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diff --git a/zlib/FAQ b/zlib/FAQ new file mode 100644 index 00000000000..7115ec38d60 --- /dev/null +++ b/zlib/FAQ @@ -0,0 +1,315 @@ + + 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. + See the file win32/DLL_FAQ.txt in the zlib distribution. + Pointers to the precompiled DLL are found in the zlib web site at + http://www.zlib.org. + + 3. Where can I get a Visual Basic interface to zlib? + + See + * http://www.winimage.com/zLibDll/ + * 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 contrib/delphi directory 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. How do I install a shared zlib library on Unix? + + 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. + +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 internally, but have different + headers and trailers around the compressed data. + +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? + + 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. + + Note that you cannot specify special gzip header contents (e.g. a file + name or modification date), nor will inflate tell you what was in the + gzip header. If you need to customize the header or see what's in it, + you can use the raw deflate and inflate operations and the crc32() + function and roll your own gzip encoding and decoding. Read the gzip + RFC 1952 for details of the header and trailer format. + +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. 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. + +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. 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 "1.2.3.4", 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. + +23. Will zlib work on a big-endian or little-endian architecture, and can I + exchange compressed data between them? + + Yes and yes. + +24. 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 + +25. 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. + +26. 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. + +27. 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. + +28. 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. + +29. 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 + +30. 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. + +31. 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(), 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: + + http://www.ijs.si/software/snprintf/ + + 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. + +32. 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 package. 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/ + +33. I get this or that compiler or source-code scanner warning when I crank it + up to maximally-pendantic. 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. + +34. 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. + +35. 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 gpg ( http://www.gnupg.org/ ) which already includes zlib compression. + For PKZIP compatible "encryption", look at http://www.info-zip.org/ + +36. 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. + +37. 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. + +38. 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. |