Mercurial > flash_v2
annotate packages/services/compress/zlib/current/src/FAQ @ 1651:91a37e2314f4
Upgrade to zlib-1.2.1
| author | gthomas |
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| date | Mon, 24 May 2004 19:33:34 +0000 |
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| 1651 | 2 Frequently Asked Questions about zlib |
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| 1651 | 5 If your question is not there, please check the zlib home page |
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6 http://www.zlib.org which may have more recent information. |
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7 The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html |
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8 |
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9 |
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10 1. Is zlib Y2K-compliant? |
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11 |
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12 Yes. zlib doesn't handle dates. |
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13 |
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14 2. Where can I get a Windows DLL version? |
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| 1651 | 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. | |
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20 |
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21 3. Where can I get a Visual Basic interface to zlib? |
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23 See |
| 1651 | 24 * http://www.winimage.com/zLibDll/ |
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25 * http://www.dogma.net/markn/articles/zlibtool/zlibtool.htm |
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26 * contrib/visual-basic.txt in the zlib distribution |
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27 |
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28 4. compress() returns Z_BUF_ERROR |
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29 |
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30 Make sure that before the call of compress, the length of the compressed |
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31 buffer is equal to the total size of the compressed buffer and not |
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32 zero. For Visual Basic, check that this parameter is passed by reference |
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33 ("as any"), not by value ("as long"). |
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34 |
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35 5. deflate() or inflate() returns Z_BUF_ERROR |
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36 |
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37 Before making the call, make sure that avail_in and avail_out are not |
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38 zero. When setting the parameter flush equal to Z_FINISH, also make sure |
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39 that avail_out is big enough to allow processing all pending input. |
| 1651 | 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. | |
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45 |
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46 6. Where's the zlib documentation (man pages, etc.)? |
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47 |
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48 It's in zlib.h for the moment, and Francis S. Lin has converted it to a |
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49 web page zlib.html. Volunteers to transform this to Unix-style man pages, |
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50 please contact Jean-loup Gailly (jloup@gzip.org). Examples of zlib usage |
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51 are in the files example.c and minigzip.c. |
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52 |
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53 7. Why don't you use GNU autoconf or libtool or ...? |
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54 |
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55 Because we would like to keep zlib as a very small and simple |
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56 package. zlib is rather portable and doesn't need much configuration. |
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57 |
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58 8. I found a bug in zlib. |
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59 |
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60 Most of the time, such problems are due to an incorrect usage of |
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61 zlib. Please try to reproduce the problem with a small program and send |
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62 the corresponding source to us at zlib@gzip.org . Do not send |
