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Frequently Asked Questions about zlib |
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If your question is not there, please check the zlib home page |
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https://zlib.net/ which may have more recent information. |
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The latest zlib FAQ is at https://zlib.net/zlib_faq.html |
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1. Is zlib Y2K-compliant? |
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Yes. zlib doesn't handle dates. |
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2. Where can I get a Windows DLL version? |
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The zlib sources can be compiled without change to produce a DLL. See the |
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file win32/DLL_FAQ.txt in the zlib distribution. |
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3. Where can I get a Visual Basic interface to zlib? |
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See |
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* https://zlib.net/nelson/ |
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* win32/DLL_FAQ.txt in the zlib distribution |
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4. compress() returns Z_BUF_ERROR. |
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Make sure that before the call of compress(), the length of the compressed |
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buffer is equal to the available size of the compressed buffer and not |
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zero. For Visual Basic, check that this parameter is passed by reference |
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("as any"), not by value ("as long"). |
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5. deflate() or inflate() returns Z_BUF_ERROR. |
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Before making the call, make sure that avail_in and avail_out are not zero. |
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When setting the parameter flush equal to Z_FINISH, also make sure that |
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avail_out is big enough to allow processing all pending input. Note that a |
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Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be |
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made with more input or output space. A Z_BUF_ERROR may in fact be |
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unavoidable depending on how the functions are used, since it is not |
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possible to tell whether or not there is more output pending when |
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strm.avail_out returns with zero. See https://zlib.net/zlib_how.html for a |
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heavily annotated example. |
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6. Where's the zlib documentation (man pages, etc.)? |
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It's in zlib.h . Examples of zlib usage are in the files test/example.c |
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and test/minigzip.c, with more in examples/ . |
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7. Why don't you use GNU autoconf or libtool or ...? |
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Because we would like to keep zlib as a very small and simple package. |
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zlib is rather portable and doesn't need much configuration. |
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8. I found a bug in zlib. |
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Most of the time, such problems are due to an incorrect usage of zlib. |
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Please try to reproduce the problem with a small program and send the |
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corresponding source to us at [email protected] . Do not send multi-megabyte |
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data files without prior agreement. |
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9. Why do I get "undefined reference to gzputc"? |
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If "make test" produces something like |
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example.o(.text+0x154): undefined reference to `gzputc' |
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check that you don't have old files libz.* in /usr/lib, /usr/local/lib or |
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/usr/X11R6/lib. Remove any old versions, then do "make install". |
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10. I need a Delphi interface to zlib. |
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See the contrib/delphi directory in the zlib distribution. |
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11. Can zlib handle .zip archives? |
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Not by itself, no. See the directory contrib/minizip in the zlib |
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distribution. |
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12. Can zlib handle .Z files? |
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No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt |
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the code of uncompress on your own. |
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13. How can I make a Unix shared library? |
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By default a shared (and a static) library is built for Unix. So: |
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make distclean |
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./configure |
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make |
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14. How do I install a shared zlib library on Unix? |
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After the above, then: |
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make install |
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However, many flavors of Unix come with a shared zlib already installed. |
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Before going to the trouble of compiling a shared version of zlib and |
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trying to install it, you may want to check if it's already there! If you |
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can #include <zlib.h>, it's there. The -lz option will probably link to |
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it. You can check the version at the top of zlib.h or with the |
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ZLIB_VERSION symbol defined in zlib.h . |
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15. I have a question about OttoPDF. |
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We are not the authors of OttoPDF. The real author is on the OttoPDF web |
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site: Joel Hainley, [email protected]. |
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16. Can zlib decode Flate data in an Adobe PDF file? |
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Yes. See https://www.pdflib.com/ . To modify PDF forms, see |
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https://sourceforge.net/projects/acroformtool/ . |
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17. Why am I getting this "register_frame_info not found" error on Solaris? |
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After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib |
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generates an error such as: |
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ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so: |
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symbol __register_frame_info: referenced symbol not found |
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The symbol __register_frame_info is not part of zlib, it is generated by |
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the C compiler (cc or gcc). You must recompile applications using zlib |
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which have this problem. This problem is specific to Solaris. See |
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http://www.sunfreeware.com for Solaris versions of zlib and applications |
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using zlib. |
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18. Why does gzip give an error on a file I make with compress/deflate? |
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The compress and deflate functions produce data in the zlib format, which |
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is different and incompatible with the gzip format. The gz* functions in |
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zlib on the other hand use the gzip format. Both the zlib and gzip formats |
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use the same compressed data format internally, but have different headers |
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and trailers around the compressed data. |
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19. Ok, so why are there two different formats? |
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The gzip format was designed to retain the directory information about a |
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single file, such as the name and last modification date. The zlib format |
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on the other hand was designed for in-memory and communication channel |
