|
| 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 ( [email protected]). 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 [email protected] . 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, [email protected]. |
| 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 [email protected] |
| 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. |
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