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Diffstat (limited to 'zlib/FAQ')
| -rw-r--r-- | zlib/FAQ | 267 |
1 files changed, 148 insertions, 119 deletions
@@ -3,8 +3,8 @@ 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 +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? @@ -13,54 +13,51 @@ The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html 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. + 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://www.dogma.net/markn/articles/zlibtool/zlibtool.htm - * contrib/visual-basic.txt in the zlib distribution + * 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 total size of the compressed buffer and not - zero. For Visual Basic, check that this parameter is passed by reference + 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. + 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 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 us (zlib@gzip.org). Examples of zlib usage are in the files - example.c and minigzip.c. + 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. + 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. + 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"? @@ -82,13 +79,15 @@ The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html 12. Can zlib handle .Z files? - No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt + 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 + 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? @@ -99,8 +98,10 @@ The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html 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. + 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. @@ -109,8 +110,8 @@ The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html 16. Can zlib decode Flate data in an Adobe PDF file? - Yes. See http://www.fastio.com/ (ClibPDF), or http://www.pdflib.com/ . - To modify PDF forms, see http://sourceforge.net/projects/acroformtool/ . + 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? @@ -121,67 +122,67 @@ The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html 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 + 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. + 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. + 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. + 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* + 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. + 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. + Yes. Please read the license in zlib.h. 23. Is zlib under the GNU license? - No. Please read the license in zlib.h. + 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 + 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 + 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 + 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 + 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 + 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. @@ -197,105 +198,112 @@ The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html 26. 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 + 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 + 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. + 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.? - 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. + 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? +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 + 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: + 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. + 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 + 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 + 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. + 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. + 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 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(). + 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. + 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://www.zlib.org/ + 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. So now, we simply make sure that the code always - works. + 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. + 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? @@ -305,20 +313,21 @@ The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html 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/ + 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 + "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 + 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 @@ -328,12 +337,32 @@ The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html 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. + 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. -41. Can you please sign these lengthy legal documents and fax them back to us +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. |