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1.4 +
1.5 + Frequently Asked Questions about zlib
1.6 +
1.7 +
1.8 +If your question is not there, please check the zlib home page
1.9 +http://zlib.net/ which may have more recent information.
1.10 +The lastest zlib FAQ is at http://zlib.net/zlib_faq.html
1.11 +
1.12 +
1.13 + 1. Is zlib Y2K-compliant?
1.14 +
1.15 + Yes. zlib doesn't handle dates.
1.16 +
1.17 + 2. Where can I get a Windows DLL version?
1.18 +
1.19 + The zlib sources can be compiled without change to produce a DLL. See the
1.20 + file win32/DLL_FAQ.txt in the zlib distribution. Pointers to the
1.21 + precompiled DLL are found in the zlib web site at http://zlib.net/ .
1.22 +
1.23 + 3. Where can I get a Visual Basic interface to zlib?
1.24 +
1.25 + See
1.26 + * http://marknelson.us/1997/01/01/zlib-engine/
1.27 + * win32/DLL_FAQ.txt in the zlib distribution
1.28 +
1.29 + 4. compress() returns Z_BUF_ERROR.
1.30 +
1.31 + Make sure that before the call of compress(), the length of the compressed
1.32 + buffer is equal to the available size of the compressed buffer and not
1.33 + zero. For Visual Basic, check that this parameter is passed by reference
1.34 + ("as any"), not by value ("as long").
1.35 +
1.36 + 5. deflate() or inflate() returns Z_BUF_ERROR.
1.37 +
1.38 + Before making the call, make sure that avail_in and avail_out are not zero.
1.39 + When setting the parameter flush equal to Z_FINISH, also make sure that
1.40 + avail_out is big enough to allow processing all pending input. Note that a
1.41 + Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be
1.42 + made with more input or output space. A Z_BUF_ERROR may in fact be
1.43 + unavoidable depending on how the functions are used, since it is not
1.44 + possible to tell whether or not there is more output pending when
1.45 + strm.avail_out returns with zero. See http://zlib.net/zlib_how.html for a
1.46 + heavily annotated example.
1.47 +
1.48 + 6. Where's the zlib documentation (man pages, etc.)?
1.49 +
1.50 + It's in zlib.h . Examples of zlib usage are in the files test/example.c
1.51 + and test/minigzip.c, with more in examples/ .
1.52 +
1.53 + 7. Why don't you use GNU autoconf or libtool or ...?
1.54 +
1.55 + Because we would like to keep zlib as a very small and simple package.
1.56 + zlib is rather portable and doesn't need much configuration.
1.57 +
1.58 + 8. I found a bug in zlib.
1.59 +
1.60 + Most of the time, such problems are due to an incorrect usage of zlib.
1.61 + Please try to reproduce the problem with a small program and send the
1.62 + corresponding source to us at zlib@gzip.org . Do not send multi-megabyte
1.63 + data files without prior agreement.
1.64 +
1.65 + 9. Why do I get "undefined reference to gzputc"?
1.66 +
1.67 + If "make test" produces something like
1.68 +
1.69 + example.o(.text+0x154): undefined reference to `gzputc'
1.70 +
1.71 + check that you don't have old files libz.* in /usr/lib, /usr/local/lib or
1.72 + /usr/X11R6/lib. Remove any old versions, then do "make install".
1.73 +
1.74 +10. I need a Delphi interface to zlib.
1.75 +
1.76 + See the contrib/delphi directory in the zlib distribution.
1.77 +
1.78 +11. Can zlib handle .zip archives?
1.79 +
1.80 + Not by itself, no. See the directory contrib/minizip in the zlib
1.81 + distribution.
1.82 +
1.83 +12. Can zlib handle .Z files?
1.84 +
1.85 + No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt
1.86 + the code of uncompress on your own.
1.87 +
1.88 +13. How can I make a Unix shared library?
1.89 +
1.90 + By default a shared (and a static) library is built for Unix. So:
1.91 +
1.92 + make distclean
1.93 + ./configure
1.94 + make
1.95 +
1.96 +14. How do I install a shared zlib library on Unix?
1.97 +
1.98 + After the above, then:
1.99 +
1.100 + make install
1.101 +
1.102 + However, many flavors of Unix come with a shared zlib already installed.
1.103 + Before going to the trouble of compiling a shared version of zlib and
1.104 + trying to install it, you may want to check if it's already there! If you
1.105 + can #include <zlib.h>, it's there. The -lz option will probably link to
1.106 + it. You can check the version at the top of zlib.h or with the
1.107 + ZLIB_VERSION symbol defined in zlib.h .
