reactos/lib/3rdparty/zlib/FAQ

   1
   2                 Frequently Asked Questions about zlib
   3
   4
   5 If your question is not there, please check the zlib home page
   6 http://www.zlib.org which may have more recent information.
   7 The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html
   8
   9
  10  1. Is zlib Y2K-compliant?
  11
  12     Yes. zlib doesn't handle dates.
  13
  14  2. Where can I get a Windows DLL version?
  15
  16     The zlib sources can be compiled without change to produce a DLL.
  17     See the file win32/DLL_FAQ.txt in the zlib distribution.
  18     Pointers to the precompiled DLL are found in the zlib web site at
  19     http://www.zlib.org.
  20
  21  3. Where can I get a Visual Basic interface to zlib?
  22
  23     See
  24         * http://www.dogma.net/markn/articles/zlibtool/zlibtool.htm
  25         * contrib/visual-basic.txt in the zlib distribution
  26         * win32/DLL_FAQ.txt in the zlib distribution
  27
  28  4. compress() returns Z_BUF_ERROR.
  29
  30     Make sure that before the call of compress, the length of the compressed
  31     buffer is equal to the total size of the compressed buffer and not
  32     zero. For Visual Basic, check that this parameter is passed by reference
  33     ("as any"), not by value ("as long").
  34
  35  5. deflate() or inflate() returns Z_BUF_ERROR.
  36
  37     Before making the call, make sure that avail_in and avail_out are not
  38     zero. When setting the parameter flush equal to Z_FINISH, also make sure
  39     that avail_out is big enough to allow processing all pending input.
  40     Note that a Z_BUF_ERROR is not fatal--another call to deflate() or
  41     inflate() can be made with more input or output space. A Z_BUF_ERROR
  42     may in fact be unavoidable depending on how the functions are used, since
  43     it is not possible to tell whether or not there is more output pending
  44     when strm.avail_out returns with zero.
  45
  46  6. Where's the zlib documentation (man pages, etc.)?
  47
  48     It's in zlib.h for the moment, and Francis S. Lin has converted it to a
  49     web page zlib.html. Volunteers to transform this to Unix-style man pages,
  50     please contact us (zlib@gzip.org). Examples of zlib usage are in the files
  51     example.c and minigzip.c.
  52
  53  7. Why don't you use GNU autoconf or libtool or ...?
  54
  55     Because we would like to keep zlib as a very small and simple
  56     package. zlib is rather portable and doesn't need much configuration.
  57
  58  8. I found a bug in zlib.
  59
  60     Most of the time, such problems are due to an incorrect usage of
  61     zlib. Please try to reproduce the problem with a small program and send
  62     the corresponding source to us at zlib@gzip.org . Do not send
  63     multi-megabyte data files without prior agreement.
  64
  65  9. Why do I get "undefined reference to gzputc"?
  66
  67     If "make test" produces something like
  68
  69        example.o(.text+0x154): undefined reference to `gzputc'
  70
  71     check that you don't have old files libz.* in /usr/lib, /usr/local/lib or
  72     /usr/X11R6/lib. Remove any old versions, then do "make install".
  73
  74 10. I need a Delphi interface to zlib.
  75
  76     See the contrib/delphi directory in the zlib distribution.
  77
  78 11. Can zlib handle .zip archives?
  79
  80     Not by itself, no.  See the directory contrib/minizip in the zlib
  81     distribution.
  82
  83 12. Can zlib handle .Z files?
  84
  85     No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt
  86     the code of uncompress on your own.
  87
  88 13. How can I make a Unix shared library?
  89
  90     make clean
  91     ./configure -s
  92     make
  93
  94 14. How do I install a shared zlib library on Unix?
  95
  96     After the above, then:
  97
  98     make install
  99
 100     However, many flavors of Unix come with a shared zlib already installed.
 101     Before going to the trouble of compiling a shared version of zlib and
 102     trying to install it, you may want to check if it's already there! If you
 103     can #include <zlib.h>, it's there. The -lz option will probably link to it.
 104
 105 15. I have a question about OttoPDF.
 106
 107     We are not the authors of OttoPDF. The real author is on the OttoPDF web
 108     site: Joel Hainley, jhainley@myndkryme.com.
 109
 110 16. Can zlib decode Flate data in an Adobe PDF file?
 111
 112     Yes. See http://www.fastio.com/ (ClibPDF), or http://www.pdflib.com/ .
 113     To modify PDF forms, see http://sourceforge.net/projects/acroformtool/ .
 114
 115 17. Why am I getting this "register_frame_info not found" error on Solaris?
 116
 117     After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
 118     generates an error such as:
 119
 120         ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
 121         symbol __register_frame_info: referenced symbol not found
 122
 123     The symbol __register_frame_info is not part of zlib, it is generated by
 124     the C compiler (cc or gcc). You must recompile applications using zlib
 125     which have this problem. This problem is specific to Solaris. See
 126     http://www.sunfreeware.com for Solaris versions of zlib and applications
 127     using zlib.