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63 multi-megabyte data files without prior agreement. |
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64 |
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65 9. Why do I get "undefined reference to gzputc"? |
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66 |
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67 If "make test" produces something like |
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68 |
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69 example.o(.text+0x154): undefined reference to `gzputc' |
| 1651 | 70 |
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71 check that you don't have old files libz.* in /usr/lib, /usr/local/lib or |
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72 /usr/X11R6/lib. Remove any old versions, then do "make install". |
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73 |
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74 10. I need a Delphi interface to zlib. |
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75 |
| 1651 | 76 See the contrib/delphi directory in the zlib distribution. |
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77 |
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78 11. Can zlib handle .zip archives? |
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79 |
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80 See the directory contrib/minizip in the zlib distribution. |
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81 |
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82 12. Can zlib handle .Z files? |
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83 |
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84 No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt |
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85 the code of uncompress on your own. |
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86 |
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87 13. How can I make a Unix shared library? |
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88 |
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89 make clean |
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90 ./configure -s |
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91 make |
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92 |
| 1651 | 93 14. How do I install a shared zlib library on Unix? |
| 94 | |
| 95 make install | |
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96 |
| 1651 | 97 However, many flavors of Unix come with a shared zlib already installed. |
| 98 Before going to the trouble of compiling a shared version of zlib and | |
| 99 trying to install it, you may want to check if it's already there! If you | |
| 100 can #include <zlib.h>, it's there. The -lz option will probably link to it. | |
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101 |
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102 15. I have a question about OttoPDF |
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103 |
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104 We are not the authors of OttoPDF. The real author is on the OttoPDF web |
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105 site Joel Hainley jhainley@myndkryme.com. |
| 1651 | 106 |
| 107 16. Why does gzip give an error on a file I make with compress/deflate? | |
| 108 | |
| 109 The compress and deflate functions produce data in the zlib format, which | |
| 110 is different and incompatible with the gzip format. The gz* functions in | |
| 111 zlib on the other hand use the gzip format. Both the zlib and gzip | |
| 112 formats use the same compressed data format internally, but have different | |
| 113 headers and trailers around the compressed data. | |
| 114 | |
| 115 17. Ok, so why are there two different formats? | |
| 116 | |
| 117 The gzip format was designed to retain the directory information about | |
| 118 a single file, such as the name and last modification date. The zlib | |
| 119 format on the other hand was designed for in-memory and communication | |
| 120 channel applications, and has a much more compact header and trailer and | |
| 121 uses a faster integrity check than gzip. | |
| 122 | |
| 123 18. Well that's nice, but how do I make a gzip file in memory? | |
| 124 | |
| 125 You can request that deflate write the gzip format instead of the zlib | |
| 126 format using deflateInit2(). You can also request that inflate decode | |
| 127 the gzip format using inflateInit2(). Read zlib.h for more details. | |
| 128 | |
| 129 Note that you cannot specify special gzip header contents (e.g. a file | |
| 130 name or modification date), nor will inflate tell you what was in the | |
| 131 gzip header. If you need to customize the header or see what's in it, | |
| 132 you can use the raw deflate and inflate operations and the crc32() | |
| 133 function and roll your own gzip encoding and decoding. Read the gzip | |
| 134 RFC 1952 for details of the header and trailer format. | |