1.108 +
1.109 +15. I have a question about OttoPDF.
1.110 +
1.111 + We are not the authors of OttoPDF. The real author is on the OttoPDF web
1.112 + site: Joel Hainley, jhainley@myndkryme.com.
1.113 +
1.114 +16. Can zlib decode Flate data in an Adobe PDF file?
1.115 +
1.116 + Yes. See http://www.pdflib.com/ . To modify PDF forms, see
1.117 + http://sourceforge.net/projects/acroformtool/ .
1.118 +
1.119 +17. Why am I getting this "register_frame_info not found" error on Solaris?
1.120 +
1.121 + After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
1.122 + generates an error such as:
1.123 +
1.124 + ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
1.125 + symbol __register_frame_info: referenced symbol not found
1.126 +
1.127 + The symbol __register_frame_info is not part of zlib, it is generated by
1.128 + the C compiler (cc or gcc). You must recompile applications using zlib
1.129 + which have this problem. This problem is specific to Solaris. See
1.130 + http://www.sunfreeware.com for Solaris versions of zlib and applications
1.131 + using zlib.
1.132 +
1.133 +18. Why does gzip give an error on a file I make with compress/deflate?
1.134 +
1.135 + The compress and deflate functions produce data in the zlib format, which
1.136 + is different and incompatible with the gzip format. The gz* functions in
1.137 + zlib on the other hand use the gzip format. Both the zlib and gzip formats
1.138 + use the same compressed data format internally, but have different headers
1.139 + and trailers around the compressed data.
1.140 +
1.141 +19. Ok, so why are there two different formats?
1.142 +
1.143 + The gzip format was designed to retain the directory information about a
1.144 + single file, such as the name and last modification date. The zlib format
1.145 + on the other hand was designed for in-memory and communication channel
1.146 + applications, and has a much more compact header and trailer and uses a
1.147 + faster integrity check than gzip.
1.148 +
1.149 +20. Well that's nice, but how do I make a gzip file in memory?
1.150 +
1.151 + You can request that deflate write the gzip format instead of the zlib
1.152 + format using deflateInit2(). You can also request that inflate decode the
1.153 + gzip format using inflateInit2(). Read zlib.h for more details.
1.154 +
1.155 +21. Is zlib thread-safe?
1.156 +
1.157 + Yes. However any library routines that zlib uses and any application-
1.158 + provided memory allocation routines must also be thread-safe. zlib's gz*
1.159 + functions use stdio library routines, and most of zlib's functions use the
1.160 + library memory allocation routines by default. zlib's *Init* functions
1.161 + allow for the application to provide custom memory allocation routines.
1.162 +
1.163 + Of course, you should only operate on any given zlib or gzip stream from a
1.164 + single thread at a time.
1.165 +
1.166 +22. Can I use zlib in my commercial application?
1.167 +
1.168 + Yes. Please read the license in zlib.h.
1.169 +
1.170 +23. Is zlib under the GNU license?
1.171 +
1.172 + No. Please read the license in zlib.h.
1.173 +
1.174 +24. The license says that altered source versions must be "plainly marked". So
1.175 + what exactly do I need to do to meet that requirement?
1.176 +
1.177 + You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In
1.178 + particular, the final version number needs to be changed to "f", and an
1.179 + identification string should be appended to ZLIB_VERSION. Version numbers
1.180 + x.x.x.f are reserved for modifications to zlib by others than the zlib
1.181 + maintainers. For example, if the version of the base zlib you are altering
1.182 + is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
1.183 + ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also
1.184 + update the version strings in deflate.c and inftrees.c.
1.185 +
1.186 + For altered source distributions, you should also note the origin and
1.187 + nature of the changes in zlib.h, as well as in ChangeLog and README, along
1.188 + with the dates of the alterations. The origin should include at least your
1.189 + name (or your company's name), and an email address to contact for help or
1.190 + issues with the library.
1.191 +
1.192 + Note that distributing a compiled zlib library along with zlib.h and
1.193 + zconf.h is also a source distribution, and so you should change
1.194 + ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes
1.195 + in zlib.h as you would for a full source distribution.
1.196 +
1.197 +25. Will zlib work on a big-endian or little-endian architecture, and can I
1.198 + exchange compressed data between them?
1.199 +
1.200 + Yes and yes.
1.201 +
1.202 +26. Will zlib work on a 64-bit machine?
