| 1 | |
| 2 | Frequently Asked Questions about zlib |
| 3 | |
| 4 | |
| 5 | If your question is not there, please check the zlib home page |
| 6 | http://zlib.net/ which may have more recent information. |
| 7 | The latest zlib FAQ is at http://zlib.net/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. See the |
| 17 | file win32/DLL_FAQ.txt in the zlib distribution. |
| 18 | |
| 19 | 3. Where can I get a Visual Basic interface to zlib? |
| 20 | |
| 21 | See |
| 22 | * http://marknelson.us/1997/01/01/zlib-engine/ |
| 23 | * win32/DLL_FAQ.txt in the zlib distribution |
| 24 | |
| 25 | 4. compress() returns Z_BUF_ERROR. |
| 26 | |
| 27 | Make sure that before the call of compress(), the length of the compressed |
| 28 | buffer is equal to the available size of the compressed buffer and not |
| 29 | zero. For Visual Basic, check that this parameter is passed by reference |
| 30 | ("as any"), not by value ("as long"). |
| 31 | |
| 32 | 5. deflate() or inflate() returns Z_BUF_ERROR. |
| 33 | |
| 34 | Before making the call, make sure that avail_in and avail_out are not zero. |
| 35 | When setting the parameter flush equal to Z_FINISH, also make sure that |
| 36 | avail_out is big enough to allow processing all pending input. Note that a |
| 37 | Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be |
| 38 | made with more input or output space. A Z_BUF_ERROR may in fact be |
| 39 | unavoidable depending on how the functions are used, since it is not |
| 40 | possible to tell whether or not there is more output pending when |
| 41 | strm.avail_out returns with zero. See http://zlib.net/zlib_how.html for a |
| 42 | heavily annotated example. |
| 43 | |
| 44 | 6. Where's the zlib documentation (man pages, etc.)? |
| 45 | |
| 46 | It's in zlib.h . Examples of zlib usage are in the files test/example.c |
| 47 | and test/minigzip.c, with more in examples/ . |
| 48 | |
| 49 | 7. Why don't you use GNU autoconf or libtool or ...? |
| 50 | |
| 51 | Because we would like to keep zlib as a very small and simple package. |
| 52 | zlib is rather portable and doesn't need much configuration. |
| 53 | |
| 54 | 8. I found a bug in zlib. |
| 55 | |
| 56 | Most of the time, such problems are due to an incorrect usage of zlib. |
| 57 | Please try to reproduce the problem with a small program and send the |
| 58 | corresponding source to us at zlib@gzip.org . Do not send multi-megabyte |
| 59 | data files without prior agreement. |
| 60 | |
| 61 | 9. Why do I get "undefined reference to gzputc"? |
| 62 | |
| 63 | If "make test" produces something like |
| 64 | |
| 65 | example.o(.text+0x154): undefined reference to `gzputc' |
| 66 | |
| 67 | check that you don't have old files libz.* in /usr/lib, /usr/local/lib or |
| 68 | /usr/X11R6/lib. Remove any old versions, then do "make install". |
| 69 | |
| 70 | 10. I need a Delphi interface to zlib. |
| 71 | |
| 72 | See the contrib/delphi directory in the zlib distribution. |
| 73 | |
| 74 | 11. Can zlib handle .zip archives? |
| 75 | |
| 76 | Not by itself, no. See the directory contrib/minizip in the zlib |
| 77 | distribution. |
| 78 | |
| 79 | 12. Can zlib handle .Z files? |
| 80 | |
| 81 | No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt |
| 82 | the code of uncompress on your own. |
| 83 | |
| 84 | 13. How can I make a Unix shared library? |
| 85 | |
| 86 | By default a shared (and a static) library is built for Unix. So: |
| 87 | |
| 88 | make distclean |
| 89 | ./configure |
| 90 | make |
| 91 | |
| 92 | 14. How do I install a shared zlib library on Unix? |
| 93 | |
| 94 | After the above, then: |
| 95 | |
| 96 | make install |
| 97 | |
| 98 | However, many flavors of Unix come with a shared zlib already installed. |
| 99 | Before going to the trouble of compiling a shared version of zlib and |
| 100 | trying to install it, you may want to check if it's already there! If you |
| 101 | can #include <zlib.h>, it's there. The -lz option will probably link to |
| 102 | it. You can check the version at the top of zlib.h or with the |
| 103 | ZLIB_VERSION symbol defined in zlib.h . |
| 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.pdflib.com/ . To modify PDF forms, see |
| 113 | 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 formats |
| 134 | use the same compressed data format internally, but have different headers |
| 135 | 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 a |
| 140 | single file, such as the name and last modification date. The zlib format |
| 141 | on the other hand was designed for in-memory and communication channel |
| 142 | applications, and has a much more compact header and trailer and uses a |
