+
−
                Frequently Asked Questions about zlib+
−
+
−
+
−
If your question is not there, please check the zlib home page+
−
http://www.zlib.org which may have more recent information.+
−
The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html+
−
+
−
+
−
 1. Is zlib Y2K-compliant?+
−
+
−
    Yes. zlib doesn't handle dates.+
−
+
−
 2. Where can I get a Windows DLL version?+
−
+
−
    The zlib sources can be compiled without change to produce a DLL.+
−
    See the file win32/DLL_FAQ.txt in the zlib distribution.+
−
    Pointers to the precompiled DLL are found in the zlib web site at+
−
    http://www.zlib.org.+
−
+
−
 3. Where can I get a Visual Basic interface to zlib?+
−
+
−
    See+
−
        * http://www.dogma.net/markn/articles/zlibtool/zlibtool.htm+
−
        * contrib/visual-basic.txt in the zlib distribution+
−
        * win32/DLL_FAQ.txt in the zlib distribution+
−
+
−
 4. compress() returns Z_BUF_ERROR.+
−
+
−
    Make sure that before the call of compress, the length of the compressed+
−
    buffer is equal to the total size of the compressed buffer and not+
−
    zero. For Visual Basic, check that this parameter is passed by reference+
−
    ("as any"), not by value ("as long").+
−
+
−
 5. deflate() or inflate() returns Z_BUF_ERROR.+
−
+
−
    Before making the call, make sure that avail_in and avail_out are not+
−
    zero. When setting the parameter flush equal to Z_FINISH, also make sure+
−
    that avail_out is big enough to allow processing all pending input.+
−
    Note that a Z_BUF_ERROR is not fatal--another call to deflate() or+
−
    inflate() can be made with more input or output space. A Z_BUF_ERROR+
−
    may in fact be unavoidable depending on how the functions are used, since+
−
    it is not possible to tell whether or not there is more output pending+
−
    when strm.avail_out returns with zero.+
−
+
−
 6. Where's the zlib documentation (man pages, etc.)?+
−
+
−
    It's in zlib.h for the moment, and Francis S. Lin has converted it to a+
−
    web page zlib.html. Volunteers to transform this to Unix-style man pages,+
−
    please contact us (zlib@gzip.org). Examples of zlib usage are in the files+
−
    example.c and minigzip.c.+
−
+
−
 7. Why don't you use GNU autoconf or libtool or ...?+
−
+
−
    Because we would like to keep zlib as a very small and simple+
−
    package. zlib is rather portable and doesn't need much configuration.+
−
+
−
 8. I found a bug in zlib.+
−
+
−
    Most of the time, such problems are due to an incorrect usage of+
−
    zlib. Please try to reproduce the problem with a small program and send+
−
    the corresponding source to us at zlib@gzip.org . Do not send+
−
    multi-megabyte data files without prior agreement.+
−
+
−
 9. Why do I get "undefined reference to gzputc"?+
−
+
−
    If "make test" produces something like+
−
+
−
       example.o(.text+0x154): undefined reference to `gzputc'+
−
+
−
    check that you don't have old files libz.* in /usr/lib, /usr/local/lib or+
−
    /usr/X11R6/lib. Remove any old versions, then do "make install".+
−
+
−
10. I need a Delphi interface to zlib.+
−
+
−
    See the contrib/delphi directory in the zlib distribution.+
−
+
−
11. Can zlib handle .zip archives?+
−
+
−
    Not by itself, no.  See the directory contrib/minizip in the zlib+
−
    distribution.+
−
+
−
12. Can zlib handle .Z files?+
−
+
−
    No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt+
−
    the code of uncompress on your own.+
−
+
−
13. How can I make a Unix shared library?+
−
+
−
    make clean+
−
    ./configure -s+
−
    make+
−
+
−
14. How do I install a shared zlib library on Unix?+
−
+
−
    After the above, then:+
−
+
−
    make install+
−
+
−
    However, many flavors of Unix come with a shared zlib already installed.+
−
    Before going to the trouble of compiling a shared version of zlib and+
−
    trying to install it, you may want to check if it's already there! If you+
−
    can #include <zlib.h>, it's there. The -lz option will probably link to it.+
−
+
−
15. I have a question about OttoPDF.+
−
+
−
    We are not the authors of OttoPDF. The real author is on the OttoPDF web+
−
    site: Joel Hainley, jhainley@myndkryme.com.+
−
+
−
16. Can zlib decode Flate data in an Adobe PDF file?+
−
+
−
    Yes. See http://www.fastio.com/ (ClibPDF), or http://www.pdflib.com/ .+
−
    To modify PDF forms, see http://sourceforge.net/projects/acroformtool/ .+
−
+
−
17. Why am I getting this "register_frame_info not found" error on Solaris?+
−
+
−
    After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib+
−
    generates an error such as:+
−
+
−
        ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:+
−
        symbol __register_frame_info: referenced symbol not found+
−
+
−
    The symbol __register_frame_info is not part of zlib, it is generated by+
−
    the C compiler (cc or gcc). You must recompile applications using zlib+
−
    which have this problem. This problem is specific to Solaris. See+
−
    http://www.sunfreeware.com for Solaris versions of zlib and applications+
−
    using zlib.+
−
+
−
18. Why does gzip give an error on a file I make with compress/deflate?+
−
+
−
    The compress and deflate functions produce data in the zlib format, which+
−
    is different and incompatible with the gzip format. The gz* functions in+
