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    1 
    2                 Frequently Asked Questions about zlib
    3 
    4 
    5 If your question is not there, please check the zlib home page
    6 http://www.zlib.org which may have more recent information.
    7 The 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 
   74 10. I need a Delphi interface to zlib.
   75 
   76     See the contrib/delphi directory in the zlib distribution.
   77 
   78 11. Can zlib handle .zip archives?
   79 
   80     Not by itself, no.  See the directory contrib/minizip in the zlib
   81     distribution.
   82 
   83 12. 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 
   88 13. How can I make a Unix shared library?
   89 
   90     make clean
   91     ./configure -s
   92     make
   93 
   94 14. 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 
  105 15. 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 
  110 16. 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 
  115 17. 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 
  129 18. 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 
  137 19. 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 
  145 20. 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 
  151 21. 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 
  162 22. Can I use zlib in my commercial application?
  163 
  164     Yes. Please read the license in zlib.h.
  165 
  166 23. Is zlib under the GNU license?
  167 
  168     No. Please read the license in zlib.h.
  169 
  170 24. 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 
  193 25. 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 
  198 26. 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 
  204 27. 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 
  210 28. 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 
  218 29. 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 
  225 30. 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 
  231 31. 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 
  238 32. 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 
  256 33. 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 
  276 34. 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 
  283 35. 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 
  291 36. 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 
  300 37. 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 
  306 38. 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 
  313 39. 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 
  329 40. 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 
  336 41. 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.