 128
 129 18. Why does gzip give an error on a file I make with compress/deflate?
 130
 131     The compress and deflate functions produce data in the zlib format, which
 132     is different and incompatible with the gzip format. The gz* functions in
 133     zlib on the other hand use the gzip format. Both the zlib and gzip
 134     formats use the same compressed data format internally, but have different
 135     headers and trailers around the compressed data.
 136
 137 19. Ok, so why are there two different formats?
 138
 139     The gzip format was designed to retain the directory information about
 140     a single file, such as the name and last modification date. The zlib
 141     format on the other hand was designed for in-memory and communication
 142     channel applications, and has a much more compact header and trailer and
 143     uses a faster integrity check than gzip.
 144
 145 20. Well that's nice, but how do I make a gzip file in memory?
 146
 147     You can request that deflate write the gzip format instead of the zlib
 148     format using deflateInit2(). You can also request that inflate decode
 149     the gzip format using inflateInit2(). Read zlib.h for more details.
 150
 151 21. Is zlib thread-safe?
 152
 153     Yes. However any library routines that zlib uses and any application-
 154     provided memory allocation routines must also be thread-safe. zlib's gz*
 155     functions use stdio library routines, and most of zlib's functions use the
 156     library memory allocation routines by default. zlib's Init functions allow
 157     for the application to provide custom memory allocation routines.
 158
 159     Of course, you should only operate on any given zlib or gzip stream from a
 160     single thread at a time.
 161
 162 22. Can I use zlib in my commercial application?
 163
 164     Yes. Please read the license in zlib.h.
 165
 166 23. Is zlib under the GNU license?
 167
 168     No. Please read the license in zlib.h.
 169
 170 24. The license says that altered source versions must be "plainly marked". So
 171     what exactly do I need to do to meet that requirement?
 172
 173     You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In
 174     particular, the final version number needs to be changed to "f", and an
 175     identification string should be appended to ZLIB_VERSION. Version numbers
 176     x.x.x.f are reserved for modifications to zlib by others than the zlib
 177     maintainers. For example, if the version of the base zlib you are altering
 178     is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
 179     ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also
 180     update the version strings in deflate.c and inftrees.c.
 181
 182     For altered source distributions, you should also note the origin and
 183     nature of the changes in zlib.h, as well as in ChangeLog and README, along
 184     with the dates of the alterations. The origin should include at least your
 185     name (or your company's name), and an email address to contact for help or
 186     issues with the library.
 187
 188     Note that distributing a compiled zlib library along with zlib.h and
 189     zconf.h is also a source distribution, and so you should change
 190     ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes
 191     in zlib.h as you would for a full source distribution.
 192
 193 25. Will zlib work on a big-endian or little-endian architecture, and can I
 194     exchange compressed data between them?
 195
 196     Yes and yes.
 197
 198 26. Will zlib work on a 64-bit machine?
 199
 200     It should. It has been tested on 64-bit machines, and has no dependence
 201     on any data types being limited to 32-bits in length. If you have any
 202     difficulties, please provide a complete problem report to zlib@gzip.org
 203
 204 27. Will zlib decompress data from the PKWare Data Compression Library?
 205
 206     No. The PKWare DCL uses a completely different compressed data format
 207     than does PKZIP and zlib. However, you can look in zlib's contrib/blast
 208     directory for a possible solution to your problem.
 209
 210 28. Can I access data randomly in a compressed stream?
 211
 212     No, not without some preparation. If when compressing you periodically
 213     use Z_FULL_FLUSH, carefully write all the pending data at those points,
 214     and keep an index of those locations, then you can start decompression
 215     at those points. You have to be careful to not use Z_FULL_FLUSH too
 216     often, since it can significantly degrade compression.
 217
 218 29. Does zlib work on MVS, OS/390, CICS, etc.?
 219
 220     We don't know for sure. We have heard occasional reports of success on
 221     these systems. If you do use it on one of these, please provide us with
 222     a report, instructions, and patches that we can reference when we get
 223     these questions. Thanks.
 224
 225 30. Is there some simpler, easier to read version of inflate I can look at
 226     to understand the deflate format?
 227
 228     First off, you should read RFC 1951. Second, yes. Look in zlib's
 229     contrib/puff directory.
 230
 231 31. Does zlib infringe on any patents?
 232
 233     As far as we know, no. In fact, that was originally the whole point behind
 234     zlib. Look here for some more information:
 235
 236     http://www.gzip.org/#faq11
 237
 238 32. Can zlib work with greater than 4 GB of data?