−
    zlib on the other hand use the gzip format. Both the zlib and gzip+
−
    formats use the same compressed data format internally, but have different+
−
    headers and trailers around the compressed data.+
−
+
−
19. Ok, so why are there two different formats?+
−
+
−
    The gzip format was designed to retain the directory information about+
−
    a single file, such as the name and last modification date. The zlib+
−
    format on the other hand was designed for in-memory and communication+
−
    channel applications, and has a much more compact header and trailer and+
−
    uses a faster integrity check than gzip.+
−
+
−
20. Well that's nice, but how do I make a gzip file in memory?+
−
+
−
    You can request that deflate write the gzip format instead of the zlib+
−
    format using deflateInit2(). You can also request that inflate decode+
−
    the gzip format using inflateInit2(). Read zlib.h for more details.+
−
+
−
21. Is zlib thread-safe?+
−
+
−
    Yes. However any library routines that zlib uses and any application-+
−
    provided memory allocation routines must also be thread-safe. zlib's gz*+
−
    functions use stdio library routines, and most of zlib's functions use the+
−
    library memory allocation routines by default. zlib's Init functions allow+
−
    for the application to provide custom memory allocation routines.+
−
+
−
    Of course, you should only operate on any given zlib or gzip stream from a+
−
    single thread at a time.+
−
+
−
22. Can I use zlib in my commercial application?+
−
+
−
    Yes. Please read the license in zlib.h.+
−
+
−
23. Is zlib under the GNU license?+
−
+
−
    No. Please read the license in zlib.h.+
−
+
−
24. The license says that altered source versions must be "plainly marked". So+
−
    what exactly do I need to do to meet that requirement?+
−
+
−
    You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In+
−
    particular, the final version number needs to be changed to "f", and an+
−
    identification string should be appended to ZLIB_VERSION. Version numbers+
−
    x.x.x.f are reserved for modifications to zlib by others than the zlib+
−
    maintainers. For example, if the version of the base zlib you are altering+
−
    is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and+
−
    ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also+
−
    update the version strings in deflate.c and inftrees.c.+
−
+
−
    For altered source distributions, you should also note the origin and+
−
    nature of the changes in zlib.h, as well as in ChangeLog and README, along+
−
    with the dates of the alterations. The origin should include at least your+
−
    name (or your company's name), and an email address to contact for help or+
−
    issues with the library.+
−
+
−
    Note that distributing a compiled zlib library along with zlib.h and+
−
    zconf.h is also a source distribution, and so you should change+
−
    ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes+
−
    in zlib.h as you would for a full source distribution.+
−
+
−
25. Will zlib work on a big-endian or little-endian architecture, and can I+
−
    exchange compressed data between them?+
−
+
−
    Yes and yes.+
−
+
−
26. Will zlib work on a 64-bit machine?+
−
+
−
    It should. It has been tested on 64-bit machines, and has no dependence+
−
    on any data types being limited to 32-bits in length. If you have any+
−
    difficulties, please provide a complete problem report to zlib@gzip.org+
−
+
−
27. Will zlib decompress data from the PKWare Data Compression Library?+
−
+
−
    No. The PKWare DCL uses a completely different compressed data format+
−
    than does PKZIP and zlib. However, you can look in zlib's contrib/blast+
−
    directory for a possible solution to your problem.+
−
+
−
28. Can I access data randomly in a compressed stream?+
−
+
−
    No, not without some preparation. If when compressing you periodically+
−
    use Z_FULL_FLUSH, carefully write all the pending data at those points,+
−
    and keep an index of those locations, then you can start decompression+
−
    at those points. You have to be careful to not use Z_FULL_FLUSH too+
−
    often, since it can significantly degrade compression.+
−
+
−
29. Does zlib work on MVS, OS/390, CICS, etc.?+
−
+
−
    We don't know for sure. We have heard occasional reports of success on+
−
    these systems. If you do use it on one of these, please provide us with+
−
    a report, instructions, and patches that we can reference when we get+
−
    these questions. Thanks.+
−
+
−
30. Is there some simpler, easier to read version of inflate I can look at+
−
    to understand the deflate format?+
−
+
−
    First off, you should read RFC 1951. Second, yes. Look in zlib's+
−
    contrib/puff directory.+
−
+
−
31. Does zlib infringe on any patents?+
−
+
−
    As far as we know, no. In fact, that was originally the whole point behind+
−
    zlib. Look here for some more information:+
−
+
−
    http://www.gzip.org/#faq11+
−
+
−
32. Can zlib work with greater than 4 GB of data?+
−
+
−
    Yes. inflate() and deflate() will process any amount of data correctly.+
−
    Each call of inflate() or deflate() is limited to input and output chunks+
−
    of the maximum value that can be stored in the compiler's "unsigned int"+
−
