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author | Mark Adler <madler@alumni.caltech.edu> | 2011-09-09 23:21:47 -0700 |
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committer | Mark Adler <madler@alumni.caltech.edu> | 2011-09-09 23:21:47 -0700 |
commit | 7c2a874e50b871d04fbd19501f7b42cff55e5abc (patch) | |
tree | 1879cd29182ababb17cde77cee5ce74505db4006 /FAQ | |
parent | a383133c4e7b93113cee912f213cf9502d785fa7 (diff) | |
download | zlib-7c2a874e50b871d04fbd19501f7b42cff55e5abc.tar.gz zlib-7c2a874e50b871d04fbd19501f7b42cff55e5abc.tar.bz2 zlib-7c2a874e50b871d04fbd19501f7b42cff55e5abc.zip |
zlib 1.2.0v1.2.0
Diffstat (limited to 'FAQ')
-rw-r--r-- | FAQ | 173 |
1 files changed, 168 insertions, 5 deletions
@@ -36,6 +36,11 @@ The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html | |||
36 | Before making the call, make sure that avail_in and avail_out are not | 36 | Before making the call, make sure that avail_in and avail_out are not |
37 | zero. When setting the parameter flush equal to Z_FINISH, also make sure | 37 | zero. When setting the parameter flush equal to Z_FINISH, also make sure |
38 | that avail_out is big enough to allow processing all pending input. | 38 | that avail_out is big enough to allow processing all pending input. |
39 | Note that a Z_BUF_ERROR is not fatal--another call to deflate() or | ||
40 | inflate() can be made with more input or output space. A Z_BUF_ERROR | ||
41 | may in fact be unavoidable depending on how the functions are used, since | ||
42 | it is not possible to tell whether or not there is more output pending | ||
43 | when strm.avail_out returns with zero. | ||
39 | 44 | ||
40 | 6. Where's the zlib documentation (man pages, etc.)? | 45 | 6. Where's the zlib documentation (man pages, etc.)? |
41 | 46 | ||
@@ -88,13 +93,171 @@ The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html | |||
88 | 14. Why does "make test" fail on Mac OS X? | 93 | 14. Why does "make test" fail on Mac OS X? |
89 | 94 | ||
90 | Mac OS X already includes zlib as a shared library, and so -lz links the | 95 | Mac OS X already includes zlib as a shared library, and so -lz links the |
91 | shared library instead of the one that the "make" compiled. For zlib | 96 | shared library instead of the one that the "make" compiled. The two are |
92 | 1.1.3, the two are incompatible due to different compile-time | 97 | incompatible due to different compile-time options. Simply change the -lz |
93 | options. Simply change the -lz in the Makefile to libz.a, and it will use | 98 | in the Makefile to libz.a, and it will use the compiled library instead |
94 | the compiled library instead of the shared one and the "make test" will | 99 | of the shared one and the "make test" will succeed. |
95 | succeed. | ||
96 | 100 | ||
97 | 15. I have a question about OttoPDF | 101 | 15. I have a question about OttoPDF |
98 | 102 | ||
99 | We are not the authors of OttoPDF. The real author is on the OttoPDF web | 103 | We are not the authors of OttoPDF. The real author is on the OttoPDF web |
100 | site Joel Hainley jhainley@myndkryme.com. | 104 | site Joel Hainley jhainley@myndkryme.com. |
105 | |||
106 | 16. Why does gzip give an error on a file I make with compress/deflate? | ||
107 | |||
108 | The compress and deflate functions produce data in the zlib format, which | ||
109 | is different and incompatible with the gzip format. The gz* functions in | ||
110 | zlib on the other hand use the gzip format. Both the zlib and gzip | ||
111 | formats use the same compressed data format, but have different headers | ||
112 | and trailers. | ||
113 | |||
114 | 17. Ok, so why are there two different formats? | ||
115 | |||
116 | The gzip format was designed to retain the directory information about | ||
117 | a single file, such as the name and last modification date. The zlib | ||
118 | format on the other hand was designed for in-memory and communication | ||
119 | channel applications, and has a much more compact header and trailer and | ||
120 | uses a faster integrity check than gzip. | ||
121 | |||
122 | 18. Well that's nice, but how do I make a gzip file in memory? | ||
123 | |||
124 | Read RFC 1952 for the gzip header and trailer format, and roll your own | ||
125 | gzip formatted data using raw deflate and crc32(). | ||
126 | |||
127 | 19. Is zlib thread-safe? | ||
128 | |||
129 | Yes. However any library routines that zlib uses and any application- | ||
130 | provided memory allocation routines must also be thread-safe. Of course, | ||
131 | you should only operate on any given zlib or gzip stream from a single | ||
132 | thread. zlib's gz* functions use stdio library routines, and most of | ||
133 | zlib's functions use the library memory allocation routines by default. | ||
134 | zlib's Init functions allow for the application to provide custom memory | ||
135 | allocation routines. | ||
136 | |||
137 | 20. Can I use zlib in my commercial application? | ||
138 | |||
139 | Yes. Please read the license in zlib.h. | ||
140 | |||
141 | 21. Is zlib under the GNU license? | ||
142 | |||
143 | No. Please read the license in zlib.h. | ||
144 | |||
145 | 22. Will zlib work on a big-endian or little-endian architecture, and can I | ||
146 | exchange compressed data between them? | ||
147 | |||
148 | Yes and yes. | ||
149 | |||
150 | 23. Will zlib work on a 64-bit machine? | ||
151 | |||
152 | It should. It has been tested on 64-bit machines, and has no dependence | ||
153 | on any data types being limited to 32-bits in length. If you have any | ||
154 | difficulties, please provide a complete problem report to zlib@gzip.org | ||
155 | |||
156 | 24. Will zlib decompress data from the PKWare Data Compression Library? | ||
157 | |||
