1 2 Frequently Asked Questions about zlib 3 4 5If your question is not there, please check the zlib home page 6https://zlib.net/ which may have more recent information. 7The latest zlib FAQ is at https://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 * https://zlib.net/nelson/ 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 https://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 7010. I need a Delphi interface to zlib. 71 72 See the contrib/delphi directory in the zlib distribution. 73 7411. Can zlib handle .zip archives? 75 76 Not by itself, no. See the directory contrib/minizip in the zlib 77 distribution. 78 7912. 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 8413. 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 9214. 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 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, jhainley@myndkryme.com. 109 11016. Can zlib decode Flate data in an Adobe PDF file? 111 112 Yes. See https://www.pdflib.com/ . To modify PDF forms, see 113 https://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 formats 134 use the same compressed data format internally, but have different headers 135 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 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 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 the 149 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 157 allow for the application to provide custom memory allocation routines. 158 159 If the non-default BUILDFIXED or DYNAMIC_CRC_TABLE defines are used on a 160 system without atomics (e.g. pre-C11), then inflate() and crc32() will not 161 be thread safe. 162 163 Of course, you should only operate on any given zlib or gzip stream from a 164 single thread at a time. 165 16622. Can I use zlib in my commercial application? 167 168 Yes. Please read the license in zlib.h. 169 17023. Is zlib under the GNU license? 171 172 No. Please read the license in zlib.h. 173 17424. The license says that altered source versions must be "plainly marked". So 175 what exactly do I need to do to meet that requirement? 176 177 You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In 178 particular, the final version number needs to be changed to "f", and an 179 identification string should be appended to ZLIB_VERSION. Version numbers 180 x.x.x.f are reserved for modifications to zlib by others than the zlib 181 maintainers. For example, if the version of the base zlib you are altering 182 is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and 183 ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also 184 update the version strings in deflate.c and inftrees.c. 185 186 For altered source distributions, you should also note the origin and 187 nature of the changes in zlib.h, as well as in ChangeLog and README, along 188 with the dates of the alterations. The origin should include at least your 189 name (or your company's name), and an email address to contact for help or 190 issues with the library. 191 192 Note that distributing a compiled zlib library along with zlib.h and 193 zconf.h is also a source distribution, and so you should change 194 ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes 195 in zlib.h as you would for a full source distribution. 196 19725. Will zlib work on a big-endian or little-endian architecture, and can I 198 exchange compressed data between them? 199 200 Yes and yes. 201 20226. Will zlib work on a 64-bit machine? 203 204 Yes. It has been tested on 64-bit machines, and has no dependence on any 205 data types being limited to 32-bits in length. If you have any 206 difficulties, please provide a complete problem report to zlib@gzip.org 207 20827. Will zlib decompress data from the PKWare Data Compression Library? 209 210 No. The PKWare DCL uses a completely different compressed data format than 211 does PKZIP and zlib. However, you can look in zlib's contrib/blast 212 directory for a possible solution to your problem. 213 21428. Can I access data randomly in a compressed stream? 215 216 No, not without some preparation. If when compressing you periodically use 217 Z_FULL_FLUSH, carefully write all the pending data at those points, and 218 keep an index of those locations, then you can start decompression at those 219 points. You have to be careful to not use Z_FULL_FLUSH too often, since it 220 can significantly degrade compression. Alternatively, you can scan a 221 deflate stream once to generate an index, and then use that index for 222 random access. See examples/zran.c . 223 22429. Does zlib work on MVS, OS/390, CICS, etc.? 225 226 It has in the past, but we have not heard of any recent evidence. There 227 were working ports of zlib 1.1.4 to MVS, but those links no longer work. 228 If you know of recent, successful applications of zlib on these operating 229 systems, please let us know. Thanks. 230 23130. Is there some simpler, easier to read version of inflate I can look at to 232 understand the deflate format? 233 234 First off, you should read RFC 1951. Second, yes. Look in zlib's 235 contrib/puff directory. 236 23731. Does zlib infringe on any patents? 238 239 As far as we know, no. In fact, that was originally the whole point behind 240 zlib. Look here for some more information: 241 242 https://web.archive.org/web/20180729212847/http://www.gzip.org/#faq11 243 24432. Can zlib work with greater than 4 GB of data? 245 246 Yes. inflate() and deflate() will process any amount of data correctly. 247 Each call of inflate() or deflate() is limited to input and output chunks 248 of the maximum value that can be stored in the compiler's "unsigned int" 249 type, but there is no limit to the number of chunks. Note however that the 250 strm.total_in and strm_total_out counters may be limited to 4 GB. These 251 counters are provided as a convenience and are not used internally by 252 inflate() or deflate(). The application can easily set up its own counters 253 updated after each call of inflate() or deflate() to count beyond 4 GB. 254 compress() and uncompress() may be limited to 4 GB, since they operate in a 255 single call. gzseek() and gztell() may be limited to 4 GB depending on how 256 zlib is compiled. See the zlibCompileFlags() function in zlib.h. 257 258 The word "may" appears several times above since there is a 4 GB limit only 259 if the compiler's "long" type is 32 bits. If the compiler's "long" type is 260 64 bits, then the limit is 16 exabytes. 