    type, but there is no limit to the number of chunks. Note however that the+
−
    strm.total_in and strm_total_out counters may be limited to 4 GB. These+
−
    counters are provided as a convenience and are not used internally by+
−
    inflate() or deflate(). The application can easily set up its own counters+
−
    updated after each call of inflate() or deflate() to count beyond 4 GB.+
−
    compress() and uncompress() may be limited to 4 GB, since they operate in a+
−
    single call. gzseek() and gztell() may be limited to 4 GB depending on how+
−
    zlib is compiled. See the zlibCompileFlags() function in zlib.h.+
−
+
−
    The word "may" appears several times above since there is a 4 GB limit+
−
    only if the compiler's "long" type is 32 bits. If the compiler's "long"+
−
    type is 64 bits, then the limit is 16 exabytes.+
−
+
−
33. Does zlib have any security vulnerabilities?+
−
+
−
    The only one that we are aware of is potentially in gzprintf(). If zlib+
−
    is compiled to use sprintf() or vsprintf(), then there is no protection+
−
    against a buffer overflow of a 4K string space, other than the caller of+
−
    gzprintf() assuring that the output will not exceed 4K. On the other+
−
    hand, if zlib is compiled to use snprintf() or vsnprintf(), which should+
−
    normally be the case, then there is no vulnerability. The ./configure+
−
    script will display warnings if an insecure variation of sprintf() will+
−
    be used by gzprintf(). Also the zlibCompileFlags() function will return+
−
    information on what variant of sprintf() is used by gzprintf().+
−
+
−
    If you don't have snprintf() or vsnprintf() and would like one, you can+
−
    find a portable implementation here:+
−
+
−
        http://www.ijs.si/software/snprintf/+
−
+
−
    Note that you should be using the most recent version of zlib. Versions+
−
    1.1.3 and before were subject to a double-free vulnerability.+
−
+
−
34. Is there a Java version of zlib?+
−
+
−
    Probably what you want is to use zlib in Java. zlib is already included+
−
    as part of the Java SDK in the java.util.zip package. If you really want+
−
    a version of zlib written in the Java language, look on the zlib home+
−
    page for links: http://www.zlib.org/+
−
+
−
35. I get this or that compiler or source-code scanner warning when I crank it+
−
    up to maximally-pedantic. Can't you guys write proper code?+
−
+
−
    Many years ago, we gave up attempting to avoid warnings on every compiler+
−
    in the universe. It just got to be a waste of time, and some compilers+
−
    were downright silly. So now, we simply make sure that the code always+
−
    works.+
−
+
−
36. Valgrind (or some similar memory access checker) says that deflate is+
−
    performing a conditional jump that depends on an uninitialized value.+
−
    Isn't that a bug?+
−
+
−
    No.  That is intentional for performance reasons, and the output of+
−
    deflate is not affected.  This only started showing up recently since+
−
    zlib 1.2.x uses malloc() by default for allocations, whereas earlier+
−
    versions used calloc(), which zeros out the allocated memory.+
−
+
−
37. Will zlib read the (insert any ancient or arcane format here) compressed+
−
    data format?+
−
+
−
    Probably not. Look in the comp.compression FAQ for pointers to various+
−
    formats and associated software.+
−
+
−
38. How can I encrypt/decrypt zip files with zlib?+
−
+
−
    zlib doesn't support encryption. The original PKZIP encryption is very weak+
−
    and can be broken with freely available programs. To get strong encryption,+
−
    use GnuPG, http://www.gnupg.org/ , which already includes zlib compression.+
−
    For PKZIP compatible "encryption", look at http://www.info-zip.org/+
−
+
−
39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?+
−
+
−
    "gzip" is the gzip format, and "deflate" is the zlib format. They should+
−
    probably have called the second one "zlib" instead to avoid confusion+
−
    with the raw deflate compressed data format. While the HTTP 1.1 RFC 2616+
−
    correctly points to the zlib specification in RFC 1950 for the "deflate"+
−
    transfer encoding, there have been reports of servers and browsers that+
−
    incorrectly produce or expect raw deflate data per the deflate+
−
    specficiation in RFC 1951, most notably Microsoft. So even though the+
−
    "deflate" transfer encoding using the zlib format would be the more+
−
    efficient approach (and in fact exactly what the zlib format was designed+
−
    for), using the "gzip" transfer encoding is probably more reliable due to+
−
    an unfortunate choice of name on the part of the HTTP 1.1 authors.+
−
+
−
    Bottom line: use the gzip format for HTTP 1.1 encoding.+
−
+
−
40. Does zlib support the new "Deflate64" format introduced by PKWare?+
−
+
−
    No. PKWare has apparently decided to keep that format proprietary, since+
−
    they have not documented it as they have previous compression formats.+
−
    In any case, the compression improvements are so modest compared to other+
−
    more modern approaches, that it's not worth the effort to implement.+
−
+
−
41. Can you please sign these lengthy legal documents and fax them back to us+
−
    so that we can use your software in our product?+
−
+
−
    No. Go away. Shoo.+
−