2
Frequently Asked Questions about zlib
5
If your question is not there, please check the zlib home page
6
http://zlib.net/ which may have more recent information.
7
The lastest zlib FAQ is at http://zlib.net/zlib_faq.html
10
1. Is zlib Y2K-compliant?
12
Yes. zlib doesn't handle dates.
14
2. Where can I get a Windows DLL version?
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/ .
20
3. Where can I get a Visual Basic interface to zlib?
23
* http://marknelson.us/1997/01/01/zlib-engine/
24
* win32/DLL_FAQ.txt in the zlib distribution
26
4. compress() returns Z_BUF_ERROR.
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
31
("as any"), not by value ("as long").
33
5. deflate() or inflate() returns Z_BUF_ERROR.
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.
45
6. Where's the zlib documentation (man pages, etc.)?
47
It's in zlib.h . Examples of zlib usage are in the files example.c and
48
minigzip.c, with more in examples/ .
50
7. Why don't you use GNU autoconf or libtool or ...?
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.
55
8. I found a bug in zlib.
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.
62
9. Why do I get "undefined reference to gzputc"?
64
If "make test" produces something like
66
example.o(.text+0x154): undefined reference to `gzputc'
68
check that you don't have old files libz.* in /usr/lib, /usr/local/lib or
69
/usr/X11R6/lib. Remove any old versions, then do "make install".
71
10. I need a Delphi interface to zlib.
73
See the contrib/delphi directory in the zlib distribution.
75
11. Can zlib handle .zip archives?
77
Not by itself, no. See the directory contrib/minizip in the zlib
80
12. Can zlib handle .Z files?
82
No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt
83
the code of uncompress on your own.
85
13. How can I make a Unix shared library?
91
14. How do I install a shared zlib library on Unix?
93
After the above, then:
97
However, many flavors of Unix come with a shared zlib already installed.
98
Before going to the trouble of compiling a shared version of zlib and
99
trying to install it, you may want to check if it's already there! If you
100
can #include <zlib.h>, it's there. The -lz option will probably link to
101
it. You can check the version at the top of zlib.h or with the
102
ZLIB_VERSION symbol defined in zlib.h .
104
15. I have a question about OttoPDF.
106
We are not the authors of OttoPDF. The real author is on the OttoPDF web
107
site: Joel Hainley, jhainley@myndkryme.com.
109
16. Can zlib decode Flate data in an Adobe PDF file?
111
Yes. See http://www.pdflib.com/ . To modify PDF forms, see
112
http://sourceforge.net/projects/acroformtool/ .
114
17. Why am I getting this "register_frame_info not found" error on Solaris?
116
After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
117
generates an error such as:
119
ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
120
symbol __register_frame_info: referenced symbol not found
122
The symbol __register_frame_info is not part of zlib, it is generated by
123
the C compiler (cc or gcc). You must recompile applications using zlib
124
which have this problem. This problem is specific to Solaris. See
125
http://www.sunfreeware.com for Solaris versions of zlib and applications
128
18. Why does gzip give an error on a file I make with compress/deflate?
130
The compress and deflate functions produce data in the zlib format, which
131
is different and incompatible with the gzip format. The gz* functions in
132
zlib on the other hand use the gzip format. Both the zlib and gzip formats
133
use the same compressed data format internally, but have different headers
134
and trailers around the compressed data.
136
19. Ok, so why are there two different formats?
138
The gzip format was designed to retain the directory information about a
139
single file, such as the name and last modification date. The zlib format
140
on the other hand was designed for in-memory and communication channel
141
applications, and has a much more compact header and trailer and uses a
142
faster integrity check than gzip.
144
20. Well that's nice, but how do I make a gzip file in memory?
146
You can request that deflate write the gzip format instead of the zlib
147
format using deflateInit2(). You can also request that inflate decode the
148
gzip format using inflateInit2(). Read zlib.h for more details.
150
21. Is zlib thread-safe?
152
Yes. However any library routines that zlib uses and any application-
153
provided memory allocation routines must also be thread-safe. zlib's gz*
154
functions use stdio library routines, and most of zlib's functions use the
155
library memory allocation routines by default. zlib's *Init* functions
156
allow for the application to provide custom memory allocation routines.
158
Of course, you should only operate on any given zlib or gzip stream from a
159
single thread at a time.
