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 lastest 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. Pointers to the 18 precompiled DLL are found in the zlib web site at http://zlib.net/ . 19 20 3. Where can I get a Visual Basic interface to zlib? 21 22 See 23 * http://marknelson.us/1997/01/01/zlib-engine/ 24 * win32/DLL_FAQ.txt in the zlib distribution 25 26 4. compress() returns Z_BUF_ERROR. 27 28 Make sure that before the call of compress(), the length of the compressed 29 buffer is equal to the available size of the compressed buffer and not 30 zero. For Visual Basic, check that this parameter is passed by reference 31 ("as any"), not by value ("as long"). 32 33 5. deflate() or inflate() returns Z_BUF_ERROR. 34 35 Before making the call, make sure that avail_in and avail_out are not zero. 36 When setting the parameter flush equal to Z_FINISH, also make sure that 37 avail_out is big enough to allow processing all pending input. Note that a 38 Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be 39 made with more input or output space. A Z_BUF_ERROR may in fact be 40 unavoidable depending on how the functions are used, since it is not 41 possible to tell whether or not there is more output pending when 42 strm.avail_out returns with zero. See http://zlib.net/zlib_how.html for a 43 heavily annotated example. 44 45 6. Where's the zlib documentation (man pages, etc.)? 46 47 It's in zlib.h . Examples of zlib usage are in the files example.c and 48 minigzip.c, with more in examples/ . 49 50 7. Why don't you use GNU autoconf or libtool or ...? 51 52 Because we would like to keep zlib as a very small and simple package. 53 zlib is rather portable and doesn't need much configuration. 54 55 8. I found a bug in zlib. 56 57 Most of the time, such problems are due to an incorrect usage of zlib. 58 Please try to reproduce the problem with a small program and send the 59 corresponding source to us at zlib (a] gzip.org . Do not send multi-megabyte 60 data files without prior agreement. 61 62 9. Why do I get "undefined reference to gzputc"? 63 64 If "make test" produces something like 65 66 example.o(.text+0x154): undefined reference to `gzputc' 67 68 check that you don't have old files libz.* in /usr/lib, /usr/local/lib or 69 /usr/X11R6/lib. Remove any old versions, then do "make install". 70 71 10. I need a Delphi interface to zlib. 72 73 See the contrib/delphi directory in the zlib distribution. 74 75 11. Can zlib handle .zip archives? 76 77 Not by itself, no. See the directory contrib/minizip in the zlib 78 distribution. 79 80 12. Can zlib handle .Z files? 81 82 No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt 83 the code of uncompress on your own. 84 85 13. How can I make a Unix shared library? 86 87 make clean 88 ./configure -s 89 make 90 91 14. How do I install a shared zlib library on Unix? 92 93 After the above, then: 94 95 make install 96 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 101 it. You can check the version at the top of zlib.h or with the 102 ZLIB_VERSION symbol defined in zlib.h . 103 104 15. I have a question about OttoPDF. 105 106 We are not the authors of OttoPDF. The real author is on the OttoPDF web 107 site: Joel Hainley, jhainley (a] myndkryme.com. 108 109 16. Can zlib decode Flate data in an Adobe PDF file? 110 111 Yes. See http://www.pdflib.com/ . To modify PDF forms, see 112 http://sourceforge.net/projects/acroformtool/ . 113 114 17. Why am I getting this "register_frame_info not found" error on Solaris? 115 116 After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib 117 generates an error such as: 118 119 ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so: 120 symbol __register_frame_info: referenced symbol not found 121 122 The symbol __register_frame_info is not part of zlib, it is generated by 123 the C compiler (cc or gcc). You must recompile applications using zlib 124 which have this problem. This problem is specific to Solaris. See 125 http://www.sunfreeware.com for Solaris versions of zlib and applications 126 using zlib. 127 128 18. Why does gzip give an error on a file I make with compress/deflate? 129 130 The compress and deflate functions produce data in the zlib format, which 131 is different and incompatible with the gzip format. The gz* functions in 132 zlib on the other hand use the gzip format. Both the zlib and gzip formats 133 use the same compressed data format internally, but have different headers 134 and trailers around the compressed data. 135 136 19. Ok, so why are there two different formats? 137 138 The gzip format was designed to retain the directory information about a 139 single file, such as the name and last modification date. The zlib format 140 on the other hand was designed for in-memory and communication channel 141 applications, and has a much more compact header and trailer and uses a 142 faster integrity check than gzip. 143 144 20. Well that's nice, but how do I make a gzip file in memory? 145 146 You can request that deflate write the gzip format instead of the zlib 147 format using deflateInit2(). You can also request that inflate decode the 148 gzip format using inflateInit2(). Read zlib.h for more details. 