1.203 +
1.204 + Yes. It has been tested on 64-bit machines, and has no dependence on any
1.205 + data types being limited to 32-bits in length. If you have any
1.206 + difficulties, please provide a complete problem report to zlib@gzip.org
1.207 +
1.208 +27. Will zlib decompress data from the PKWare Data Compression Library?
1.209 +
1.210 + No. The PKWare DCL uses a completely different compressed data format than
1.211 + does PKZIP and zlib. However, you can look in zlib's contrib/blast
1.212 + directory for a possible solution to your problem.
1.213 +
1.214 +28. Can I access data randomly in a compressed stream?
1.215 +
1.216 + No, not without some preparation. If when compressing you periodically use
1.217 + Z_FULL_FLUSH, carefully write all the pending data at those points, and
1.218 + keep an index of those locations, then you can start decompression at those
1.219 + points. You have to be careful to not use Z_FULL_FLUSH too often, since it
1.220 + can significantly degrade compression. Alternatively, you can scan a
1.221 + deflate stream once to generate an index, and then use that index for
1.222 + random access. See examples/zran.c .
1.223 +
1.224 +29. Does zlib work on MVS, OS/390, CICS, etc.?
1.225 +
1.226 + It has in the past, but we have not heard of any recent evidence. There
1.227 + were working ports of zlib 1.1.4 to MVS, but those links no longer work.
1.228 + If you know of recent, successful applications of zlib on these operating
1.229 + systems, please let us know. Thanks.
1.230 +
1.231 +30. Is there some simpler, easier to read version of inflate I can look at to
1.232 + understand the deflate format?
1.233 +
1.234 + First off, you should read RFC 1951. Second, yes. Look in zlib's
1.235 + contrib/puff directory.
1.236 +
1.237 +31. Does zlib infringe on any patents?
1.238 +
1.239 + As far as we know, no. In fact, that was originally the whole point behind
1.240 + zlib. Look here for some more information:
1.241 +
1.242 + http://www.gzip.org/#faq11
1.243 +
1.244 +32. Can zlib work with greater than 4 GB of data?
1.245 +
1.246 + Yes. inflate() and deflate() will process any amount of data correctly.
1.247 + Each call of inflate() or deflate() is limited to input and output chunks
1.248 + of the maximum value that can be stored in the compiler's "unsigned int"
1.249 + type, but there is no limit to the number of chunks. Note however that the
1.250 + strm.total_in and strm_total_out counters may be limited to 4 GB. These
1.251 + counters are provided as a convenience and are not used internally by
1.252 + inflate() or deflate(). The application can easily set up its own counters
1.253 + updated after each call of inflate() or deflate() to count beyond 4 GB.
1.254 + compress() and uncompress() may be limited to 4 GB, since they operate in a
1.255 + single call. gzseek() and gztell() may be limited to 4 GB depending on how
1.256 + zlib is compiled. See the zlibCompileFlags() function in zlib.h.
1.257 +
1.258 + The word "may" appears several times above since there is a 4 GB limit only
1.259 + if the compiler's "long" type is 32 bits. If the compiler's "long" type is
1.260 + 64 bits, then the limit is 16 exabytes.
1.261 +
1.262 +33. Does zlib have any security vulnerabilities?
1.263 +
1.264 + The only one that we are aware of is potentially in gzprintf(). If zlib is
1.265 + compiled to use sprintf() or vsprintf(), then there is no protection
1.266 + against a buffer overflow of an 8K string space (or other value as set by
1.267 + gzbuffer()), other than the caller of gzprintf() assuring that the output
1.268 + will not exceed 8K. On the other hand, if zlib is compiled to use
1.269 + snprintf() or vsnprintf(), which should normally be the case, then there is
1.270 + no vulnerability. The ./configure script will display warnings if an
1.271 + insecure variation of sprintf() will be used by gzprintf(). Also the
1.272 + zlibCompileFlags() function will return information on what variant of
1.273 + sprintf() is used by gzprintf().
1.274 +
1.275 + If you don't have snprintf() or vsnprintf() and would like one, you can
1.276 + find a portable implementation here:
1.277 +
1.278 + http://www.ijs.si/software/snprintf/
1.279 +
1.280 + Note that you should be using the most recent version of zlib. Versions
1.281 + 1.1.3 and before were subject to a double-free vulnerability, and versions
1.282 + 1.2.1 and 1.2.2 were subject to an access exception when decompressing
1.283 + invalid compressed data.