161
22. Can I use zlib in my commercial application?
163
Yes. Please read the license in zlib.h.
165
23. Is zlib under the GNU license?
167
No. Please read the license in zlib.h.
169
24. The license says that altered source versions must be "plainly marked". So
170
what exactly do I need to do to meet that requirement?
172
You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In
173
particular, the final version number needs to be changed to "f", and an
174
identification string should be appended to ZLIB_VERSION. Version numbers
175
x.x.x.f are reserved for modifications to zlib by others than the zlib
176
maintainers. For example, if the version of the base zlib you are altering
177
is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
178
ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also
179
update the version strings in deflate.c and inftrees.c.
181
For altered source distributions, you should also note the origin and
182
nature of the changes in zlib.h, as well as in ChangeLog and README, along
183
with the dates of the alterations. The origin should include at least your
184
name (or your company's name), and an email address to contact for help or
185
issues with the library.
187
Note that distributing a compiled zlib library along with zlib.h and
188
zconf.h is also a source distribution, and so you should change
189
ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes
190
in zlib.h as you would for a full source distribution.
192
25. Will zlib work on a big-endian or little-endian architecture, and can I
193
exchange compressed data between them?
197
26. Will zlib work on a 64-bit machine?
199
Yes. It has been tested on 64-bit machines, and has no dependence on any
200
data types being limited to 32-bits in length. If you have any
201
difficulties, please provide a complete problem report to zlib@gzip.org
203
27. Will zlib decompress data from the PKWare Data Compression Library?
205
No. The PKWare DCL uses a completely different compressed data format than
206
does PKZIP and zlib. However, you can look in zlib's contrib/blast
207
directory for a possible solution to your problem.
209
28. Can I access data randomly in a compressed stream?
211
No, not without some preparation. If when compressing you periodically use
212
Z_FULL_FLUSH, carefully write all the pending data at those points, and
213
keep an index of those locations, then you can start decompression at those
214
points. You have to be careful to not use Z_FULL_FLUSH too often, since it
215
can significantly degrade compression. Alternatively, you can scan a
216
deflate stream once to generate an index, and then use that index for
217
random access. See examples/zran.c .
219
29. Does zlib work on MVS, OS/390, CICS, etc.?
221
It has in the past, but we have not heard of any recent evidence. There
222
were working ports of zlib 1.1.4 to MVS, but those links no longer work.
223
If you know of recent, successful applications of zlib on these operating
224
systems, please let us know. Thanks.
226
30. Is there some simpler, easier to read version of inflate I can look at to
227
understand the deflate format?
229
First off, you should read RFC 1951. Second, yes. Look in zlib's
230
contrib/puff directory.
232
31. Does zlib infringe on any patents?
234
As far as we know, no. In fact, that was originally the whole point behind
235
zlib. Look here for some more information:
237
http://www.gzip.org/#faq11
239
32. Can zlib work with greater than 4 GB of data?
241
Yes. inflate() and deflate() will process any amount of data correctly.
242
Each call of inflate() or deflate() is limited to input and output chunks
243
of the maximum value that can be stored in the compiler's "unsigned int"
244
type, but there is no limit to the number of chunks. Note however that the
245
strm.total_in and strm_total_out counters may be limited to 4 GB. These
246
counters are provided as a convenience and are not used internally by
247
inflate() or deflate(). The application can easily set up its own counters
248
updated after each call of inflate() or deflate() to count beyond 4 GB.
249
compress() and uncompress() may be limited to 4 GB, since they operate in a
250
single call. gzseek() and gztell() may be limited to 4 GB depending on how
251
zlib is compiled. See the zlibCompileFlags() function in zlib.h.
253
The word "may" appears several times above since there is a 4 GB limit only
254
if the compiler's "long" type is 32 bits. If the compiler's "long" type is
255
64 bits, then the limit is 16 exabytes.
257
33. Does zlib have any security vulnerabilities?
259
The only one that we are aware of is potentially in gzprintf(). If zlib is
260
compiled to use sprintf() or vsprintf(), then there is no protection
261
against a buffer overflow of an 8K string space (or other value as set by
262
gzbuffer()), other than the caller of gzprintf() assuring that the output
263
will not exceed 8K. On the other hand, if zlib is compiled to use
264
snprintf() or vsnprintf(), which should normally be the case, then there is
265
no vulnerability. The ./configure script will display warnings if an
266
insecure variation of sprintf() will be used by gzprintf(). Also the
267
zlibCompileFlags() function will return information on what variant of
268
sprintf() is used by gzprintf().
