in contrib has its own license.
  359 43. Is zlib subject to export controls?  What is its ECCN?
  361     zlib is not subject to export controls, and so is classified as EAR99.
  363 44. Can you please sign these lengthy legal documents and fax them back to us
  364     so that we can use your software in our product?
  366     No. Go away. Shoo.