michael@0: michael@0: Frequently Asked Questions about zlib michael@0: michael@0: michael@0: If your question is not there, please check the zlib home page michael@0: http://zlib.net/ which may have more recent information. michael@0: The lastest zlib FAQ is at http://zlib.net/zlib_faq.html michael@0: michael@0: michael@0: 1. Is zlib Y2K-compliant? michael@0: michael@0: Yes. zlib doesn't handle dates. michael@0: michael@0: 2. Where can I get a Windows DLL version? michael@0: michael@0: The zlib sources can be compiled without change to produce a DLL. See the michael@0: file win32/DLL_FAQ.txt in the zlib distribution. Pointers to the michael@0: precompiled DLL are found in the zlib web site at http://zlib.net/ . michael@0: michael@0: 3. Where can I get a Visual Basic interface to zlib? michael@0: michael@0: See michael@0: * http://marknelson.us/1997/01/01/zlib-engine/ michael@0: * win32/DLL_FAQ.txt in the zlib distribution michael@0: michael@0: 4. compress() returns Z_BUF_ERROR. michael@0: michael@0: Make sure that before the call of compress(), the length of the compressed michael@0: buffer is equal to the available size of the compressed buffer and not michael@0: zero. For Visual Basic, check that this parameter is passed by reference michael@0: ("as any"), not by value ("as long"). michael@0: michael@0: 5. deflate() or inflate() returns Z_BUF_ERROR. michael@0: michael@0: Before making the call, make sure that avail_in and avail_out are not zero. michael@0: When setting the parameter flush equal to Z_FINISH, also make sure that michael@0: avail_out is big enough to allow processing all pending input. Note that a michael@0: Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be michael@0: made with more input or output space. A Z_BUF_ERROR may in fact be michael@0: unavoidable depending on how the functions are used, since it is not michael@0: possible to tell whether or not there is more output pending when michael@0: strm.avail_out returns with zero. See http://zlib.net/zlib_how.html for a michael@0: heavily annotated example. michael@0: michael@0: 6. Where's the zlib documentation (man pages, etc.)? michael@0: michael@0: It's in zlib.h . Examples of zlib usage are in the files test/example.c michael@0: and test/minigzip.c, with more in examples/ . michael@0: michael@0: 7. Why don't you use GNU autoconf or libtool or ...? michael@0: michael@0: Because we would like to keep zlib as a very small and simple package. michael@0: zlib is rather portable and doesn't need much configuration. michael@0: michael@0: 8. I found a bug in zlib. michael@0: michael@0: Most of the time, such problems are due to an incorrect usage of zlib. michael@0: Please try to reproduce the problem with a small program and send the michael@0: corresponding source to us at zlib@gzip.org . Do not send multi-megabyte michael@0: data files without prior agreement. michael@0: michael@0: 9. Why do I get "undefined reference to gzputc"? michael@0: michael@0: If "make test" produces something like michael@0: michael@0: example.o(.text+0x154): undefined reference to `gzputc' michael@0: michael@0: check that you don't have old files libz.* in /usr/lib, /usr/local/lib or michael@0: /usr/X11R6/lib. Remove any old versions, then do "make install". michael@0: michael@0: 10. I need a Delphi interface to zlib. michael@0: michael@0: See the contrib/delphi directory in the zlib distribution. michael@0: michael@0: 11. Can zlib handle .zip archives? michael@0: michael@0: Not by itself, no. See the directory contrib/minizip in the zlib michael@0: distribution. michael@0: michael@0: 12. Can zlib handle .Z files? michael@0: michael@0: No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt michael@0: the code of uncompress on your own. michael@0: michael@0: 13. How can I make a Unix shared library? michael@0: michael@0: By default a shared (and a static) library is built for Unix. So: michael@0: michael@0: make distclean michael@0: ./configure michael@0: make michael@0: michael@0: 14. How do I install a shared zlib library on Unix? michael@0: michael@0: After the above, then: michael@0: michael@0: make install michael@0: michael@0: However, many flavors of Unix come with a shared zlib already installed. michael@0: Before going to the trouble of compiling a shared version of zlib and michael@0: trying to install it, you may want to check if it's already there! If you michael@0: can #include , it's there. The -lz option will probably link to michael@0: it. You can check the version at the top of zlib.h or with the michael@0: ZLIB_VERSION symbol defined in zlib.h . michael@0: michael@0: 15. I have a question about OttoPDF. michael@0: michael@0: We are not the authors of OttoPDF. The real author is on the OttoPDF web michael@0: site: Joel Hainley, jhainley@myndkryme.com. michael@0: michael@0: 16. Can zlib