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1 1
2 Frequently Asked Questions about zlib 2 Frequently Asked Questions about zlib
3 3
4 4
5 If your question is not there, please check the zlib home page 5 If your question is not there, please check the zlib home page
6 http://www.zlib.org which may have more recent information. 6 http://zlib.net/ which may have more recent information.
7 The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html 7 The lastest zlib FAQ is at http://zlib.net/zlib_faq.html
8 8
9 9
10 1. Is zlib Y2K-compliant? 10 1. Is zlib Y2K-compliant?
11 11
12 Yes. zlib doesn't handle dates. 12 Yes. zlib doesn't handle dates.
13 13
14 2. Where can I get a Windows DLL version? 14 2. Where can I get a Windows DLL version?
15 15
16 The zlib sources can be compiled without change to produce a DLL. 16 The zlib sources can be compiled without change to produce a DLL. See the
17 See the file win32/DLL_FAQ.txt in the zlib distribution. 17 file win32/DLL_FAQ.txt in the zlib distribution. Pointers to the
18 Pointers to the precompiled DLL are found in the zlib web site at 18 precompiled DLL are found in the zlib web site at http://zlib.net/ .
19 http://www.zlib.org.
20 19
21 3. Where can I get a Visual Basic interface to zlib? 20 3. Where can I get a Visual Basic interface to zlib?
22 21
23 See 22 See
24 * http://www.dogma.net/markn/articles/zlibtool/zlibtool.htm 23 * http://marknelson.us/1997/01/01/zlib-engine/
25 * contrib/visual-basic.txt in the zlib distribution
26 * win32/DLL_FAQ.txt in the zlib distribution 24 * win32/DLL_FAQ.txt in the zlib distribution
27 25
28 4. compress() returns Z_BUF_ERROR. 26 4. compress() returns Z_BUF_ERROR.
29 27
30 Make sure that before the call of compress, the length of the compressed 28 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 29 buffer is equal to the available size of the compressed buffer and not
32 zero. For Visual Basic, check that this parameter is passed by reference 30 zero. For Visual Basic, check that this parameter is passed by reference
33 ("as any"), not by value ("as long"). 31 ("as any"), not by value ("as long").
34 32
35 5. deflate() or inflate() returns Z_BUF_ERROR. 33 5. deflate() or inflate() returns Z_BUF_ERROR.
36 34
37 Before making the call, make sure that avail_in and avail_out are not 35 Before making the call, make sure that avail_in and avail_out are not zero.
38 zero. When setting the parameter flush equal to Z_FINISH, also make sure 36 When setting the parameter flush equal to Z_FINISH, also make sure that
39 that avail_out is big enough to allow processing all pending input. 37 avail_out is big enough to allow processing all pending input. Note that a
40 Note that a Z_BUF_ERROR is not fatal--another call to deflate() or 38 Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be
41 inflate() can be made with more input or output space. A Z_BUF_ERROR 39 made with more input or output space. A Z_BUF_ERROR may in fact be
42 may in fact be unavoidable depending on how the functions are used, since 40 unavoidable depending on how the functions are used, since it is not
43 it is not possible to tell whether or not there is more output pending 41 possible to tell whether or not there is more output pending when
44 when strm.avail_out returns with zero. 42 strm.avail_out returns with zero. See http://zlib.net/zlib_how.html for a
43 heavily annotated example.
45 44
46 6. Where's the zlib documentation (man pages, etc.)? 45 6. Where's the zlib documentation (man pages, etc.)?
47 46
48 It's in zlib.h for the moment, and Francis S. Lin has converted it to a 47 It's in zlib.h . Examples of zlib usage are in the files test/example.c
49 web page zlib.html. Volunteers to transform this to Unix-style man pages, 48 and test/minigzip.c, with more in examples/ .
50 please contact us (zlib@gzip.org). Examples of zlib usage are in the files
51 example.c and minigzip.c.
52 49
53 7. Why don't you use GNU autoconf or libtool or ...? 50 7. Why don't you use GNU autoconf or libtool or ...?
54 51
55 Because we would like to keep zlib as a very small and simple 52 Because we would like to keep zlib as a very small and simple package.
56 package. zlib is rather portable and doesn't need much configuration. 53 zlib is rather portable and doesn't need much configuration.
57 54
58 8. I found a bug in zlib. 55 8. I found a bug in zlib.
59 56
60 Most of the time, such problems are due to an incorrect usage of 57 Most of the time, such problems are due to an incorrect usage of zlib.