270
If you don't have snprintf() or vsnprintf() and would like one, you can
271
find a portable implementation here:
273
http://www.ijs.si/software/snprintf/
275
Note that you should be using the most recent version of zlib. Versions
276
1.1.3 and before were subject to a double-free vulnerability, and versions
277
1.2.1 and 1.2.2 were subject to an access exception when decompressing
278
invalid compressed data.
280
34. Is there a Java version of zlib?
282
Probably what you want is to use zlib in Java. zlib is already included
283
as part of the Java SDK in the java.util.zip package. If you really want
284
a version of zlib written in the Java language, look on the zlib home
285
page for links: http://zlib.net/ .
287
35. I get this or that compiler or source-code scanner warning when I crank it
288
up to maximally-pedantic. Can't you guys write proper code?
290
Many years ago, we gave up attempting to avoid warnings on every compiler
291
in the universe. It just got to be a waste of time, and some compilers
292
were downright silly as well as contradicted each other. So now, we simply
293
make sure that the code always works.
295
36. Valgrind (or some similar memory access checker) says that deflate is
296
performing a conditional jump that depends on an uninitialized value.
299
No. That is intentional for performance reasons, and the output of deflate
300
is not affected. This only started showing up recently since zlib 1.2.x
301
uses malloc() by default for allocations, whereas earlier versions used
302
calloc(), which zeros out the allocated memory. Even though the code was
303
correct, versions 1.2.4 and later was changed to not stimulate these
306
37. Will zlib read the (insert any ancient or arcane format here) compressed
309
Probably not. Look in the comp.compression FAQ for pointers to various
310
formats and associated software.
312
38. How can I encrypt/decrypt zip files with zlib?
314
zlib doesn't support encryption. The original PKZIP encryption is very
315
weak and can be broken with freely available programs. To get strong
316
encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib
317
compression. For PKZIP compatible "encryption", look at
318
http://www.info-zip.org/
320
39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
322
"gzip" is the gzip format, and "deflate" is the zlib format. They should
323
probably have called the second one "zlib" instead to avoid confusion with
324
the raw deflate compressed data format. While the HTTP 1.1 RFC 2616
325
correctly points to the zlib specification in RFC 1950 for the "deflate"
326
transfer encoding, there have been reports of servers and browsers that
327
incorrectly produce or expect raw deflate data per the deflate
328
specficiation in RFC 1951, most notably Microsoft. So even though the
329
"deflate" transfer encoding using the zlib format would be the more
330
efficient approach (and in fact exactly what the zlib format was designed
331
for), using the "gzip" transfer encoding is probably more reliable due to
332
an unfortunate choice of name on the part of the HTTP 1.1 authors.
334
Bottom line: use the gzip format for HTTP 1.1 encoding.
336
40. Does zlib support the new "Deflate64" format introduced by PKWare?
338
No. PKWare has apparently decided to keep that format proprietary, since
339
they have not documented it as they have previous compression formats. In
340
any case, the compression improvements are so modest compared to other more
341
modern approaches, that it's not worth the effort to implement.
343
41. I'm having a problem with the zip functions in zlib, can you help?
345
There are no zip functions in zlib. You are probably using minizip by
346
Giles Vollant, which is found in the contrib directory of zlib. It is not
347
part of zlib. In fact none of the stuff in contrib is part of zlib. The
348
files in there are not supported by the zlib authors. You need to contact
349
the authors of the respective contribution for help.
351
42. The match.asm code in contrib is under the GNU General Public License.
352
Since it's part of zlib, doesn't that mean that all of zlib falls under the
355
No. The files in contrib are not part of zlib. They were contributed by
356
other authors and are provided as a convenience to the user within the zlib
357
distribution. Each item in contrib has its own license.
359
43. Is zlib subject to export controls? What is its ECCN?
361
zlib is not subject to export controls, and so is classified as EAR99.
363
44. Can you please sign these lengthy legal documents and fax them back to us
364
so that we can use your software in our product?