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+
+                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.