61 zlib. Please try to reproduce the problem with a small program and send 58 Please try to reproduce the problem with a small program and send the
62 the corresponding source to us at zlib@gzip.org . Do not send 59 corresponding source to us at zlib@gzip.org . Do not send multi-megabyte
63 multi-megabyte data files without prior agreement. 60 data files without prior agreement.
64 61
65 9. Why do I get "undefined reference to gzputc"? 62 9. Why do I get "undefined reference to gzputc"?
66 63
67 If "make test" produces something like 64 If "make test" produces something like
68 65
80 Not by itself, no. See the directory contrib/minizip in the zlib 77 Not by itself, no. See the directory contrib/minizip in the zlib
81 distribution. 78 distribution.
82 79
83 12. Can zlib handle .Z files? 80 12. Can zlib handle .Z files?
84 81
85 No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt 82 No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt
86 the code of uncompress on your own. 83 the code of uncompress on your own.
87 84
88 13. How can I make a Unix shared library? 85 13. How can I make a Unix shared library?
89 86
90 make clean 87 By default a shared (and a static) library is built for Unix. So:
91 ./configure -s 88
89 make distclean
90 ./configure
92 make 91 make
93 92
94 14. How do I install a shared zlib library on Unix? 93 14. How do I install a shared zlib library on Unix?
95 94
96 After the above, then: 95 After the above, then:
97 96
98 make install 97 make install
99 98
100 However, many flavors of Unix come with a shared zlib already installed. 99 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 100 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 101 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. 102 can #include <zlib.h>, it's there. The -lz option will probably link to
103 it. You can check the version at the top of zlib.h or with the
104 ZLIB_VERSION symbol defined in zlib.h .
104 105
105 15. I have a question about OttoPDF. 106 15. I have a question about OttoPDF.
106 107
107 We are not the authors of OttoPDF. The real author is on the OttoPDF web 108 We are not the authors of OttoPDF. The real author is on the OttoPDF web
108 site: Joel Hainley, jhainley@myndkryme.com. 109 site: Joel Hainley, jhainley@myndkryme.com.
109 110
110 16. Can zlib decode Flate data in an Adobe PDF file? 111 16. Can zlib decode Flate data in an Adobe PDF file?
111 112
112 Yes. See http://www.fastio.com/ (ClibPDF), or http://www.pdflib.com/ . 113 Yes. See http://www.pdflib.com/ . To modify PDF forms, see
113 To modify PDF forms, see http://sourceforge.net/projects/acroformtool/ . 114 http://sourceforge.net/projects/acroformtool/ .
114 115
115 17. Why am I getting this "register_frame_info not found" error on Solaris? 116 17. Why am I getting this "register_frame_info not found" error on Solaris?
116 117
117 After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib 118 After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
118 generates an error such as: 119 generates an error such as:
119 120
120 ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so: 121 ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
121 symbol __register_frame_info: referenced symbol not found 122 symbol __register_frame_info: referenced symbol not found
122 123
123 The symbol __register_frame_info is not part of zlib, it is generated by 124 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 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 which have this problem. This problem is specific to Solaris. See
126 http://www.sunfreeware.com for Solaris versions of zlib and applications 127 http://www.sunfreeware.com for Solaris versions of zlib and applications
127 using zlib. 128 using zlib.
128 129
129 18. Why does gzip give an error on a file I make with compress/deflate? 130 18. Why does gzip give an error on a file I make with compress/deflate?
130 131
131 The compress and deflate functions produce data in the zlib format, which 132 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 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 zlib on the other hand use the gzip format. Both the zlib and gzip formats
134 formats use the same compressed data format internally, but have different 135 use the same compressed data format internally, but have different headers
135 headers and trailers around the compressed data. 136 and trailers around the compressed data.
136 137
137 19. Ok, so why are there two different formats? 138 19. Ok, so why are there two different formats?
138 139
139 The gzip format was designed to retain the directory information about 140 The gzip format was designed to retain the directory information about a
140 a single file, such as the name and last modification date. The zlib 141 single file, such as the name and last modification date. The zlib format
141 format on the other hand was designed for in-memory and communication 142 on the other hand was designed for in-memory and communication channel
142 channel applications, and has a much more compact header and trailer and 143 applications, and has a much more compact header and trailer and uses a
143 uses a faster integrity check than gzip. 144 faster integrity check than gzip.
