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1 2 Frequently Asked Questions about zlib 3 4 5If your question is not there, please check the zlib home page 6http://www.zlib.org which may have more recent information. 7The 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@example.com). 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 firstname.lastname@example.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 7410. I need a Delphi interface to zlib. 75 76 See the contrib/delphi directory in the zlib distribution. 77 7811. Can zlib handle .zip archives? 79 80 Not by itself, no. See the directory contrib/minizip in the zlib 81 distribution. 82 8312. 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 8813. How can I make a Unix shared library? 89 90 make clean 91 ./configure -s 92 make 93 9414. 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 10515. 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@example.com. 109 11016. 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 11517. 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 12918. 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 13719. 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 14520. 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 15121. 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 16222. Can I use zlib in my commercial application? 163 164 Yes. Please read the license in zlib.h. 165 16623. Is zlib under the GNU license? 167 168 No. Please read the license in zlib.h. 169 17024. 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 "188.8.131.52", 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 19325. 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 19826. 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 firstname.lastname@example.org 203 20427. 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 21028. 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 21829. 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 22530. 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 23131. 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 23832. 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 25633. 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 27634. 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 28335. 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 29136. 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 30037. 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 30638. 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 31339. 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 32940. 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 33641. 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.