decode Flate data in an Adobe PDF file? michael@0: michael@0: Yes. See http://www.pdflib.com/ . To modify PDF forms, see michael@0: http://sourceforge.net/projects/acroformtool/ . michael@0: michael@0: 17. Why am I getting this "register_frame_info not found" error on Solaris? michael@0: michael@0: After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib michael@0: generates an error such as: michael@0: michael@0: ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so: michael@0: symbol __register_frame_info: referenced symbol not found michael@0: michael@0: The symbol __register_frame_info is not part of zlib, it is generated by michael@0: the C compiler (cc or gcc). You must recompile applications using zlib michael@0: which have this problem. This problem is specific to Solaris. See michael@0: http://www.sunfreeware.com for Solaris versions of zlib and applications michael@0: using zlib. michael@0: michael@0: 18. Why does gzip give an error on a file I make with compress/deflate? michael@0: michael@0: The compress and deflate functions produce data in the zlib format, which michael@0: is different and incompatible with the gzip format. The gz* functions in michael@0: zlib on the other hand use the gzip format. Both the zlib and gzip formats michael@0: use the same compressed data format internally, but have different headers michael@0: and trailers around the compressed data. michael@0: michael@0: 19. Ok, so why are there two different formats? michael@0: michael@0: The gzip format was designed to retain the directory information about a michael@0: single file, such as the name and last modification date. The zlib format michael@0: on the other hand was designed for in-memory and communication channel michael@0: applications, and has a much more compact header and trailer and uses a michael@0: faster integrity check than gzip. michael@0: michael@0: 20. Well that's nice, but how do I make a gzip file in memory? michael@0: michael@0: You can request that deflate write the gzip format instead of the zlib michael@0: format using deflateInit2(). You can also request that inflate decode the michael@0: gzip format using inflateInit2(). Read zlib.h for more details. michael@0: michael@0: 21. Is zlib thread-safe? michael@0: michael@0: Yes. However any library routines that zlib uses and any application- michael@0: provided memory allocation routines must also be thread-safe. zlib's gz* michael@0: functions use stdio library routines, and most of zlib's functions use the michael@0: library memory allocation routines by default. zlib's *Init* functions michael@0: allow for the application to provide custom memory allocation routines. michael@0: michael@0: Of course, you should only operate on any given zlib or gzip stream from a michael@0: single thread at a time. michael@0: michael@0: 22. Can I use zlib in my commercial application? michael@0: michael@0: Yes. Please read the license in zlib.h. michael@0: michael@0: 23. Is zlib under the GNU license? michael@0: michael@0: No. Please read the license in zlib.h. michael@0: michael@0: 24. The license says that altered source versions must be "plainly marked". So michael@0: what exactly do I need to do to meet that requirement? michael@0: michael@0: You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In michael@0: particular, the final version number needs to be changed to "f", and an michael@0: identification string should be appended to ZLIB_VERSION. Version numbers michael@0: x.x.x.f are reserved for modifications to zlib by others than the zlib michael@0: maintainers. For example, if the version of the base zlib you are altering michael@0: is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and michael@0: ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also michael@0: update the version strings in deflate.c and inftrees.c. michael@0: michael@0: For altered source distributions, you should also note the origin and michael@0: nature of the changes in zlib.h, as well as in ChangeLog and README, along michael@0: with the dates of the alterations. The origin should include at least your michael@0: name (or your company's name), and an email address to contact for help or michael@0: issues with the library. michael@0: michael@0: Note that distributing a compiled zlib library along with zlib.h and michael@0: zconf.h is also a source distribution, and so you should change michael@0: ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes michael@0: in zlib.h as you would for a full source distribution. michael@0: michael@0: 25. Will zlib work on a big-endian or little-endian architecture, and can I michael@0: exchange compressed data between them? michael@0: michael@0: Yes and yes. michael@0: michael@0: 26. Will zlib work on a 64-bit machine? michael@0: michael@0: Yes. It has been tested on 64-bit