Home | History | Annotate | Download | only in src
      1 
      2                 Frequently Asked Questions about zlib
      3 
      4 
      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
      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.  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/ .
     19 
     20  3. Where can I get a Visual Basic interface to zlib?
     21 
     22     See
     23         * http://marknelson.us/1997/01/01/zlib-engine/
     24         * win32/DLL_FAQ.txt in the zlib distribution
     25 
     26  4. compress() returns Z_BUF_ERROR.
     27 
     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").
     32 
     33  5. deflate() or inflate() returns Z_BUF_ERROR.
     34 
     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.
     44 
     45  6. Where's the zlib documentation (man pages, etc.)?
     46 
     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/ .
     49 
     50  7. Why don't you use GNU autoconf or libtool or ...?
     51 
     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.
     54 
     55  8. I found a bug in zlib.
     56 
     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 (a] gzip.org .  Do not send multi-megabyte
     60     data files without prior agreement.
     61 
     62  9. Why do I get "undefined reference to gzputc"?
     63 
     64     If "make test" produces something like
     65 
     66        example.o(.text+0x154): undefined reference to `gzputc'
     67 
     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".
     70 
     71 10. I need a Delphi interface to zlib.
     72 
     73     See the contrib/delphi directory in the zlib distribution.
     74 
     75 11. Can zlib handle .zip archives?
     76 
     77     Not by itself, no.  See the directory contrib/minizip in the zlib
     78     distribution.
     79 
     80 12. Can zlib handle .Z files?
     81 
     82     No, sorry.  You have to spawn an uncompress or gunzip subprocess, or adapt
     83     the code of uncompress on your own.
     84 
     85 13. How can I make a Unix shared library?
     86 
     87     By default a shared (and a static) library is built for Unix.  So:
     88 
     89     make distclean
     90     ./configure
     91     make
     92 
     93 14. How do I install a shared zlib library on Unix?
     94 
     95     After the above, then:
     96 
     97     make install
     98 
     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 .
    105 
    106 15. I have a question about OttoPDF.
    107 
    108     We are not the authors of OttoPDF. The real author is on the OttoPDF web
    109     site: Joel Hainley, jhainley (a] myndkryme.com.
    110 
    111 16. Can zlib decode Flate data in an Adobe PDF file?
    112 
    113     Yes. See http://www.pdflib.com/ . To modify PDF forms, see
    114     http://sourceforge.net/projects/acroformtool/ .
    115 
    116 17. Why am I getting this "register_frame_info not found" error on Solaris?
    117 
    118     After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
    119     generates an error such as:
    120 
    121         ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
    122         symbol __register_frame_info: referenced symbol not found
    123 
    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.
    129 
    130 18. Why does gzip give an error on a file I make with compress/deflate?
    131 
    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.
    137 
    138 19. Ok, so why are there two different formats?
    139 
    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.
    145 
    146 20. Well that's nice, but how do I make a gzip file in memory?
    147 
    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.
    151 
    152 21. Is zlib thread-safe?
    153 
    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.
    159 
    160     Of course, you should only operate on any given zlib or gzip stream from a
    161     single thread at a time.
    162 
    163 22. Can I use zlib in my commercial application?
    164 
    165     Yes.  Please read the license in zlib.h.
    166 
    167 23. Is zlib under the GNU license?
    168 
    169     No.  Please read the license in zlib.h.
    170 
    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?
    173 
    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 "1.2.3.4", 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.
    182 
    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.
    188 
    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.
    193 
    194 25. Will zlib work on a big-endian or little-endian architecture, and can I
    195     exchange compressed data between them?
    196 
    197     Yes and yes.
    198 
    199 26. Will zlib work on a 64-bit machine?
    200 
    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 (a] gzip.org
    204 
    205 27. Will zlib decompress data from the PKWare Data Compression Library?
    206 
    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.
    210 
    211 28. Can I access data randomly in a compressed stream?
    212 
    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 .
    220 
    221 29. Does zlib work on MVS, OS/390, CICS, etc.?
    222 
    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.
    227 
    228 30. Is there some simpler, easier to read version of inflate I can look at to
    229     understand the deflate format?
    230 
    231     First off, you should read RFC 1951.  Second, yes.  Look in zlib's
    232     contrib/puff directory.
    233 
    234 31. Does zlib infringe on any patents?
    235 
    236     As far as we know, no.  In fact, that was originally the whole point behind
    237     zlib.  Look here for some more information:
    238 
    239     http://www.gzip.org/#faq11
    240 
    241 32. Can zlib work with greater than 4 GB of data?
    242 
    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.
    254 
    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.
    258 
    259 33. Does zlib have any security vulnerabilities?
    260 
    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().
    271 
    272     If you don't have snprintf() or vsnprintf() and would like one, you can
    273     find a portable implementation here:
    274 
    275         http://www.ijs.si/software/snprintf/
    276 
    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.
    281 
    282 34. Is there a Java version of zlib?
    283 
    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/ .
    288 
    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?
    291 
    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.
    296 
    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?
    300 
    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.
    307 
    308 37. Will zlib read the (insert any ancient or arcane format here) compressed
    309     data format?
    310 
    311     Probably not. Look in the comp.compression FAQ for pointers to various
    312     formats and associated software.
    313 
    314 38. How can I encrypt/decrypt zip files with zlib?
    315 
    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/
    321 
    322 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
    323 
    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.
    335 
    336     Bottom line: use the gzip format for HTTP 1.1 encoding.
    337 
    338 40. Does zlib support the new "Deflate64" format introduced by PKWare?
    339 
    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.
    344 
    345 41. I'm having a problem with the zip functions in zlib, can you help?
    346 
    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.
    352 
    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?
    356 
    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.
    360 
    361 43. Is zlib subject to export controls?  What is its ECCN?
    362 
    363     zlib is not subject to export controls, and so is classified as EAR99.
    364 
    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?
    367 
    368     No. Go away. Shoo.
    369