149 150 21. Is zlib thread-safe? 151 152 Yes. However any library routines that zlib uses and any application- 153 provided memory allocation routines must also be thread-safe. zlib's gz* 154 functions use stdio library routines, and most of zlib's functions use the 155 library memory allocation routines by default. zlib's *Init* functions 156 allow for the application to provide custom memory allocation routines. 157 158 Of course, you should only operate on any given zlib or gzip stream from a 159 single thread at a time. 160 161 22. Can I use zlib in my commercial application? 162 163 Yes. Please read the license in zlib.h. 164 165 23. Is zlib under the GNU license? 166 167 No. Please read the license in zlib.h. 168 169 24. The license says that altered source versions must be "plainly marked". So 170 what exactly do I need to do to meet that requirement? 171 172 You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In 173 particular, the final version number needs to be changed to "f", and an 174 identification string should be appended to ZLIB_VERSION. Version numbers 175 x.x.x.f are reserved for modifications to zlib by others than the zlib 176 maintainers. For example, if the version of the base zlib you are altering 177 is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and 178 ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also 179 update the version strings in deflate.c and inftrees.c. 180 181 For altered source distributions, you should also note the origin and 182 nature of the changes in zlib.h, as well as in ChangeLog and README, along 183 with the dates of the alterations. The origin should include at least your 184 name (or your company's name), and an email address to contact for help or 185 issues with the library. 186 187 Note that distributing a compiled zlib library along with zlib.h and 188 zconf.h is also a source distribution, and so you should change 189 ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes 190 in zlib.h as you would for a full source distribution. 191 192 25. Will zlib work on a big-endian or little-endian architecture, and can I 193 exchange compressed data between them? 194 195 Yes and yes. 196 197 26. Will zlib work on a 64-bit machine? 198 199 Yes. It has been tested on 64-bit machines, and has no dependence on any 200 data types being limited to 32-bits in length. If you have any 201 difficulties, please provide a complete problem report to zlib (a] gzip.org 202 203 27. Will zlib decompress data from the PKWare Data Compression Library? 204 205 No. The PKWare DCL uses a completely different compressed data format than 206 does PKZIP and zlib. However, you can look in zlib's contrib/blast 207 directory for a possible solution to your problem. 208 209 28. Can I access data randomly in a compressed stream? 210 211 No, not without some preparation. If when compressing you periodically use 212 Z_FULL_FLUSH, carefully write all the pending data at those points, and 213 keep an index of those locations, then you can start decompression at those 214 points. You have to be careful to not use Z_FULL_FLUSH too often, since it 215 can significantly degrade compression. Alternatively, you can scan a 216 deflate stream once to generate an index, and then use that index for 217 random access. See examples/zran.c . 218 219 29. Does zlib work on MVS, OS/390, CICS, etc.? 220 221 It has in the past, but we have not heard of any recent evidence. There 222 were working ports of zlib 1.1.4 to MVS, but those links no longer work. 223 If you know of recent, successful applications of zlib on these operating 224 systems, please let us know. Thanks. 225 226 30. Is there some simpler, easier to read version of inflate I can look at to 227 understand the deflate format? 228 229 First off, you should read RFC 1951. Second, yes. Look in zlib's 230 contrib/puff directory. 231 232 31. Does zlib infringe on any patents? 233 234 As far as we know, no. In fact, that was originally the whole point behind 235 zlib. Look here for some more information: 236 237 http://www.gzip.org/#faq11 238 239 32. Can zlib work with greater than 4 GB of data? 240 241 Yes. inflate() and deflate() will process any amount of data correctly. 242 Each call of inflate() or deflate() is limited to input and output chunks 243 of the maximum value that can be stored in the compiler's "unsigned int" 244 type, but there is no limit to the number of chunks. Note however that the 245 strm.total_in and strm_total_out counters may be limited to 4 GB. These 246 counters are provided as a convenience and are not used internally by 247 inflate() or deflate(). The application can easily set up its own counters 248 updated after each call of inflate() or deflate() to count beyond 4 GB. 249 compress() and uncompress() may be limited to 4 GB, since they operate in a 250 single call. gzseek() and gztell() may be limited to 4 GB depending on how 251 zlib is compiled. See the zlibCompileFlags() function in zlib.h. 252 253 The word "may" appears several times above since there is a 4 GB limit only 254 if the compiler's "long" type is 32 bits. If the compiler's "long" type is 255 64 bits, then the limit is 16 exabytes. 256 257 33. Does zlib have any security vulnerabilities? 