144 145
145 20. Well that's nice, but how do I make a gzip file in memory? 146 20. Well that's nice, but how do I make a gzip file in memory?
146 147
147 You can request that deflate write the gzip format instead of the zlib 148 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 format using deflateInit2(). You can also request that inflate decode the
149 the gzip format using inflateInit2(). Read zlib.h for more details. 150 gzip format using inflateInit2(). Read zlib.h for more details.
150 151
151 21. Is zlib thread-safe? 152 21. Is zlib thread-safe?
152 153
153 Yes. However any library routines that zlib uses and any application- 154 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 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 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 library memory allocation routines by default. zlib's *Init* functions
157 for the application to provide custom memory allocation routines. 158 allow for the application to provide custom memory allocation routines.
158 159
159 Of course, you should only operate on any given zlib or gzip stream from a 160 Of course, you should only operate on any given zlib or gzip stream from a
160 single thread at a time. 161 single thread at a time.
161 162
162 22. Can I use zlib in my commercial application? 163 22. Can I use zlib in my commercial application?
163 164
164 Yes. Please read the license in zlib.h. 165 Yes. Please read the license in zlib.h.
165 166
166 23. Is zlib under the GNU license? 167 23. Is zlib under the GNU license?
167 168
168 No. Please read the license in zlib.h. 169 No. Please read the license in zlib.h.
169 170
170 24. The license says that altered source versions must be "plainly marked". So 171 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 what exactly do I need to do to meet that requirement?
172 173
173 You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In 174 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 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 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 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 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 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 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 update the version strings in deflate.c and inftrees.c.
181 182
182 For altered source distributions, you should also note the origin and 183 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 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 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 name (or your company's name), and an email address to contact for help or
186 issues with the library. 187 issues with the library.
187 188
188 Note that distributing a compiled zlib library along with zlib.h and 189 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 zconf.h is also a source distribution, and so you should change
195 196
196 Yes and yes. 197 Yes and yes.
197 198
198 26. Will zlib work on a 64-bit machine? 199 26. Will zlib work on a 64-bit machine?
199 200
200 It should. It has been tested on 64-bit machines, and has no dependence 201 Yes. It has been tested on 64-bit machines, and has no dependence on any
201 on any data types being limited to 32-bits in length. If you have any 202 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 difficulties, please provide a complete problem report to zlib@gzip.org
203 204
204 27. Will zlib decompress data from the PKWare Data Compression Library? 205 27. Will zlib decompress data from the PKWare Data Compression Library?
205 206
206 No. The PKWare DCL uses a completely different compressed data format 207 No. The PKWare DCL uses a completely different compressed data format than
207 than does PKZIP and zlib. However, you can look in zlib's contrib/blast 208 does PKZIP and zlib. However, you can look in zlib's contrib/blast
208 directory for a possible solution to your problem. 209 directory for a possible solution to your problem.
209 210
210 28. Can I access data randomly in a compressed stream? 211 28. Can I access data randomly in a compressed stream?
211 212
212 No, not without some preparation. If when compressing you periodically 213 No, not without some preparation. If when compressing you periodically use
213 use Z_FULL_FLUSH, carefully write all the pending data at those points, 214 Z_FULL_FLUSH, carefully write all the pending data at those points, and
214 and keep an index of those locations, then you can start decompression 215 keep an index of those locations, then you can start decompression at those
215 at those points. You have to be careful to not use Z_FULL_FLUSH too 216 points. You have to be careful to not use Z_FULL_FLUSH too often, since it
216 often, since it can significantly degrade compression. 217 can significantly degrade compression. Alternatively, you can scan a
218 deflate stream once to generate an index, and then use that index for
219 random access. See examples/zran.c .
217 220
218 29. Does zlib work on MVS, OS/390, CICS, etc.? 221 29. Does zlib work on MVS, OS/390, CICS, etc.?
219 222
220 We don't know for sure. We have heard occasional reports of success on 223 It has in the past, but we have not heard of any recent evidence. There
221 these systems. If you do use it on one of these, please provide us with 224 were working ports of zlib 1.1.4 to MVS, but those links no longer work.
222 a report, instructions, and patches that we can reference when we get 225 If you know of recent, successful applications of zlib on these operating
223 these questions. Thanks. 226 systems, please let us know. Thanks.
224 227
225 30. Is there some simpler, easier to read version of inflate I can look at 228 30. Is there some simpler, easier to read version of inflate I can look at to
226 to understand the deflate format? 229 understand the deflate format?