158 | No. The PKWare DCL uses a completely different compressed data format | ||
159 | than does PKZIP and zlib. However, you can look in zlib's contrib/blast | ||
160 | directory for a possible solution to your problem. | ||
161 | |||
162 | 25. Can I access data randomly in a compressed stream? | ||
163 | |||
164 | No, not without some preparation. If when compressing you periodically | ||
165 | use Z_FULL_FLUSH, carefully write all the pending data at those points, | ||
166 | and keep an index of those locations, then you can start decompression | ||
167 | at those points. You have to be careful to not use Z_FULL_FLUSH too | ||
168 | often, since it can significantly degrade compression. | ||
169 | |||
170 | 26. Does zlib work on MVS, OS/390, CICS, etc.? | ||
171 | |||
172 | We don't know for sure. We have heard occasional reports of success on | ||
173 | these systems. If you do use it on one of these, please provide us with | ||
174 | a report, instructions, and patches that we can reference when we get | ||
175 | these questions. Thanks. | ||
176 | |||
177 | 27. Is there some simpler, easier to read version of inflate I can look at | ||
178 | to understand the deflate format? | ||
179 | |||
180 | First off, you should read RFC 1951. Second, yes. Look in zlib's | ||
181 | contrib/puff directory. | ||
182 | |||
183 | 28. Does zlib infringe on any patents? | ||
184 | |||
185 | As far as we know, no. In fact, that was originally the whole point behind | ||
186 | zlib. Look here for some more information: | ||
187 | |||
188 | http://www.gzip.org/#faq11 | ||
189 | |||
190 | 29. Can zlib work with greater than 4 GB of data? | ||
191 | |||
192 | Yes. inflate() and deflate() will process any amount of data correctly. | ||
193 | However the strm.total_in and strm_total_out counters may be limited to | ||
194 | 4 GB. The user can easily set up their own counters updated after each | ||
195 | call of inflate() or deflate() to count beyond 4 GB. compress() and | ||
196 | uncompress() may be limited to 4 GB, since they operate in a single | ||
197 | call using unsigned long lengths. gzseek() may be limited to 4 GB | ||
198 | depending on how zlib is compiled. | ||
199 | |||
200 | 30. Does zlib have any security vulnerabilities? | ||
201 | |||
202 | The only one that we are aware of is potentially in gzprintf(). If zlib | ||
203 | is compiled to use sprintf() or vsprintf(), then there is no protection | ||
204 | against a buffer overflow of a 4K string space, other than the caller of | ||
205 | gzprintf() assuring that the output will not exceed 4K. On the other | ||
206 | hand, if zlib is compiled to use snprintf() or vsnprintf(), then there is | ||
207 | no vulnerability. | ||
208 | |||
209 | Note that you should be using the most recent version of zlib. Versions | ||
210 | 1.1.3 and before were subject to a double-free vulnerability. | ||
211 | |||
212 | 31. Is there a Java version of zlib? | ||
213 | |||
214 | Probably what you want is to use zlib in Java. zlib is already included | ||
215 | as part of the Java SDK in the java.util.zip class. If you really want | ||
216 | a version of zlib written in the Java language, look on the zlib home | ||
217 | page for links: http://www.zlib.org/ | ||
218 | |||
219 | 32. I get this or that compiler or source-code scanner warning. Can't you guys | ||
220 | write proper code? | ||
221 | |||
222 | Many years ago, we gave up attempting to avoid warnings on every compiler | ||
223 | in the universe. It just got to be a waste of time, and some compilers | ||
224 | were downright silly. So now, we simply make sure that the code always | ||
225 | works. | ||
226 | |||
227 | 33. Will zlib read the (insert any ancient or arcane format here) compressed | ||
228 | data format? | ||
229 | |||
230 | Probably not. Look in the comp.compression FAQ for pointers to various | ||
231 | formats and associated software. | ||
232 | |||
233 | 34. How can I encrypt/decrypt zip files with zlib? | ||
234 | |||
235 | zlib doesn't support encryption. PKZIP encryption is very weak and can be | ||
236 | broken with freely available programs. To get strong encryption, use gpg | ||
237 | which already includes zlib compression. | ||
238 | |||
239 | 35. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings? | ||
240 | |||
241 | "gzip" is the gzip format, and "deflate" is the zlib format. They should | ||
242 | probably have called the second one "zlib" instead to avoid confusion | ||
243 | with the raw deflate compressed data format. While the HTTP 1.1 RFC 2616 | ||
244 | correctly points to the zlib specification in RFC 1950 for the "deflate" | ||
245 | transfer encoding, there have been reports of servers and browsers that | ||
246 | incorrectly produce or expect raw deflate data per the deflate | ||
247 | specficiation in RFC 1951, most notably Microsoft. So even though the | ||
248 | "deflate" transfer encoding using the zlib format would be the more | ||
249 | efficient approach (and in fact exactly what the zlib format was designed | ||
250 | for), using the "gzip" transfer encoding is probably more reliable due to | ||
251 | an unfortunate choice of name on the part of the HTTP 1.1 authors. | ||
252 | |||
253 | 36. Does zlib support the new "Deflate64" format introduced by PKWare? | ||
254 | |||
255 | No. PKWare has apparently decided to keep that format proprietary, since | ||
256 | they have not documented it as they have previous compression formats. | ||
257 | In any case, the compression improvements are so modest compared to other | ||
258 | more modern approaches, that it's not worth the effort to implement. | ||
259 | |||
260 | 37. Can you please sign these lengthy legal documents and fax them back to us | ||
261 | so that we can use your software in our product? | ||
262 | |||
263 | No. | ||