258 259 The only one that we are aware of is potentially in gzprintf(). If zlib is 260 compiled to use sprintf() or vsprintf(), then there is no protection 261 against a buffer overflow of an 8K string space (or other value as set by 262 gzbuffer()), other than the caller of gzprintf() assuring that the output 263 will not exceed 8K. On the other hand, if zlib is compiled to use 264 snprintf() or vsnprintf(), which should normally be the case, then there is 265 no vulnerability. The ./configure script will display warnings if an 266 insecure variation of sprintf() will be used by gzprintf(). Also the 267 zlibCompileFlags() function will return information on what variant of 268 sprintf() is used by gzprintf(). 269 270 If you don't have snprintf() or vsnprintf() and would like one, you can 271 find a portable implementation here: 272 273 http://www.ijs.si/software/snprintf/ 274 275 Note that you should be using the most recent version of zlib. Versions 276 1.1.3 and before were subject to a double-free vulnerability, and versions 277 1.2.1 and 1.2.2 were subject to an access exception when decompressing 278 invalid compressed data. 279 280 34. Is there a Java version of zlib? 281 282 Probably what you want is to use zlib in Java. zlib is already included 283 as part of the Java SDK in the java.util.zip package. If you really want 284 a version of zlib written in the Java language, look on the zlib home 285 page for links: http://zlib.net/ . 286 287 35. I get this or that compiler or source-code scanner warning when I crank it 288 up to maximally-pedantic. Can't you guys write proper code? 289 290 Many years ago, we gave up attempting to avoid warnings on every compiler 291 in the universe. It just got to be a waste of time, and some compilers 292 were downright silly as well as contradicted each other. So now, we simply 293 make sure that the code always works. 294 295 36. Valgrind (or some similar memory access checker) says that deflate is 296 performing a conditional jump that depends on an uninitialized value. 297 Isn't that a bug? 298 299 No. That is intentional for performance reasons, and the output of deflate 300 is not affected. This only started showing up recently since zlib 1.2.x 301 uses malloc() by default for allocations, whereas earlier versions used 302 calloc(), which zeros out the allocated memory. Even though the code was 303 correct, versions 1.2.4 and later was changed to not stimulate these 304 checkers. 305 306 37. Will zlib read the (insert any ancient or arcane format here) compressed 307 data format? 308 309 Probably not. Look in the comp.compression FAQ for pointers to various 310 formats and associated software. 311 312 38. How can I encrypt/decrypt zip files with zlib? 313 314 zlib doesn't support encryption. The original PKZIP encryption is very 315 weak and can be broken with freely available programs. To get strong 316 encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib 317 compression. For PKZIP compatible "encryption", look at 318 http://www.info-zip.org/ 319 320 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings? 321 322 "gzip" is the gzip format, and "deflate" is the zlib format. They should 323 probably have called the second one "zlib" instead to avoid confusion with 324 the raw deflate compressed data format. While the HTTP 1.1 RFC 2616 325 correctly points to the zlib specification in RFC 1950 for the "deflate" 326 transfer encoding, there have been reports of servers and browsers that 327 incorrectly produce or expect raw deflate data per the deflate 328 specficiation in RFC 1951, most notably Microsoft. So even though the 329 "deflate" transfer encoding using the zlib format would be the more 330 efficient approach (and in fact exactly what the zlib format was designed 331 for), using the "gzip" transfer encoding is probably more reliable due to 332 an unfortunate choice of name on the part of the HTTP 1.1 authors. 333 334 Bottom line: use the gzip format for HTTP 1.1 encoding. 335 336 40. Does zlib support the new "Deflate64" format introduced by PKWare? 337 338 No. PKWare has apparently decided to keep that format proprietary, since 339 they have not documented it as they have previous compression formats. In 340 any case, the compression improvements are so modest compared to other more 341 modern approaches, that it's not worth the effort to implement. 342 343 41. I'm having a problem with the zip functions in zlib, can you help? 344 345 There are no zip functions in zlib. You are probably using minizip by 346 Giles Vollant, which is found in the contrib directory of zlib. It is not 347 part of zlib. In fact none of the stuff in contrib is part of zlib. The 348 files in there are not supported by the zlib authors. You need to contact 349 the authors of the respective contribution for help. 350 351 42. The match.asm code in contrib is under the GNU General Public License. 352 Since it's part of zlib, doesn't that mean that all of zlib falls under the 353 GNU GPL? 354 355 No. The files in contrib are not part of zlib. They were contributed by 356 other authors and are provided as a convenience to the user within the zlib 357 distribution. Each item in contrib has its own license. 358 359 43. Is zlib subject to export controls? What is its ECCN? 360 361 zlib is not subject to export controls, and so is classified as EAR99. 362 363 44. Can you please sign these lengthy legal documents and fax them back to us 364 so that we can use your software in our product? 365 366 No. Go away. Shoo. 367