| 143 | 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 the |
| 149 | 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 |
| 157 | allow 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 | Yes. It has been tested on 64-bit machines, and has no dependence on any |
| 201 | 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 than |
| 207 | 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 use |
| 213 | Z_FULL_FLUSH, carefully write all the pending data at those points, and |
| 214 | keep an index of those locations, then you can start decompression at those |
| 215 | points. You have to be careful to not use Z_FULL_FLUSH too often, since it |
| 216 | can significantly degrade compression. Alternatively, you can scan a |
| 217 | deflate stream once to generate an index, and then use that index for |
| 218 | random access. See examples/zran.c . |
| 219 | |
| 220 | 29. Does zlib work on MVS, OS/390, CICS, etc.? |
| 221 | |
| 222 | It has in the past, but we have not heard of any recent evidence. There |
| 223 | were working ports of zlib 1.1.4 to MVS, but those links no longer work. |
| 224 | If you know of recent, successful applications of zlib on these operating |
| 225 | systems, please let us know. Thanks. |
| 226 | |
| 227 | 30. Is there some simpler, easier to read version of inflate I can look at to |
| 228 | understand the deflate format? |
| 229 | |
| 230 | First off, you should read RFC 1951. Second, yes. Look in zlib's |
| 231 | contrib/puff directory. |
| 232 | |
| 233 | 31. Does zlib infringe on any patents? |
| 234 | |
| 235 | As far as we know, no. In fact, that was originally the whole point behind |
| 236 | zlib. Look here for some more information: |
| 237 | |
| 238 | http://www.gzip.org/#faq11 |
| 239 | |
| 240 | 32. Can zlib work with greater than 4 GB of data? |
| 241 | |
| 242 | Yes. inflate() and deflate() will process any amount of data correctly. |
| 243 | Each call of inflate() or deflate() is limited to input and output chunks |
| 244 | of the maximum value that can be stored in the compiler's "unsigned int" |
| 245 | type, but there is no limit to the number of chunks. Note however that the |
| 246 | strm.total_in and strm_total_out counters may be limited to 4 GB. These |
| 247 | counters are provided as a convenience and are not used internally by |
| 248 | inflate() or deflate(). The application can easily set up its own counters |
| 249 | updated after each call of inflate() or deflate() to count beyond 4 GB. |
| 250 | compress() and uncompress() may be limited to 4 GB, since they operate in a |
| 251 | single call. gzseek() and gztell() may be limited to 4 GB depending on how |
| 252 | zlib is compiled. See the zlibCompileFlags() function in zlib.h. |
| 253 | |
| 254 | The word "may" appears several times above since there is a 4 GB limit only |
| 255 | if the compiler's "long" type is 32 bits. If the compiler's "long" type is |
| 256 | 64 bits, then the limit is 16 exabytes. |
| 257 | |
| 258 | 33. Does zlib have any security vulnerabilities? |
| 259 | |
| 260 | The only one that we are aware of is potentially in gzprintf(). If zlib is |
| 261 | compiled to use sprintf() or vsprintf(), then there is no protection |
| 262 | against a buffer overflow of an 8K string space (or other value as set by |
| 263 | gzbuffer()), other than the caller of gzprintf() assuring that the output |
| 264 | will not exceed 8K. On the other hand, if zlib is compiled to use |
| 265 | snprintf() or vsnprintf(), which should normally be the case, then there is |
| 266 | no vulnerability. The ./configure script will display warnings if an |
| 267 | insecure variation of sprintf() will be used by gzprintf(). Also the |
| 268 | zlibCompileFlags() function will return information on what variant of |
| 269 | sprintf() is used by gzprintf(). |
| 270 | |
| 271 | If you don't have snprintf() or vsnprintf() and would like one, you can |
| 272 | find a portable implementation here: |
| 273 | |
| 274 | http://www.ijs.si/software/snprintf/ |
| 275 | |
| 276 | Note that you should be using the most recent version of zlib. Versions |
| 277 | 1.1.3 and before were subject to a double-free vulnerability, and versions |
| 278 | 1.2.1 and 1.2.2 were subject to an access exception when decompressing |
| 279 | invalid compressed data. |
| 280 | |
| 281 | 34. Is there a Java version of zlib? |
| 282 | |
| 283 | Probably what you want is to use zlib in Java. zlib is already included |
| 284 | as part of the Java SDK in the java.util.zip package. If you really want |
| 285 | a version of zlib written in the Java language, look on the zlib home |
