author Sam Lantinga <slouken@libsdl.org>
Fri, 26 Oct 2018 14:58:01 -0700
changeset 617 d64228a395fc
parent 521 9ed2f7d4a251
permissions -rw-r--r--
Fixed webp library detection when cross-compiling for Windows
     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.