227 230
228 First off, you should read RFC 1951. Second, yes. Look in zlib's 231 First off, you should read RFC 1951. Second, yes. Look in zlib's
229 contrib/puff directory. 232 contrib/puff directory.
230 233
231 31. Does zlib infringe on any patents? 234 31. Does zlib infringe on any patents?
232 235
233 As far as we know, no. In fact, that was originally the whole point behind 236 As far as we know, no. In fact, that was originally the whole point behind
234 zlib. Look here for some more information: 237 zlib. Look here for some more information:
235 238
236 http://www.gzip.org/#faq11 239 http://www.gzip.org/#faq11
237 240
238 32. Can zlib work with greater than 4 GB of data? 241 32. Can zlib work with greater than 4 GB of data?
239 242
240 Yes. inflate() and deflate() will process any amount of data correctly. 243 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 244 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" 245 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 246 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 247 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 248 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 249 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. 250 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 251 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 252 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. 253 zlib is compiled. See the zlibCompileFlags() function in zlib.h.
251 254
252 The word "may" appears several times above since there is a 4 GB limit 255 The word "may" appears several times above since there is a 4 GB limit only
253 only if the compiler's "long" type is 32 bits. If the compiler's "long" 256 if the compiler's "long" type is 32 bits. If the compiler's "long" type is
254 type is 64 bits, then the limit is 16 exabytes. 257 64 bits, then the limit is 16 exabytes.
255 258
256 33. Does zlib have any security vulnerabilities? 259 33. Does zlib have any security vulnerabilities?
257 260
258 The only one that we are aware of is potentially in gzprintf(). If zlib 261 The only one that we are aware of is potentially in gzprintf(). If zlib is
259 is compiled to use sprintf() or vsprintf(), then there is no protection 262 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 263 against a buffer overflow of an 8K string space (or other value as set by
261 gzprintf() assuring that the output will not exceed 4K. On the other 264 gzbuffer()), other than the caller of gzprintf() assuring that the output
262 hand, if zlib is compiled to use snprintf() or vsnprintf(), which should 265 will not exceed 8K. On the other hand, if zlib is compiled to use
263 normally be the case, then there is no vulnerability. The ./configure 266 snprintf() or vsnprintf(), which should normally be the case, then there is
264 script will display warnings if an insecure variation of sprintf() will 267 no vulnerability. The ./configure script will display warnings if an
265 be used by gzprintf(). Also the zlibCompileFlags() function will return 268 insecure variation of sprintf() will be used by gzprintf(). Also the
266 information on what variant of sprintf() is used by gzprintf(). 269 zlibCompileFlags() function will return information on what variant of
270 sprintf() is used by gzprintf().
267 271
268 If you don't have snprintf() or vsnprintf() and would like one, you can 272 If you don't have snprintf() or vsnprintf() and would like one, you can
269 find a portable implementation here: 273 find a portable implementation here:
270 274
271 http://www.ijs.si/software/snprintf/ 275 http://www.ijs.si/software/snprintf/
272 276
273 Note that you should be using the most recent version of zlib. Versions 277 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. 278 1.1.3 and before were subject to a double-free vulnerability, and versions
279 1.2.1 and 1.2.2 were subject to an access exception when decompressing
280 invalid compressed data.
275 281
276 34. Is there a Java version of zlib? 282 34. Is there a Java version of zlib?
277 283
278 Probably what you want is to use zlib in Java. zlib is already included 284 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 285 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 286 a version of zlib written in the Java language, look on the zlib home
281 page for links: http://www.zlib.org/ 287 page for links: http://zlib.net/ .
282 288
283 35. I get this or that compiler or source-code scanner warning when I crank it 289 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? 290 up to maximally-pedantic. Can't you guys write proper code?
285 291
286 Many years ago, we gave up attempting to avoid warnings on every compiler 292 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 293 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 294 were downright silly as well as contradicted each other. So now, we simply
289 works. 295 make sure that the code always works.
290 296
291 36. Valgrind (or some similar memory access checker) says that deflate is 297 36. Valgrind (or some similar memory access checker) says that deflate is
292 performing a conditional jump that depends on an uninitialized value. 298 performing a conditional jump that depends on an uninitialized value.
