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