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+ Frequently Asked Questions about zlib
+If your question is not there, please check the zlib home page
+ which may have more recent information.
+The lastest zlib FAQ is at
+ 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 .
+ 3. Where can I get a Visual Basic interface to zlib?
+ See
+ *
+ * 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 available 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. See for a
+ heavily annotated example.
+ 6. Where's the zlib documentation (man pages, etc.)?
+ It's in zlib.h . Examples of zlib usage are in the files example.c and
+ minigzip.c, with more in examples/ .
+ 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 . 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. You can check the version at the top of zlib.h or with the
+ ZLIB_VERSION symbol defined in zlib.h .
+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,
+16. Can zlib decode Flate data in an Adobe PDF file?
+ Yes. See . To modify PDF forms, see
+ .
+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:
+ rpm: fatal: relocation error: file /usr/local/lib/
+ 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
+ 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 "", 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?
+ Yes. 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
+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. Alternatively, you can scan a
+ deflate stream once to generate an index, and then use that index for
+ random access. See examples/zran.c .
+29. Does zlib work on MVS, OS/390, CICS, etc.?
+ It has in the past, but we have not heard of any recent evidence. There
+ were working ports of zlib 1.1.4 to MVS, but those links no longer work.
+ If you know of recent, successful applications of zlib on these operating
+ systems, please let us know. 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:
+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 an 8K string space (or other value as set by
+ gzbuffer()), other than the caller of gzprintf() assuring that the output
+ will not exceed 8K. 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:
+ 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, and versions
+ 1.2.1 and 1.2.2 were subject to an access exception when decompressing
+ invalid compressed data.
+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 package. If you really want
+ a version of zlib written in the Java language, look on the zlib home
+ page for links: .
+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 as well as contradicted each other. 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. Even though the code was
+ correct, versions 1.2.4 and later was changed to not stimulate these
+ checkers.
+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, , which already includes zlib
+ compression. For PKZIP compatible "encryption", look at
+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. I'm having a problem with the zip functions in zlib, can you help?
+ There are no zip functions in zlib. You are probably using minizip by
+ Giles Vollant, which is found in the contrib directory of zlib. It is not
+ part of zlib. In fact none of the stuff in contrib is part of zlib. The
+ files in there are not supported by the zlib authors. You need to contact
+ the authors of the respective contribution for help.
+42. The match.asm code in contrib is under the GNU General Public License.
+ Since it's part of zlib, doesn't that mean that all of zlib falls under the
+ No. The files in contrib are not part of zlib. They were contributed by
+ other authors and are provided as a convenience to the user within the zlib
+ distribution. Each item in contrib has its own license.
+43. Is zlib subject to export controls? What is its ECCN?
+ zlib is not subject to export controls, and so is classified as EAR99.
+44. 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.