| 135 | |
| 136 19. Is zlib thread-safe? | |
| 137 | |
| 138 Yes. However any library routines that zlib uses and any application- | |
| 139 provided memory allocation routines must also be thread-safe. zlib's gz* | |
| 140 functions use stdio library routines, and most of zlib's functions use the | |
| 141 library memory allocation routines by default. zlib's Init functions allow | |
| 142 for the application to provide custom memory allocation routines. | |
| 143 | |
| 144 Of course, you should only operate on any given zlib or gzip stream from a | |
| 145 single thread at a time. | |
| 146 | |
| 147 20. Can I use zlib in my commercial application? | |
| 148 | |
| 149 Yes. Please read the license in zlib.h. | |
| 150 | |
| 151 21. Is zlib under the GNU license? | |
| 152 | |
| 153 No. Please read the license in zlib.h. | |
| 154 | |
| 155 22. The license says that altered source versions must be "plainly marked". So | |
| 156 what exactly do I need to do to meet that requirement? | |
| 157 | |
| 158 You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In | |
| 159 particular, the final version number needs to be changed to "f", and an | |
| 160 identification string should be appended to ZLIB_VERSION. Version numbers | |
| 161 x.x.x.f are reserved for modifications to zlib by others than the zlib | |
| 162 maintainers. For example, if the version of the base zlib you are altering | |
| 163 is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and | |
| 164 ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also | |
| 165 update the version strings in deflate.c and inftrees.c. | |
| 166 | |
| 167 For altered source distributions, you should also note the origin and | |
| 168 nature of the changes in zlib.h, as well as in ChangeLog and README, along | |
| 169 with the dates of the alterations. The origin should include at least your | |
| 170 name (or your company's name), and an email address to contact for help or | |
| 171 issues with the library. | |
| 172 | |
| 173 Note that distributing a compiled zlib library along with zlib.h and | |
| 174 zconf.h is also a source distribution, and so you should change | |
| 175 ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes | |
| 176 in zlib.h as you would for a full source distribution. | |
| 177 | |
| 178 23. Will zlib work on a big-endian or little-endian architecture, and can I | |
| 179 exchange compressed data between them? | |
| 180 | |
| 181 Yes and yes. | |
| 182 | |
| 183 24. Will zlib work on a 64-bit machine? | |
| 184 | |
| 185 It should. It has been tested on 64-bit machines, and has no dependence | |
| 186 on any data types being limited to 32-bits in length. If you have any | |
| 187 difficulties, please provide a complete problem report to zlib@gzip.org | |
| 188 | |
| 189 25. Will zlib decompress data from the PKWare Data Compression Library? | |
| 190 | |
| 191 No. The PKWare DCL uses a completely different compressed data format | |
| 192 than does PKZIP and zlib. However, you can look in zlib's contrib/blast | |
| 193 directory for a possible solution to your problem. | |
| 194 | |
| 195 26. Can I access data randomly in a compressed stream? | |
| 196 | |
| 197 No, not without some preparation. If when compressing you periodically | |
| 198 use Z_FULL_FLUSH, carefully write all the pending data at those points, | |
| 199 and keep an index of those locations, then you can start decompression | |
| 200 at those points. You have to be careful to not use Z_FULL_FLUSH too | |
| 201 often, since it can significantly degrade compression. | |
| 202 | |
| 203 27. Does zlib work on MVS, OS/390, CICS, etc.? | |
| 204 | |
| 205 We don't know for sure. We have heard occasional reports of success on | |
| 206 these systems. If you do use it on one of these, please provide us with | |
| 207 a report, instructions, and patches that we can reference when we get | |
| 208 these questions. Thanks. | |
| 209 | |
| 210 28. Is there some simpler, easier to read version of inflate I can look at | |
| 211 to understand the deflate format? | |
| 212 | |
| 213 First off, you should read RFC 1951. Second, yes. Look in zlib's | |
| 214 contrib/puff directory. | |
| 215 | |
| 216 29. Does zlib infringe on any patents? | |
| 217 | |
| 218 As far as we know, no. In fact, that was originally the whole point behind | |
| 219 zlib. Look here for some more information: | |
| 220 | |
| 221 http://www.gzip.org/#faq11 | |
| 222 | |
| 223 30. Can zlib work with greater than 4 GB of data? | |
| 224 | |
| 225 Yes. inflate() and deflate() will process any amount of data correctly. | |
| 226 Each call of inflate() or deflate() is limited to input and output chunks | |
| 227 of the maximum value that can be stored in the compiler's "unsigned int" | |
| 228 type, but there is no limit to the number of chunks. Note however that the | |
| 229 strm.total_in and strm_total_out counters may be limited to 4 GB. These | |