 239
 240     Yes. inflate() and deflate() will process any amount of data correctly.
 241     Each call of inflate() or deflate() is limited to input and output chunks
 242     of the maximum value that can be stored in the compiler's "unsigned int"
 243     type, but there is no limit to the number of chunks. Note however that the
 244     strm.total_in and strm_total_out counters may be limited to 4 GB. These
 245     counters are provided as a convenience and are not used internally by
 246     inflate() or deflate(). The application can easily set up its own counters
 247     updated after each call of inflate() or deflate() to count beyond 4 GB.
 248     compress() and uncompress() may be limited to 4 GB, since they operate in a
 249     single call. gzseek() and gztell() may be limited to 4 GB depending on how
 250     zlib is compiled. See the zlibCompileFlags() function in zlib.h.
 251
 252     The word "may" appears several times above since there is a 4 GB limit
 253     only if the compiler's "long" type is 32 bits. If the compiler's "long"
 254     type is 64 bits, then the limit is 16 exabytes.
 255
 256 33. Does zlib have any security vulnerabilities?
 257
 258     The only one that we are aware of is potentially in gzprintf(). If zlib
 259     is compiled to use sprintf() or vsprintf(), then there is no protection
 260     against a buffer overflow of a 4K string space, other than the caller of
 261     gzprintf() assuring that the output will not exceed 4K. On the other
 262     hand, if zlib is compiled to use snprintf() or vsnprintf(), which should
 263     normally be the case, then there is no vulnerability. The ./configure
 264     script will display warnings if an insecure variation of sprintf() will
 265     be used by gzprintf(). Also the zlibCompileFlags() function will return
 266     information on what variant of sprintf() is used by gzprintf().
 267
 268     If you don't have snprintf() or vsnprintf() and would like one, you can
 269     find a portable implementation here:
 270
 271         http://www.ijs.si/software/snprintf/
 272
 273     Note that you should be using the most recent version of zlib. Versions
 274     1.1.3 and before were subject to a double-free vulnerability.
 275
 276 34. Is there a Java version of zlib?
 277
 278     Probably what you want is to use zlib in Java. zlib is already included
 279     as part of the Java SDK in the java.util.zip package. If you really want
 280     a version of zlib written in the Java language, look on the zlib home
 281     page for links: http://www.zlib.org/
 282
 283 35. I get this or that compiler or source-code scanner warning when I crank it
 284     up to maximally-pedantic. Can't you guys write proper code?
 285
 286     Many years ago, we gave up attempting to avoid warnings on every compiler
 287     in the universe. It just got to be a waste of time, and some compilers
 288     were downright silly. So now, we simply make sure that the code always
 289     works.
 290
 291 36. Valgrind (or some similar memory access checker) says that deflate is
 292     performing a conditional jump that depends on an uninitialized value.
 293     Isn't that a bug?
 294
 295     No.  That is intentional for performance reasons, and the output of
 296     deflate is not affected.  This only started showing up recently since
 297     zlib 1.2.x uses malloc() by default for allocations, whereas earlier
 298     versions used calloc(), which zeros out the allocated memory.
 299
 300 37. Will zlib read the (insert any ancient or arcane format here) compressed
 301     data format?
 302
 303     Probably not. Look in the comp.compression FAQ for pointers to various
 304     formats and associated software.
 305
 306 38. How can I encrypt/decrypt zip files with zlib?
 307
 308     zlib doesn't support encryption. The original PKZIP encryption is very weak
 309     and can be broken with freely available programs. To get strong encryption,
 310     use GnuPG, http://www.gnupg.org/ , which already includes zlib compression.
 311     For PKZIP compatible "encryption", look at http://www.info-zip.org/
 312
 313 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
 314
 315     "gzip" is the gzip format, and "deflate" is the zlib format. They should
 316     probably have called the second one "zlib" instead to avoid confusion
 317     with the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
 318     correctly points to the zlib specification in RFC 1950 for the "deflate"
 319     transfer encoding, there have been reports of servers and browsers that
 320     incorrectly produce or expect raw deflate data per the deflate
 321     specficiation in RFC 1951, most notably Microsoft. So even though the
 322     "deflate" transfer encoding using the zlib format would be the more
 323     efficient approach (and in fact exactly what the zlib format was designed
 324     for), using the "gzip" transfer encoding is probably more reliable due to
 325     an unfortunate choice of name on the part of the HTTP 1.1 authors.
 326
 327     Bottom line: use the gzip format for HTTP 1.1 encoding.
 328
 329 40. Does zlib support the new "Deflate64" format introduced by PKWare?
 330
 331     No. PKWare has apparently decided to keep that format proprietary, since
 332     they have not documented it as they have previous compression formats.
 333     In any case, the compression improvements are so modest compared to other
 334     more modern approaches, that it's not worth the effort to implement.
 335
 336 41. Can you please sign these lengthy legal documents and fax them back to us
 337     so that we can use your software in our product?
 338
 339     No. Go away. Shoo.