| 286 | page for links: http://zlib.net/ . |
| 287 | |
| 288 | 35. I get this or that compiler or source-code scanner warning when I crank it |
| 289 | up to maximally-pedantic. Can't you guys write proper code? |
| 290 | |
| 291 | Many years ago, we gave up attempting to avoid warnings on every compiler |
| 292 | in the universe. It just got to be a waste of time, and some compilers |
| 293 | were downright silly as well as contradicted each other. So now, we simply |
| 294 | make sure that the code always works. |
| 295 | |
| 296 | 36. Valgrind (or some similar memory access checker) says that deflate is |
| 297 | performing a conditional jump that depends on an uninitialized value. |
| 298 | Isn't that a bug? |
| 299 | |
| 300 | No. That is intentional for performance reasons, and the output of deflate |
| 301 | is not affected. This only started showing up recently since zlib 1.2.x |
| 302 | uses malloc() by default for allocations, whereas earlier versions used |
| 303 | calloc(), which zeros out the allocated memory. Even though the code was |
| 304 | correct, versions 1.2.4 and later was changed to not stimulate these |
| 305 | checkers. |
| 306 | |
| 307 | 37. Will zlib read the (insert any ancient or arcane format here) compressed |
| 308 | data format? |
| 309 | |
| 310 | Probably not. Look in the comp.compression FAQ for pointers to various |
| 311 | formats and associated software. |
| 312 | |
| 313 | 38. How can I encrypt/decrypt zip files with zlib? |
| 314 | |
| 315 | zlib doesn't support encryption. The original PKZIP encryption is very |
| 316 | weak and can be broken with freely available programs. To get strong |
| 317 | encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib |
| 318 | compression. For PKZIP compatible "encryption", look at |
| 319 | http://www.info-zip.org/ |
| 320 | |
| 321 | 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings? |
| 322 | |
| 323 | "gzip" is the gzip format, and "deflate" is the zlib format. They should |
| 324 | probably have called the second one "zlib" instead to avoid confusion with |
| 325 | the raw deflate compressed data format. While the HTTP 1.1 RFC 2616 |
| 326 | correctly points to the zlib specification in RFC 1950 for the "deflate" |
| 327 | transfer encoding, there have been reports of servers and browsers that |
| 328 | incorrectly produce or expect raw deflate data per the deflate |
| 329 | specification in RFC 1951, most notably Microsoft. So even though the |
| 330 | "deflate" transfer encoding using the zlib format would be the more |
| 331 | efficient approach (and in fact exactly what the zlib format was designed |
| 332 | for), using the "gzip" transfer encoding is probably more reliable due to |
| 333 | an unfortunate choice of name on the part of the HTTP 1.1 authors. |
| 334 | |
| 335 | Bottom line: use the gzip format for HTTP 1.1 encoding. |
| 336 | |
| 337 | 40. Does zlib support the new "Deflate64" format introduced by PKWare? |
| 338 | |
| 339 | No. PKWare has apparently decided to keep that format proprietary, since |
| 340 | they have not documented it as they have previous compression formats. In |
| 341 | any case, the compression improvements are so modest compared to other more |
| 342 | modern approaches, that it's not worth the effort to implement. |
| 343 | |
| 344 | 41. I'm having a problem with the zip functions in zlib, can you help? |
| 345 | |
| 346 | There are no zip functions in zlib. You are probably using minizip by |
| 347 | Giles Vollant, which is found in the contrib directory of zlib. It is not |
| 348 | part of zlib. In fact none of the stuff in contrib is part of zlib. The |
| 349 | files in there are not supported by the zlib authors. You need to contact |
| 350 | the authors of the respective contribution for help. |
| 351 | |
| 352 | 42. The match.asm code in contrib is under the GNU General Public License. |
| 353 | Since it's part of zlib, doesn't that mean that all of zlib falls under the |
| 354 | GNU GPL? |
| 355 | |
| 356 | No. The files in contrib are not part of zlib. They were contributed by |
| 357 | other authors and are provided as a convenience to the user within the zlib |
| 358 | distribution. Each item in contrib has its own license. |
| 359 | |
| 360 | 43. Is zlib subject to export controls? What is its ECCN? |
| 361 | |
| 362 | zlib is not subject to export controls, and so is classified as EAR99. |
| 363 | |
| 364 | 44. Can you please sign these lengthy legal documents and fax them back to us |
| 365 | so that we can use your software in our product? |
| 366 | |
| 367 | No. Go away. Shoo. |