| 230 counters are provided as a convenience and are not used internally by | |
| 231 inflate() or deflate(). The application can easily set up its own counters | |
| 232 updated after each call of inflate() or deflate() to count beyond 4 GB. | |
| 233 compress() and uncompress() may be limited to 4 GB, since they operate in a | |
| 234 single call. gzseek() and gztell() may be limited to 4 GB depending on how | |
| 235 zlib is compiled. See the zlibCompileFlags() function in zlib.h. | |
| 236 | |
| 237 The word "may" appears several times above since there is a 4 GB limit | |
| 238 only if the compiler's "long" type is 32 bits. If the compiler's "long" | |
| 239 type is 64 bits, then the limit is 16 exabytes. | |
| 240 | |
| 241 31. Does zlib have any security vulnerabilities? | |
| 242 | |
| 243 The only one that we are aware of is potentially in gzprintf(). If zlib | |
| 244 is compiled to use sprintf() or vsprintf(), then there is no protection | |
| 245 against a buffer overflow of a 4K string space, other than the caller of | |
| 246 gzprintf() assuring that the output will not exceed 4K. On the other | |
| 247 hand, if zlib is compiled to use snprintf() or vsnprintf(), which should | |
| 248 normally be the case, then there is no vulnerability. The ./configure | |
| 249 script will display warnings if an insecure variation of sprintf() will | |
| 250 be used by gzprintf(). Also the zlibCompileFlags() function will return | |
| 251 information on what variant of sprintf() is used by gzprintf(). | |
| 252 | |
| 253 If you don't have snprintf() or vsnprintf() and would like one, you can | |
| 254 find a portable implementation here: | |
| 255 | |
| 256 http://www.ijs.si/software/snprintf/ | |
| 257 | |
| 258 Note that you should be using the most recent version of zlib. Versions | |
| 259 1.1.3 and before were subject to a double-free vulnerability. | |
| 260 | |
| 261 32. Is there a Java version of zlib? | |
| 262 | |
| 263 Probably what you want is to use zlib in Java. zlib is already included | |
| 264 as part of the Java SDK in the java.util.zip package. If you really want | |
| 265 a version of zlib written in the Java language, look on the zlib home | |
| 266 page for links: http://www.zlib.org/ | |
| 267 | |
| 268 33. I get this or that compiler or source-code scanner warning when I crank it | |
| 269 up to maximally-pendantic. Can't you guys write proper code? | |
| 270 | |
| 271 Many years ago, we gave up attempting to avoid warnings on every compiler | |
| 272 in the universe. It just got to be a waste of time, and some compilers | |
| 273 were downright silly. So now, we simply make sure that the code always | |
| 274 works. | |
| 275 | |
| 276 34. Will zlib read the (insert any ancient or arcane format here) compressed | |
| 277 data format? | |
| 278 | |
| 279 Probably not. Look in the comp.compression FAQ for pointers to various | |
| 280 formats and associated software. | |
| 281 | |
| 282 35. How can I encrypt/decrypt zip files with zlib? | |
| 283 | |
| 284 zlib doesn't support encryption. The original PKZIP encryption is very weak | |
| 285 and can be broken with freely available programs. To get strong encryption, | |
| 286 use gpg ( http://www.gnupg.org/ ) which already includes zlib compression. | |
| 287 For PKZIP compatible "encryption", look at http://www.info-zip.org/ | |
| 288 | |
| 289 36. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings? | |
| 290 | |
| 291 "gzip" is the gzip format, and "deflate" is the zlib format. They should | |
| 292 probably have called the second one "zlib" instead to avoid confusion | |
| 293 with the raw deflate compressed data format. While the HTTP 1.1 RFC 2616 | |
| 294 correctly points to the zlib specification in RFC 1950 for the "deflate" | |
| 295 transfer encoding, there have been reports of servers and browsers that | |
| 296 incorrectly produce or expect raw deflate data per the deflate | |
| 297 specficiation in RFC 1951, most notably Microsoft. So even though the | |
| 298 "deflate" transfer encoding using the zlib format would be the more | |
| 299 efficient approach (and in fact exactly what the zlib format was designed | |
| 300 for), using the "gzip" transfer encoding is probably more reliable due to | |
| 301 an unfortunate choice of name on the part of the HTTP 1.1 authors. | |
| 302 | |
| 303 Bottom line: use the gzip format for HTTP 1.1 encoding. | |
| 304 | |
| 305 37. Does zlib support the new "Deflate64" format introduced by PKWare? | |
| 306 | |
| 307 No. PKWare has apparently decided to keep that format proprietary, since | |
| 308 they have not documented it as they have previous compression formats. | |
| 309 In any case, the compression improvements are so modest compared to other | |
| 310 more modern approaches, that it's not worth the effort to implement. | |
| 311 | |
| 312 38. Can you please sign these lengthy legal documents and fax them back to us | |
| 313 so that we can use your software in our product? | |
| 314 | |
| 315 No. Go away. Shoo. |