261 26233. Does zlib have any security vulnerabilities? 263 264 The only one that we are aware of is potentially in gzprintf(). If zlib is 265 compiled to use sprintf() or vsprintf(), which requires that ZLIB_INSECURE 266 be defined, then there is no protection against a buffer overflow of an 8K 267 string space (or other value as set by gzbuffer()), other than the caller 268 of gzprintf() assuring that the output will not exceed 8K. On the other 269 hand, if zlib is compiled to use snprintf() or vsnprintf(), which should 270 normally be the case, then there is no vulnerability. The ./configure 271 script will display warnings if an insecure variation of sprintf() will be 272 used by gzprintf(). Also the zlibCompileFlags() function will return 273 information on what variant of sprintf() is used by gzprintf(). 274 275 If you don't have snprintf() or vsnprintf() and would like one, you can 276 find a good portable implementation in stb_sprintf.h here: 277 278 https://github.com/nothings/stb 279 280 Note that you should be using the most recent version of zlib. Versions 281 1.1.3 and before were subject to a double-free vulnerability, and versions 282 1.2.1 and 1.2.2 were subject to an access exception when decompressing 283 invalid compressed data. 284 28534. Is there a Java version of zlib? 286 287 Probably what you want is to use zlib in Java. zlib is already included 288 as part of the Java SDK in the java.util.zip package. If you really want 289 a version of zlib written in the Java language, look on the zlib home 290 page for links: https://zlib.net/ . 291 29235. I get this or that compiler or source-code scanner warning when I crank it 293 up to maximally-pedantic. Can't you guys write proper code? 294 295 Many years ago, we gave up attempting to avoid warnings on every compiler 296 in the universe. It just got to be a waste of time, and some compilers 297 were downright silly as well as contradicted each other. So now, we simply 298 make sure that the code always works. 299 30036. Valgrind (or some similar memory access checker) says that deflate is 301 performing a conditional jump that depends on an uninitialized value. 302 Isn't that a bug? 303 304 No. That is intentional for performance reasons, and the output of deflate 305 is not affected. This only started showing up recently since zlib 1.2.x 306 uses malloc() by default for allocations, whereas earlier versions used 307 calloc(), which zeros out the allocated memory. Even though the code was 308 correct, versions 1.2.4 and later was changed to not stimulate these 309 checkers. 310 31137. Will zlib read the (insert any ancient or arcane format here) compressed 312 data format? 313 314 Probably not. Look in the comp.compression FAQ for pointers to various 315 formats and associated software. 316 31738. How can I encrypt/decrypt zip files with zlib? 318 319 zlib doesn't support encryption. The original PKZIP encryption is very 320 weak and can be broken with freely available programs. To get strong 321 encryption, use GnuPG, https://www.gnupg.org/ , which already includes zlib 322 compression. For PKZIP compatible "encryption", look at 323 https://infozip.sourceforge.net/ 324 32539. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings? 326 327 "gzip" is the gzip format, and "deflate" is the zlib format. They should 328 probably have called the second one "zlib" instead to avoid confusion with 329 the raw deflate compressed data format. While the HTTP 1.1 RFC 2616 330 correctly points to the zlib specification in RFC 1950 for the "deflate" 331 transfer encoding, there have been reports of servers and browsers that 332 incorrectly produce or expect raw deflate data per the deflate 333 specification in RFC 1951, most notably Microsoft. So even though the 334 "deflate" transfer encoding using the zlib format would be the more 335 efficient approach (and in fact exactly what the zlib format was designed 336 for), using the "gzip" transfer encoding is probably more reliable due to 337 an unfortunate choice of name on the part of the HTTP 1.1 authors. 338 339 Bottom line: use the gzip format for HTTP 1.1 encoding. 340 34140. Does zlib support the new "Deflate64" format introduced by PKWare? 342 343 No. PKWare has apparently decided to keep that format proprietary, since 344 they have not documented it as they have previous compression formats. In 345 any case, the compression improvements are so modest compared to other more 346 modern approaches, that it's not worth the effort to implement. 347 34841. I'm having a problem with the zip functions in zlib, can you help? 349 350 There are no zip functions in zlib. You are probably using minizip by 351 Giles Vollant, which is found in the contrib directory of zlib. It is not 352 part of zlib. In fact none of the stuff in contrib is part of zlib. The 353 files in there are not supported by the zlib authors. You need to contact 354 the authors of the respective contribution for help. 355 35642. The match.asm code in contrib is under the GNU General Public License. 357 Since it's part of zlib, doesn't that mean that all of zlib falls under the 358 GNU GPL? 359 360 No. The files in contrib are not part of zlib. They were contributed by 361 other authors and are provided as a convenience to the user within the zlib 362 distribution. Each item in contrib has its own license. 363 36443. Is zlib subject to export controls? What is its ECCN? 365 366 zlib is not subject to export controls, and so is classified as EAR99. 367 36844. Can you please sign these lengthy legal documents and fax them back to us 369 so that we can use your software in our product? 370 371 No. Go away. Shoo. 372