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applications, and has a much more compact header and trailer and uses a |
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faster integrity check than gzip. |
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20. Well that's nice, but how do I make a gzip file in memory? |
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You can request that deflate write the gzip format instead of the zlib |
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format using deflateInit2(). You can also request that inflate decode the |
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gzip format using inflateInit2(). Read zlib.h for more details. |
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21. Is zlib thread-safe? |
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Yes. However any library routines that zlib uses and any application- |
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provided memory allocation routines must also be thread-safe. zlib's gz* |
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functions use stdio library routines, and most of zlib's functions use the |
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library memory allocation routines by default. zlib's *Init* functions |
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allow for the application to provide custom memory allocation routines. |
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If the non-default BUILDFIXED or DYNAMIC_CRC_TABLE defines are used on a |
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system without atomics (e.g. pre-C11), then inflate() and crc32() will not |
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be thread safe. |
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Of course, you should only operate on any given zlib or gzip stream from a |
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single thread at a time. |
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22. Can I use zlib in my commercial application? |
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Yes. Please read the license in zlib.h. |
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23. Is zlib under the GNU license? |
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No. Please read the license in zlib.h. |
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24. The license says that altered source versions must be "plainly marked". So |
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what exactly do I need to do to meet that requirement? |
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You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In |
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particular, the final version number needs to be changed to "f", and an |
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identification string should be appended to ZLIB_VERSION. Version numbers |
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x.x.x.f are reserved for modifications to zlib by others than the zlib |
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maintainers. For example, if the version of the base zlib you are altering |
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is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and |
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ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also |
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update the version strings in deflate.c and inftrees.c. |
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For altered source distributions, you should also note the origin and |
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nature of the changes in zlib.h, as well as in ChangeLog and README, along |
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with the dates of the alterations. The origin should include at least your |
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name (or your company's name), and an email address to contact for help or |
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issues with the library. |
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Note that distributing a compiled zlib library along with zlib.h and |
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zconf.h is also a source distribution, and so you should change |
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ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes |
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in zlib.h as you would for a full source distribution. |
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25. Will zlib work on a big-endian or little-endian architecture, and can I |
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exchange compressed data between them? |
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Yes and yes. |
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26. Will zlib work on a 64-bit machine? |
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Yes. It has been tested on 64-bit machines, and has no dependence on any |
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data types being limited to 32-bits in length. If you have any |
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difficulties, please provide a complete problem report to [email protected] |
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27. Will zlib decompress data from the PKWare Data Compression Library? |
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No. The PKWare DCL uses a completely different compressed data format than |
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does PKZIP and zlib. However, you can look in zlib's contrib/blast |
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directory for a possible solution to your problem. |
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28. Can I access data randomly in a compressed stream? |
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No, not without some preparation. If when compressing you periodically use |
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Z_FULL_FLUSH, carefully write all the pending data at those points, and |
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keep an index of those locations, then you can start decompression at those |
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points. You have to be careful to not use Z_FULL_FLUSH too often, since it |
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can significantly degrade compression. Alternatively, you can scan a |
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deflate stream once to generate an index, and then use that index for |
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random access. See examples/zran.c . |
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29. Does zlib work on MVS, OS/390, CICS, etc.? |
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It has in the past, but we have not heard of any recent evidence. There |
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were working ports of zlib 1.1.4 to MVS, but those links no longer work. |
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If you know of recent, successful applications of zlib on these operating |
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systems, please let us know. Thanks. |
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30. Is there some simpler, easier to read version of inflate I can look at to |
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understand the deflate format? |
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First off, you should read RFC 1951. Second, yes. Look in zlib's |
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contrib/puff directory. |
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31. Does zlib infringe on any patents? |
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As far as we know, no. In fact, that was originally the whole point behind |
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zlib. Look here for some more information: |
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https://web.archive.org/web/20180729212847/http://www.gzip.org/#faq11 |
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32. Can zlib work with greater than 4 GB of data? |
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Yes. inflate() and deflate() will process any amount of data correctly. |
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Each call of inflate() or deflate() is limited to input and output chunks |
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of the maximum value that can be stored in the compiler's "unsigned int" |
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type, but there is no limit to the number of chunks. Note however that the |
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strm.total_in and strm_total_out counters may be limited to 4 GB. These |
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counters are provided as a convenience and are not used internally by |
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inflate() or deflate(). The application can easily set up its own counters |
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updated after each call of inflate() or deflate() to count beyond 4 GB. |
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compress() and uncompress() may be limited to 4 GB, since they operate in a |
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single call. gzseek() and gztell() may be limited to 4 GB depending on how |
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zlib is compiled. See the zlibCompileFlags() function in zlib.h. |
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The word "may" appears several times above since there is a 4 GB limit only |