machines, and has no dependence on any michael@0: data types being limited to 32-bits in length. If you have any michael@0: difficulties, please provide a complete problem report to zlib@gzip.org michael@0: michael@0: 27. Will zlib decompress data from the PKWare Data Compression Library? michael@0: michael@0: No. The PKWare DCL uses a completely different compressed data format than michael@0: does PKZIP and zlib. However, you can look in zlib's contrib/blast michael@0: directory for a possible solution to your problem. michael@0: michael@0: 28. Can I access data randomly in a compressed stream? michael@0: michael@0: No, not without some preparation. If when compressing you periodically use michael@0: Z_FULL_FLUSH, carefully write all the pending data at those points, and michael@0: keep an index of those locations, then you can start decompression at those michael@0: points. You have to be careful to not use Z_FULL_FLUSH too often, since it michael@0: can significantly degrade compression. Alternatively, you can scan a michael@0: deflate stream once to generate an index, and then use that index for michael@0: random access. See examples/zran.c . michael@0: michael@0: 29. Does zlib work on MVS, OS/390, CICS, etc.? michael@0: michael@0: It has in the past, but we have not heard of any recent evidence. There michael@0: were working ports of zlib 1.1.4 to MVS, but those links no longer work. michael@0: If you know of recent, successful applications of zlib on these operating michael@0: systems, please let us know. Thanks. michael@0: michael@0: 30. Is there some simpler, easier to read version of inflate I can look at to michael@0: understand the deflate format? michael@0: michael@0: First off, you should read RFC 1951. Second, yes. Look in zlib's michael@0: contrib/puff directory. michael@0: michael@0: 31. Does zlib infringe on any patents? michael@0: michael@0: As far as we know, no. In fact, that was originally the whole point behind michael@0: zlib. Look here for some more information: michael@0: michael@0: http://www.gzip.org/#faq11 michael@0: michael@0: 32. Can zlib work with greater than 4 GB of data? michael@0: michael@0: Yes. inflate() and deflate() will process any amount of data correctly. michael@0: Each call of inflate() or deflate() is limited to input and output chunks michael@0: of the maximum value that can be stored in the compiler's "unsigned int" michael@0: type, but there is no limit to the number of chunks. Note however that the michael@0: strm.total_in and strm_total_out counters may be limited to 4 GB. These michael@0: counters are provided as a convenience and are not used internally by michael@0: inflate() or deflate(). The application can easily set up its own counters michael@0: updated after each call of inflate() or deflate() to count beyond 4 GB. michael@0: compress() and uncompress() may be limited to 4 GB, since they operate in a michael@0: single call. gzseek() and gztell() may be limited to 4 GB depending on how michael@0: zlib is compiled. See the zlibCompileFlags() function in zlib.h. michael@0: michael@0: The word "may" appears several times above since there is a 4 GB limit only michael@0: if the compiler's "long" type is 32 bits. If the compiler's "long" type is michael@0: 64 bits, then the limit is 16 exabytes. michael@0: michael@0: 33. Does zlib have any security vulnerabilities? michael@0: michael@0: The only one that we are aware of is potentially in gzprintf(). If zlib is michael@0: compiled to use sprintf() or vsprintf(), then there is no protection michael@0: against a buffer overflow of an 8K string space (or other value as set by michael@0: gzbuffer()), other than the caller of gzprintf() assuring that the output michael@0: will not exceed 8K. On the other hand, if zlib is compiled to use michael@0: snprintf() or vsnprintf(), which should normally be the case, then there is michael@0: no vulnerability. The ./configure script will display warnings if an michael@0: insecure variation of sprintf() will be used by gzprintf(). Also the michael@0: zlibCompileFlags() function will return information on what variant of michael@0: sprintf() is used by gzprintf(). michael@0: michael@0: If you don't have snprintf() or vsnprintf() and would like one, you can michael@0: find a portable implementation here: michael@0: michael@0: http://www.ijs.si/software/snprintf/ michael@0: michael@0: Note that you should be using the most recent version of zlib. Versions michael@0: 1.1.3 and before were subject to a double-free vulnerability, and versions michael@0: 1.2.1 and 1.2.2 were subject to an access exception when decompressing michael@0: invalid compressed data. michael@0: michael@0: 34. Is there a Java version of zlib? michael@0: michael@0: Probably what you want is to use zlib in Java. zlib is already included michael@0: as part