293 Isn't that a bug? 299 Isn't that a bug?
294 300
295 No. That is intentional for performance reasons, and the output of 301 No. That is intentional for performance reasons, and the output of deflate
296 deflate is not affected. This only started showing up recently since 302 is not affected. This only started showing up recently since zlib 1.2.x
297 zlib 1.2.x uses malloc() by default for allocations, whereas earlier 303 uses malloc() by default for allocations, whereas earlier versions used
298 versions used calloc(), which zeros out the allocated memory. 304 calloc(), which zeros out the allocated memory. Even though the code was
305 correct, versions 1.2.4 and later was changed to not stimulate these
306 checkers.
299 307
300 37. Will zlib read the (insert any ancient or arcane format here) compressed 308 37. Will zlib read the (insert any ancient or arcane format here) compressed
301 data format? 309 data format?
302 310
303 Probably not. Look in the comp.compression FAQ for pointers to various 311 Probably not. Look in the comp.compression FAQ for pointers to various
304 formats and associated software. 312 formats and associated software.
305 313
306 38. How can I encrypt/decrypt zip files with zlib? 314 38. How can I encrypt/decrypt zip files with zlib?
307 315
308 zlib doesn't support encryption. The original PKZIP encryption is very weak 316 zlib doesn't support encryption. The original PKZIP encryption is very
309 and can be broken with freely available programs. To get strong encryption, 317 weak and can be broken with freely available programs. To get strong
310 use GnuPG, http://www.gnupg.org/ , which already includes zlib compression. 318 encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib
311 For PKZIP compatible "encryption", look at http://www.info-zip.org/ 319 compression. For PKZIP compatible "encryption", look at
320 http://www.info-zip.org/
312 321
313 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings? 322 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
314 323
315 "gzip" is the gzip format, and "deflate" is the zlib format. They should 324 "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 325 probably have called the second one "zlib" instead to avoid confusion with
317 with the raw deflate compressed data format. While the HTTP 1.1 RFC 2616 326 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" 327 correctly points to the zlib specification in RFC 1950 for the "deflate"
319 transfer encoding, there have been reports of servers and browsers that 328 transfer encoding, there have been reports of servers and browsers that
320 incorrectly produce or expect raw deflate data per the deflate 329 incorrectly produce or expect raw deflate data per the deflate
321 specficiation in RFC 1951, most notably Microsoft. So even though the 330 specification in RFC 1951, most notably Microsoft. So even though the
322 "deflate" transfer encoding using the zlib format would be the more 331 "deflate" transfer encoding using the zlib format would be the more
323 efficient approach (and in fact exactly what the zlib format was designed 332 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 333 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. 334 an unfortunate choice of name on the part of the HTTP 1.1 authors.
326 335
327 Bottom line: use the gzip format for HTTP 1.1 encoding. 336 Bottom line: use the gzip format for HTTP 1.1 encoding.
328 337
329 40. Does zlib support the new "Deflate64" format introduced by PKWare? 338 40. Does zlib support the new "Deflate64" format introduced by PKWare?
330 339
331 No. PKWare has apparently decided to keep that format proprietary, since 340 No. PKWare has apparently decided to keep that format proprietary, since
332 they have not documented it as they have previous compression formats. 341 they have not documented it as they have previous compression formats. In
333 In any case, the compression improvements are so modest compared to other 342 any case, the compression improvements are so modest compared to other more
334 more modern approaches, that it's not worth the effort to implement. 343 modern approaches, that it's not worth the effort to implement.
335 344
336 41. Can you please sign these lengthy legal documents and fax them back to us 345 41. I'm having a problem with the zip functions in zlib, can you help?
346
347 There are no zip functions in zlib. You are probably using minizip by
348 Giles Vollant, which is found in the contrib directory of zlib. It is not
349 part of zlib. In fact none of the stuff in contrib is part of zlib. The
350 files in there are not supported by the zlib authors. You need to contact
351 the authors of the respective contribution for help.
352
353 42. The match.asm code in contrib is under the GNU General Public License.
354 Since it's part of zlib, doesn't that mean that all of zlib falls under the
355 GNU GPL?
356
357 No. The files in contrib are not part of zlib. They were contributed by
358 other authors and are provided as a convenience to the user within the zlib
359 distribution. Each item in contrib has its own license.
360
361 43. Is zlib subject to export controls? What is its ECCN?
362
363 zlib is not subject to export controls, and so is classified as EAR99.
364
365 44. 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? 366 so that we can use your software in our product?
338 367
339 No. Go away. Shoo. 368 No. Go away. Shoo.