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if the compiler's "long" type is 32 bits. If the compiler's "long" type is |
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64 bits, then the limit is 16 exabytes. |
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33. Does zlib have any security vulnerabilities? |
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The only one that we are aware of is potentially in gzprintf(). If zlib is |
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compiled to use sprintf() or vsprintf(), which requires that ZLIB_INSECURE |
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be defined, then there is no protection against a buffer overflow of an 8K |
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string space (or other value as set by gzbuffer()), other than the caller |
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of gzprintf() assuring that the output will not exceed 8K. On the other |
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hand, if zlib is compiled to use snprintf() or vsnprintf(), which should |
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normally be the case, then there is no vulnerability. The ./configure |
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script will display warnings if an insecure variation of sprintf() will be |
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used by gzprintf(). Also the zlibCompileFlags() function will return |
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information on what variant of sprintf() is used by gzprintf(). |
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If you don't have snprintf() or vsnprintf() and would like one, you can |
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find a good portable implementation in stb_sprintf.h here: |
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https://github.com/nothings/stb |
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Note that you should be using the most recent version of zlib. Versions |
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1.1.3 and before were subject to a double-free vulnerability, and versions |
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1.2.1 and 1.2.2 were subject to an access exception when decompressing |
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invalid compressed data. |
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34. Is there a Java version of zlib? |
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Probably what you want is to use zlib in Java. zlib is already included |
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as part of the Java SDK in the java.util.zip package. If you really want |
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a version of zlib written in the Java language, look on the zlib home |
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page for links: https://zlib.net/ . |
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35. I get this or that compiler or source-code scanner warning when I crank it |
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up to maximally-pedantic. Can't you guys write proper code? |
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Many years ago, we gave up attempting to avoid warnings on every compiler |
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in the universe. It just got to be a waste of time, and some compilers |
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were downright silly as well as contradicted each other. So now, we simply |
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make sure that the code always works. |
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36. Valgrind (or some similar memory access checker) says that deflate is |
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performing a conditional jump that depends on an uninitialized value. |
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Isn't that a bug? |
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No. That is intentional for performance reasons, and the output of deflate |
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is not affected. This only started showing up recently since zlib 1.2.x |
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uses malloc() by default for allocations, whereas earlier versions used |
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calloc(), which zeros out the allocated memory. Even though the code was |
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correct, versions 1.2.4 and later was changed to not stimulate these |
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checkers. |
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37. Will zlib read the (insert any ancient or arcane format here) compressed |
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data format? |
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Probably not. Look in the comp.compression FAQ for pointers to various |
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formats and associated software. |
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38. How can I encrypt/decrypt zip files with zlib? |
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zlib doesn't support encryption. The original PKZIP encryption is very |
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weak and can be broken with freely available programs. To get strong |
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encryption, use GnuPG, https://www.gnupg.org/ , which already includes zlib |
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compression. For PKZIP compatible "encryption", look at |
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https://infozip.sourceforge.net/ |
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39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings? |
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"gzip" is the gzip format, and "deflate" is the zlib format. They should |
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probably have called the second one "zlib" instead to avoid confusion with |
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the raw deflate compressed data format. While the HTTP 1.1 RFC 2616 |
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correctly points to the zlib specification in RFC 1950 for the "deflate" |
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transfer encoding, there have been reports of servers and browsers that |
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incorrectly produce or expect raw deflate data per the deflate |
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specification in RFC 1951, most notably Microsoft. So even though the |
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"deflate" transfer encoding using the zlib format would be the more |
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efficient approach (and in fact exactly what the zlib format was designed |
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for), using the "gzip" transfer encoding is probably more reliable due to |
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an unfortunate choice of name on the part of the HTTP 1.1 authors. |
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Bottom line: use the gzip format for HTTP 1.1 encoding. |
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40. Does zlib support the new "Deflate64" format introduced by PKWare? |
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No. PKWare has apparently decided to keep that format proprietary, since |
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they have not documented it as they have previous compression formats. In |
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any case, the compression improvements are so modest compared to other more |
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modern approaches, that it's not worth the effort to implement. |
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41. I'm having a problem with the zip functions in zlib, can you help? |
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There are no zip functions in zlib. You are probably using minizip by |
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Giles Vollant, which is found in the contrib directory of zlib. It is not |
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part of zlib. In fact none of the stuff in contrib is part of zlib. The |
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files in there are not supported by the zlib authors. You need to contact |
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the authors of the respective contribution for help. |
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42. The match.asm code in contrib is under the GNU General Public License. |
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Since it's part of zlib, doesn't that mean that all of zlib falls under the |
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GNU GPL? |
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No. The files in contrib are not part of zlib. They were contributed by |
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other authors and are provided as a convenience to the user within the zlib |
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distribution. Each item in contrib has its own license. |
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364
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43. Is zlib subject to export controls? What is its ECCN? |
|
365
|
|
|
366
|
zlib is not subject to export controls, and so is classified as EAR99. |
|
367
|
|
|
368
|
44. Can you please sign these lengthy legal documents and fax them back to us |
|
369
|
so that we can use your software in our product? |
|
370
|
|
|
371
|
No. Go away. Shoo. |
|
372
|
|