14
14
2. Where can I get a Windows DLL version?
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
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. Pointers to the
18
precompiled DLL are found in the zlib web site at http://zlib.net/ .
21
20
3. Where can I get a Visual Basic interface to zlib?
24
* http://www.dogma.net/markn/articles/zlibtool/zlibtool.htm
25
* contrib/visual-basic.txt in the zlib distribution
23
* http://marknelson.us/1997/01/01/zlib-engine/
26
24
* win32/DLL_FAQ.txt in the zlib distribution
28
26
4. compress() returns Z_BUF_ERROR.
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
28
Make sure that before the call of compress(), the length of the compressed
29
buffer is equal to the available size of the compressed buffer and not
30
zero. For Visual Basic, check that this parameter is passed by reference
33
31
("as any"), not by value ("as long").
35
33
5. deflate() or inflate() returns Z_BUF_ERROR.
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.
35
Before making the call, make sure that avail_in and avail_out are not zero.
36
When setting the parameter flush equal to Z_FINISH, also make sure that
37
avail_out is big enough to allow processing all pending input. Note that a
38
Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be
39
made with more input or output space. A Z_BUF_ERROR may in fact be
40
unavoidable depending on how the functions are used, since it is not
41
possible to tell whether or not there is more output pending when
42
strm.avail_out returns with zero. See http://zlib.net/zlib_how.html for a
43
heavily annotated example.
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45
6. Where's the zlib documentation (man pages, etc.)?
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 (zlib@gzip.org). Examples of zlib usage are in the files
51
example.c and minigzip.c.
47
It's in zlib.h . Examples of zlib usage are in the files test/example.c
48
and test/minigzip.c, with more in examples/ .
53
50
7. Why don't you use GNU autoconf or libtool or ...?
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.
52
Because we would like to keep zlib as a very small and simple package.
53
zlib is rather portable and doesn't need much configuration.
58
55
8. I found a bug in zlib.
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 zlib@gzip.org . Do not send
63
multi-megabyte data files without prior agreement.
57
Most of the time, such problems are due to an incorrect usage of zlib.
58
Please try to reproduce the problem with a small program and send the
59
corresponding source to us at zlib@gzip.org . Do not send multi-megabyte
60
data files without prior agreement.
65
62
9. Why do I get "undefined reference to gzputc"?
121
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symbol __register_frame_info: referenced symbol not found
123
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
which have this problem. This problem is specific to Solaris. See
125
the C compiler (cc or gcc). You must recompile applications using zlib
126
which have this problem. This problem is specific to Solaris. See
126
127
http://www.sunfreeware.com for Solaris versions of zlib and applications
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130
18. Why does gzip give an error on a file I make with compress/deflate?
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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
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.
133
is different and incompatible with the gzip format. The gz* functions in
134
zlib on the other hand use the gzip format. Both the zlib and gzip formats
135
use the same compressed data format internally, but have different headers
136
and trailers around the compressed data.
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138
19. Ok, so why are there two different formats?
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.
140
The gzip format was designed to retain the directory information about a
141
single file, such as the name and last modification date. The zlib format
142
on the other hand was designed for in-memory and communication channel
143
applications, and has a much more compact header and trailer and uses a
144
faster integrity check than gzip.
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146
20. Well that's nice, but how do I make a gzip file in memory?
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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
the gzip format using inflateInit2(). Read zlib.h for more details.
149
format using deflateInit2(). You can also request that inflate decode the
150
gzip format using inflateInit2(). Read zlib.h for more details.
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152
21. Is zlib thread-safe?
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*
154
Yes. However any library routines that zlib uses and any application-
155
provided memory allocation routines must also be thread-safe. zlib's gz*
155
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
for the application to provide custom memory allocation routines.
157
library memory allocation routines by default. zlib's *Init* functions
158
allow for the application to provide custom memory allocation routines.
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160
Of course, you should only operate on any given zlib or gzip stream from a
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161
single thread at a time.
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163
22. Can I use zlib in my commercial application?
164
Yes. Please read the license in zlib.h.
165
Yes. Please read the license in zlib.h.
166
167
23. Is zlib under the GNU license?
168
No. Please read the license in zlib.h.
169
No. Please read the license in zlib.h.
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171
24. The license says that altered source versions must be "plainly marked". So
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172
what exactly do I need to do to meet that requirement?
173
You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In
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You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In
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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
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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
179
is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
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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
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181
update the version strings in deflate.c and inftrees.c.
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For altered source distributions, you should also note the origin and
183
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
186
name (or your company's name), and an email address to contact for help or
186
187
issues with the library.
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199
26. Will zlib work on a 64-bit machine?
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
201
Yes. It has been tested on 64-bit machines, and has no dependence on any
202
data types being limited to 32-bits in length. If you have any
202
203
difficulties, please provide a complete problem report to zlib@gzip.org
204
205
27. Will zlib decompress data from the PKWare Data Compression Library?
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
207
No. The PKWare DCL uses a completely different compressed data format than
208
does PKZIP and zlib. However, you can look in zlib's contrib/blast
208
209
directory for a possible solution to your problem.
