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