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