1.284 +
1.285 +34. Is there a Java version of zlib?
1.286 +
1.287 + Probably what you want is to use zlib in Java. zlib is already included
1.288 + as part of the Java SDK in the java.util.zip package. If you really want
1.289 + a version of zlib written in the Java language, look on the zlib home
1.290 + page for links: http://zlib.net/ .
1.291 +
1.292 +35. I get this or that compiler or source-code scanner warning when I crank it
1.293 + up to maximally-pedantic. Can't you guys write proper code?
1.294 +
1.295 + Many years ago, we gave up attempting to avoid warnings on every compiler
1.296 + in the universe. It just got to be a waste of time, and some compilers
1.297 + were downright silly as well as contradicted each other. So now, we simply
1.298 + make sure that the code always works.
1.299 +
1.300 +36. Valgrind (or some similar memory access checker) says that deflate is
1.301 + performing a conditional jump that depends on an uninitialized value.
1.302 + Isn't that a bug?
1.303 +
1.304 + No. That is intentional for performance reasons, and the output of deflate
1.305 + is not affected. This only started showing up recently since zlib 1.2.x
1.306 + uses malloc() by default for allocations, whereas earlier versions used
1.307 + calloc(), which zeros out the allocated memory. Even though the code was
1.308 + correct, versions 1.2.4 and later was changed to not stimulate these
1.309 + checkers.
1.310 +
1.311 +37. Will zlib read the (insert any ancient or arcane format here) compressed
1.312 + data format?
1.313 +
1.314 + Probably not. Look in the comp.compression FAQ for pointers to various
1.315 + formats and associated software.
1.316 +
1.317 +38. How can I encrypt/decrypt zip files with zlib?
1.318 +
1.319 + zlib doesn't support encryption. The original PKZIP encryption is very
1.320 + weak and can be broken with freely available programs. To get strong
1.321 + encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib
1.322 + compression. For PKZIP compatible "encryption", look at
1.323 + http://www.info-zip.org/
1.324 +
1.325 +39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
1.326 +
1.327 + "gzip" is the gzip format, and "deflate" is the zlib format. They should
1.328 + probably have called the second one "zlib" instead to avoid confusion with
1.329 + the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
1.330 + correctly points to the zlib specification in RFC 1950 for the "deflate"
1.331 + transfer encoding, there have been reports of servers and browsers that
1.332 + incorrectly produce or expect raw deflate data per the deflate
1.333 + specification in RFC 1951, most notably Microsoft. So even though the
1.334 + "deflate" transfer encoding using the zlib format would be the more
1.335 + efficient approach (and in fact exactly what the zlib format was designed
1.336 + for), using the "gzip" transfer encoding is probably more reliable due to
1.337 + an unfortunate choice of name on the part of the HTTP 1.1 authors.
1.338 +
1.339 + Bottom line: use the gzip format for HTTP 1.1 encoding.
1.340 +
1.341 +40. Does zlib support the new "Deflate64" format introduced by PKWare?
1.342 +
1.343 + No. PKWare has apparently decided to keep that format proprietary, since
1.344 + they have not documented it as they have previous compression formats. In
1.345 + any case, the compression improvements are so modest compared to other more
1.346 + modern approaches, that it's not worth the effort to implement.
1.347 +
1.348 +41. I'm having a problem with the zip functions in zlib, can you help?
1.349 +
1.350 + There are no zip functions in zlib. You are probably using minizip by
1.351 + Giles Vollant, which is found in the contrib directory of zlib. It is not
1.352 + part of zlib. In fact none of the stuff in contrib is part of zlib. The
1.353 + files in there are not supported by the zlib authors. You need to contact
1.354 + the authors of the respective contribution for help.
1.355 +
1.356 +42. The match.asm code in contrib is under the GNU General Public License.
1.357 + Since it's part of zlib, doesn't that mean that all of zlib falls under the
1.358 + GNU GPL?
1.359 +
1.360 + No. The files in contrib are not part of zlib. They were contributed by
1.361 + other authors and are provided as a convenience to the user within the zlib
1.362 + distribution. Each item in contrib has its own license.
1.363 +
1.364 +43. Is zlib subject to export controls? What is its ECCN?
1.365 +
1.366 + zlib is not subject to export controls, and so is classified as EAR99.
1.367 +
1.368 +44. Can you please sign these lengthy legal documents and fax them back to us
1.369 + so that we can use your software in our product?
1.370 +
1.371 + No. Go away. Shoo.
