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. If you want a precompiled DLL, see http://www.winimage.com/zLibDll/ . Questions about the zlib DLL should be sent to Gilles Vollant (info@winimage.com). 3. Where can I get a Visual Basic interface to zlib? See * http://www.winimage.com/zLibDll/cmp-z-it.zip * 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 directories contrib/delphi and contrib/delphi2 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. Why does "make test" fail on Mac OS X? Mac OS X already includes zlib as a shared library, and so -lz links the shared library instead of the one that the "make" compiled. The two are incompatible due to different compile-time options. Simply change the -lz in the Makefile to libz.a, and it will use the compiled library instead of the shared one and the "make test" will succeed. 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, but have different headers and trailers. 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? Read RFC 1952 for the gzip header and trailer format, and roll your own gzip formatted data using raw deflate and crc32(). 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. Of course, you should only operate on any given zlib or gzip stream from a single thread. 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. 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. Will zlib work on a big-endian or little-endian architecture, and can I exchange compressed data between them? Yes and yes. 23. 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 24. 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. 25. 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. 26. 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. 27. 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. 28. 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 29. Can zlib work with greater than 4 GB of data? Yes. inflate() and deflate() will process any amount of data correctly. However the strm.total_in and strm_total_out counters may be limited to 4 GB. The user can easily set up their 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 using unsigned long lengths. gzseek() may be limited to 4 GB depending on how zlib is compiled. 30. 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(), then there is no vulnerability. 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. 31. 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 class. 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/ 32. I get this or that compiler or source-code scanner warning. 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. 33. 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. 34. How can I encrypt/decrypt zip files with zlib? zlib doesn't support encryption. PKZIP encryption is very weak and can be broken with freely available programs. To get strong encryption, use gpg which already includes zlib compression. 35. 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. 36. 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. 37. 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.