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211
28. Can I access data randomly in a compressed stream?
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.
213
No, not without some preparation. If when compressing you periodically use
214
Z_FULL_FLUSH, carefully write all the pending data at those points, and
215
keep an index of those locations, then you can start decompression at those
216
points. You have to be careful to not use Z_FULL_FLUSH too often, since it
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 .
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221
29. Does zlib work on MVS, OS/390, CICS, etc.?
220
Yes, there are working ports of zlib 1.1.4 to MVS which you can find
223
http://www.homerow.net/asm/zlib390.htm
224
http://www.homerow.net/asm/zlibLE.htm
226
If these are updated to more recent versions of zlib, please let us
229
30. Is there some simpler, easier to read version of inflate I can look at
230
to understand the deflate format?
232
First off, you should read RFC 1951. Second, yes. Look in zlib's
223
It has in the past, but we have not heard of any recent evidence. There
224
were working ports of zlib 1.1.4 to MVS, but those links no longer work.
225
If you know of recent, successful applications of zlib on these operating
226
systems, please let us know. Thanks.
228
30. Is there some simpler, easier to read version of inflate I can look at to
229
understand the deflate format?
231
First off, you should read RFC 1951. Second, yes. Look in zlib's
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232
contrib/puff directory.
235
234
31. Does zlib infringe on any patents?
237
As far as we know, no. In fact, that was originally the whole point behind
238
zlib. Look here for some more information:
236
As far as we know, no. In fact, that was originally the whole point behind
237
zlib. Look here for some more information:
240
239
http://www.gzip.org/#faq11
242
241
32. Can zlib work with greater than 4 GB of data?
244
Yes. inflate() and deflate() will process any amount of data correctly.
243
Yes. inflate() and deflate() will process any amount of data correctly.
245
244
Each call of inflate() or deflate() is limited to input and output chunks
246
245
of the maximum value that can be stored in the compiler's "unsigned int"
247
type, but there is no limit to the number of chunks. Note however that the
248
strm.total_in and strm_total_out counters may be limited to 4 GB. These
246
type, but there is no limit to the number of chunks. Note however that the
247
strm.total_in and strm_total_out counters may be limited to 4 GB. These
249
248
counters are provided as a convenience and are not used internally by
250
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
251
250
updated after each call of inflate() or deflate() to count beyond 4 GB.
252
251
compress() and uncompress() may be limited to 4 GB, since they operate in a
253
single call. gzseek() and gztell() may be limited to 4 GB depending on how
254
zlib is compiled. See the zlibCompileFlags() function in zlib.h.
252
single call. gzseek() and gztell() may be limited to 4 GB depending on how
253
zlib is compiled. See the zlibCompileFlags() function in zlib.h.
256
The word "may" appears several times above since there is a 4 GB limit
257
only if the compiler's "long" type is 32 bits. If the compiler's "long"
258
type is 64 bits, then the limit is 16 exabytes.
255
The word "may" appears several times above since there is a 4 GB limit only
256
if the compiler's "long" type is 32 bits. If the compiler's "long" type is
257
64 bits, then the limit is 16 exabytes.
260
259
33. Does zlib have any security vulnerabilities?
262
The only one that we are aware of is potentially in gzprintf(). If zlib
263
is compiled to use sprintf() or vsprintf(), then there is no protection
264
against a buffer overflow of a 4K string space, other than the caller of
265
gzprintf() assuring that the output will not exceed 4K. On the other
266
hand, if zlib is compiled to use snprintf() or vsnprintf(), which should
267
normally be the case, then there is no vulnerability. The ./configure
268
script will display warnings if an insecure variation of sprintf() will
269
be used by gzprintf(). Also the zlibCompileFlags() function will return
270
information on what variant of sprintf() is used by gzprintf().
261
The only one that we are aware of is potentially in gzprintf(). If zlib is
262
compiled to use sprintf() or vsprintf(), then there is no protection
263
against a buffer overflow of an 8K string space (or other value as set by
264
gzbuffer()), other than the caller of gzprintf() assuring that the output
265
will not exceed 8K. On the other hand, if zlib is compiled to use
266
snprintf() or vsnprintf(), which should normally be the case, then there is
267
no vulnerability. The ./configure script will display warnings if an
268
insecure variation of sprintf() will be used by gzprintf(). Also the
269
zlibCompileFlags() function will return information on what variant of
270
sprintf() is used by gzprintf().
272
272
If you don't have snprintf() or vsnprintf() and would like one, you can
273
273
find a portable implementation here:
275
275
http://www.ijs.si/software/snprintf/
277
Note that you should be using the most recent version of zlib. Versions
278
1.1.3 and before were subject to a double-free vulnerability, and version
279
1.2.1 was subject to an access exception when decompressing invalid
277
Note that you should be using the most recent version of zlib. Versions
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.
