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diff --git a/zlib/FAQ b/zlib/FAQ new file mode 100644 index 0000000..99b7cf9 --- /dev/null +++ b/zlib/FAQ @@ -0,0 +1,368 @@ + + Frequently Asked Questions about zlib + + +If your question is not there, please check the zlib home page +http://zlib.net/ which may have more recent information. +The lastest zlib FAQ is at http://zlib.net/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://zlib.net/ . + + 3. Where can I get a Visual Basic interface to zlib? + + See + * http://marknelson.us/1997/01/01/zlib-engine/ + * win32/DLL_FAQ.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 available 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. See http://zlib.net/zlib_how.html for a + heavily annotated example. + + 6. Where's the zlib documentation (man pages, etc.)? + + It's in zlib.h . Examples of zlib usage are in the files test/example.c + and test/minigzip.c, with more in examples/ . + + 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? + + Not by itself, no. 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? + + By default a shared (and a static) library is built for Unix. So: + + make distclean + ./configure + make + +14. How do I install a shared zlib library on Unix? + + After the above, then: + + 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. You can check the version at the top of zlib.h or with the + ZLIB_VERSION symbol defined in zlib.h . + +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. Can zlib decode Flate data in an Adobe PDF file? + + Yes. See http://www.pdflib.com/ . To modify PDF forms, see + http://sourceforge.net/projects/acroformtool/ . + +17. Why am I getting this "register_frame_info not found" error on Solaris? + + After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib + generates an error such as: + + ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so: + symbol __register_frame_info: referenced symbol not found + + The symbol __register_frame_info is not part of zlib, it is generated by + the C compiler (cc or gcc). You must recompile applications using zlib + which have this problem. This problem is specific to Solaris. See + http://www.sunfreeware.com for Solaris versions of zlib and applications + using zlib. + +18. 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. + +19. 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. + +20. 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. + +21. 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. + +22. Can I use zlib in my commercial application? + + Yes. Please read the license in zlib.h. + +23. Is zlib under the GNU license? + + No. Please read the license in zlib.h. + +24. 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. + +25. Will zlib work on a big-endian or little-endian architecture, and can I + exchange compressed data between them? + + Yes and yes. + +26. Will zlib work on a 64-bit machine? + + Yes. 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 + +27. 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. + +28. 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. Alternatively, you can scan a + deflate stream once to generate an index, and then use that index for + random access. See examples/zran.c . + +29. Does zlib work on MVS, OS/390, CICS, etc.? + + It has in the past, but we have not heard of any recent evidence. There + were working ports of zlib 1.1.4 to MVS, but those links no longer work. + If you know of recent, successful applications of zlib on these operating + systems, please let us know. Thanks. + +30. 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. + +31. 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 + +32. 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. + +33. 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 an 8K string space (or other value as set by + gzbuffer()), other than the caller of gzprintf() assuring that the output + will not exceed 8K. 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, and versions + 1.2.1 and 1.2.2 were subject to an access exception when decompressing + invalid compressed data. + +34. 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://zlib.net/ . + +35. I get this or that compiler or source-code scanner warning when I crank it + up to maximally-pedantic. 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 as well as contradicted each other. So now, we simply + make sure that the code always works. + +36. Valgrind (or some similar memory access checker) says that deflate is + performing a conditional jump that depends on an uninitialized value. + Isn't that a bug? + + No. That is intentional for performance reasons, and the output of deflate + is not affected. This only started showing up recently since zlib 1.2.x + uses malloc() by default for allocations, whereas earlier versions used + calloc(), which zeros out the allocated memory. Even though the code was + correct, versions 1.2.4 and later was changed to not stimulate these + checkers. + +37. 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. + +38. 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 GnuPG, http://www.gnupg.org/ , which already includes zlib + compression. For PKZIP compatible "encryption", look at + http://www.info-zip.org/ + +39. 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 + specification 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. + +40. 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. + +41. I'm having a problem with the zip functions in zlib, can you help? + + There are no zip functions in zlib. You are probably using minizip by + Giles Vollant, which is found in the contrib directory of zlib. It is not + part of zlib. In fact none of the stuff in contrib is part of zlib. The + files in there are not supported by the zlib authors. You need to contact + the authors of the respective contribution for help. + +42. The match.asm code in contrib is under the GNU General Public License. + Since it's part of zlib, doesn't that mean that all of zlib falls under the + GNU GPL? + + No. The files in contrib are not part of zlib. They were contributed by + other authors and are provided as a convenience to the user within the zlib + distribution. Each item in contrib has its own license. + +43. Is zlib subject to export controls? What is its ECCN? + + zlib is not subject to export controls, and so is classified as EAR99. + +44. 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. |