of the Java SDK in the java.util.zip package. If you really want michael@0: a version of zlib written in the Java language, look on the zlib home michael@0: page for links: http://zlib.net/ . michael@0: michael@0: 35. I get this or that compiler or source-code scanner warning when I crank it michael@0: up to maximally-pedantic. Can't you guys write proper code? michael@0: michael@0: Many years ago, we gave up attempting to avoid warnings on every compiler michael@0: in the universe. It just got to be a waste of time, and some compilers michael@0: were downright silly as well as contradicted each other. So now, we simply michael@0: make sure that the code always works. michael@0: michael@0: 36. Valgrind (or some similar memory access checker) says that deflate is michael@0: performing a conditional jump that depends on an uninitialized value. michael@0: Isn't that a bug? michael@0: michael@0: No. That is intentional for performance reasons, and the output of deflate michael@0: is not affected. This only started showing up recently since zlib 1.2.x michael@0: uses malloc() by default for allocations, whereas earlier versions used michael@0: calloc(), which zeros out the allocated memory. Even though the code was michael@0: correct, versions 1.2.4 and later was changed to not stimulate these michael@0: checkers. michael@0: michael@0: 37. Will zlib read the (insert any ancient or arcane format here) compressed michael@0: data format? michael@0: michael@0: Probably not. Look in the comp.compression FAQ for pointers to various michael@0: formats and associated software. michael@0: michael@0: 38. How can I encrypt/decrypt zip files with zlib? michael@0: michael@0: zlib doesn't support encryption. The original PKZIP encryption is very michael@0: weak and can be broken with freely available programs. To get strong michael@0: encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib michael@0: compression. For PKZIP compatible "encryption", look at michael@0: http://www.info-zip.org/ michael@0: michael@0: 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings? michael@0: michael@0: "gzip" is the gzip format, and "deflate" is the zlib format. They should michael@0: probably have called the second one "zlib" instead to avoid confusion with michael@0: the raw deflate compressed data format. While the HTTP 1.1 RFC 2616 michael@0: correctly points to the zlib specification in RFC 1950 for the "deflate" michael@0: transfer encoding, there have been reports of servers and browsers that michael@0: incorrectly produce or expect raw deflate data per the deflate michael@0: specification in RFC 1951, most notably Microsoft. So even though the michael@0: "deflate" transfer encoding using the zlib format would be the more michael@0: efficient approach (and in fact exactly what the zlib format was designed michael@0: for), using the "gzip" transfer encoding is probably more reliable due to michael@0: an unfortunate choice of name on the part of the HTTP 1.1 authors. michael@0: michael@0: Bottom line: use the gzip format for HTTP 1.1 encoding. michael@0: michael@0: 40. Does zlib support the new "Deflate64" format introduced by PKWare? michael@0: michael@0: No. PKWare has apparently decided to keep that format proprietary, since michael@0: they have not documented it as they have previous compression formats. In michael@0: any case, the compression improvements are so modest compared to other more michael@0: modern approaches, that it's not worth the effort to implement. michael@0: michael@0: 41. I'm having a problem with the zip functions in zlib, can you help? michael@0: michael@0: There are no zip functions in zlib. You are probably using minizip by michael@0: Giles Vollant, which is found in the contrib directory of zlib. It is not michael@0: part of zlib. In fact none of the stuff in contrib is part of zlib. The michael@0: files in there are not supported by the zlib authors. You need to contact michael@0: the authors of the respective contribution for help. michael@0: michael@0: 42. The match.asm code in contrib is under the GNU General Public License. michael@0: Since it's part of zlib, doesn't that mean that all of zlib falls under the michael@0: GNU GPL? michael@0: michael@0: No. The files in contrib are not part of zlib. They were contributed by michael@0: other authors and are provided as a convenience to the user within the zlib michael@0: distribution. Each item in contrib has its own license. michael@0: michael@0: 43. Is zlib subject to export controls? What is its ECCN? michael@0: michael@0: zlib is not subject to export controls, and so is classified as EAR99. michael@0: michael@0: 44. Can you please sign these lengthy legal documents and fax them back to us michael@0: so that we can use your software in our product? michael@0: michael@0: No. Go away. Shoo.