282
282
34. Is there a Java version of zlib?
284
284
Probably what you want is to use zlib in Java. zlib is already included
285
285
as part of the Java SDK in the java.util.zip package. If you really want
286
286
a version of zlib written in the Java language, look on the zlib home
287
page for links: http://www.zlib.org/
287
page for links: http://zlib.net/ .
289
289
35. I get this or that compiler or source-code scanner warning when I crank it
290
290
up to maximally-pedantic. Can't you guys write proper code?
292
292
Many years ago, we gave up attempting to avoid warnings on every compiler
293
in the universe. It just got to be a waste of time, and some compilers
294
were downright silly. So now, we simply make sure that the code always
293
in the universe. It just got to be a waste of time, and some compilers
294
were downright silly as well as contradicted each other. So now, we simply
295
make sure that the code always works.
297
297
36. Valgrind (or some similar memory access checker) says that deflate is
298
298
performing a conditional jump that depends on an uninitialized value.
299
299
Isn't that a bug?
301
No. That is intentional for performance reasons, and the output of
302
deflate is not affected. This only started showing up recently since
303
zlib 1.2.x uses malloc() by default for allocations, whereas earlier
304
versions used calloc(), which zeros out the allocated memory.
301
No. That is intentional for performance reasons, and the output of deflate
302
is not affected. This only started showing up recently since zlib 1.2.x
303
uses malloc() by default for allocations, whereas earlier versions used
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
308
37. Will zlib read the (insert any ancient or arcane format here) compressed
312
314
38. How can I encrypt/decrypt zip files with zlib?
314
zlib doesn't support encryption. The original PKZIP encryption is very weak
315
and can be broken with freely available programs. To get strong encryption,
316
use GnuPG, http://www.gnupg.org/ , which already includes zlib compression.
317
For PKZIP compatible "encryption", look at http://www.info-zip.org/
316
zlib doesn't support encryption. The original PKZIP encryption is very
317
weak and can be broken with freely available programs. To get strong
318
encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib
319
compression. For PKZIP compatible "encryption", look at
320
http://www.info-zip.org/
319
322
39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
321
"gzip" is the gzip format, and "deflate" is the zlib format. They should
322
probably have called the second one "zlib" instead to avoid confusion
323
with the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
324
"gzip" is the gzip format, and "deflate" is the zlib format. They should
325
probably have called the second one "zlib" instead to avoid confusion with
326
the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
324
327
correctly points to the zlib specification in RFC 1950 for the "deflate"
325
328
transfer encoding, there have been reports of servers and browsers that
326
329
incorrectly produce or expect raw deflate data per the deflate
327
specficiation in RFC 1951, most notably Microsoft. So even though the
330
specification in RFC 1951, most notably Microsoft. So even though the
328
331
"deflate" transfer encoding using the zlib format would be the more
329
332
efficient approach (and in fact exactly what the zlib format was designed
330
333
for), using the "gzip" transfer encoding is probably more reliable due to
335
338
40. Does zlib support the new "Deflate64" format introduced by PKWare?
337
No. PKWare has apparently decided to keep that format proprietary, since
338
they have not documented it as they have previous compression formats.
339
In any case, the compression improvements are so modest compared to other
340
more modern approaches, that it's not worth the effort to implement.
340
No. PKWare has apparently decided to keep that format proprietary, since
341
they have not documented it as they have previous compression formats. In
342
any case, the compression improvements are so modest compared to other more
343
modern approaches, that it's not worth the effort to implement.
342
345
41. I'm having a problem with the zip functions in zlib, can you help?
344
There are no zip functions in zlib. You are probably using minizip by
345
Giles Vollant, which is found in the contrib directory of zlib. It is not
346
part of zlib. In fact none of the stuff in contrib is part of zlib. The
347
files in there are not supported by the zlib authors. You need to contact
348
the authors of the contribution for help.
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.
350
353
42. The match.asm code in contrib is under the GNU General Public License.
351
354
Since it's part of zlib, doesn't that mean that all of zlib falls under the
354
No. The files in contrib are not part of zlib. They were contributed by
357
No. The files in contrib are not part of zlib. They were contributed by
355
358
other authors and are provided as a convenience to the user within the zlib
356
distribution. Each of the items in contrib have their own license.
358
43. Can you please sign these lengthy legal documents and fax them back to us
359
distribution. Each item in contrib has its own license.
361
43. Is zlib subject to export controls? What is its ECCN?
363
zlib is not subject to export controls, and so is classified as EAR99.
365
44. Can you please sign these lengthy legal documents and fax them back to us
359
366
so that we can use your software in our product?
361
368
No. Go away. Shoo.
added
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