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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.winimage.com/zLibDll/
+ * http://www.dogma.net/markn/articles/zlibtool/zlibtool.htm
+ * contrib/visual-basic.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 Jean-loup Gailly (jloup@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?
+ 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?
+ 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. 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.
+17. 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.
+18. 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.
+ Note that you cannot specify special gzip header contents (e.g. a file
+ name or modification date), nor will inflate tell you what was in the
+ gzip header. If you need to customize the header or see what's in it,
+ you can use the raw deflate and inflate operations and the crc32()
+ function and roll your own gzip encoding and decoding. Read the gzip
+ RFC 1952 for details of the header and trailer format.
+19. 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.
+20. Can I use zlib in my commercial application?
+ Yes. Please read the license in zlib.h.
+21. Is zlib under the GNU license?
+ No. Please read the license in zlib.h.
+22. 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 "", 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.
+23. Will zlib work on a big-endian or little-endian architecture, and can I
+ exchange compressed data between them?
+ Yes and yes.
+24. 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
+25. 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.
+26. 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.
+27. 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.
+28. 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.
+29. 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
+30. 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.
+31. 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.
+32. 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/
+33. I get this or that compiler or source-code scanner warning when I crank it
+ up to maximally-pendantic. 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.
+34. 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.
+35. 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 gpg ( http://www.gnupg.org/ ) which already includes zlib compression.
+ For PKZIP compatible "encryption", look at http://www.info-zip.org/
+36. 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.
+37. 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.
+38. 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.