FAQ 16.2 KB
Newer Older
M
Mark Adler 已提交
1

M
Mark Adler 已提交
2
                Frequently Asked Questions about zlib
M
Mark Adler 已提交
3 4


M
Mark Adler 已提交
5
If your question is not there, please check the zlib home page
M
Mark Adler 已提交
6 7
http://zlib.net/ which may have more recent information.
The lastest zlib FAQ is at http://zlib.net/zlib_faq.html
M
Mark Adler 已提交
8 9


M
Mark Adler 已提交
10
 1. Is zlib Y2K-compliant?
M
Mark Adler 已提交
11

M
Mark Adler 已提交
12
    Yes. zlib doesn't handle dates.
M
Mark Adler 已提交
13

M
Mark Adler 已提交
14
 2. Where can I get a Windows DLL version?
M
Mark Adler 已提交
15

M
Mark Adler 已提交
16 17 18
    The zlib sources can be compiled without change to produce a DLL.  See the
    file win32/DLL_FAQ.txt in the zlib distribution.  Pointers to the
    precompiled DLL are found in the zlib web site at http://zlib.net/ .
M
Mark Adler 已提交
19

M
Mark Adler 已提交
20
 3. Where can I get a Visual Basic interface to zlib?
M
Mark Adler 已提交
21

M
Mark Adler 已提交
22
    See
M
Mark Adler 已提交
23
        * http://marknelson.us/1997/01/01/zlib-engine/
M
Mark Adler 已提交
24
        * win32/DLL_FAQ.txt in the zlib distribution
M
Mark Adler 已提交
25

M
Mark Adler 已提交
26
 4. compress() returns Z_BUF_ERROR.
M
Mark Adler 已提交
27

M
Mark Adler 已提交
28
    Make sure that before the call of compress(), the length of the compressed
M
Mark Adler 已提交
29 30
    buffer is equal to the available size of the compressed buffer and not
    zero.  For Visual Basic, check that this parameter is passed by reference
M
Mark Adler 已提交
31
    ("as any"), not by value ("as long").
M
Mark Adler 已提交
32

M
Mark Adler 已提交
33
 5. deflate() or inflate() returns Z_BUF_ERROR.
M
Mark Adler 已提交
34

M
Mark Adler 已提交
35 36 37 38 39 40 41 42 43
    Before making the call, make sure that avail_in and avail_out are not zero.
    When setting the parameter flush equal to Z_FINISH, also make sure that
    avail_out is big enough to allow processing all pending input.  Note that a
    Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be
    made with more input or output space.  A Z_BUF_ERROR may in fact be
    unavoidable depending on how the functions are used, since it is not
    possible to tell whether or not there is more output pending when
    strm.avail_out returns with zero.  See http://zlib.net/zlib_how.html for a
    heavily annotated example.
M
Mark Adler 已提交
44

M
Mark Adler 已提交
45
 6. Where's the zlib documentation (man pages, etc.)?
M
Mark Adler 已提交
46

47 48
    It's in zlib.h .  Examples of zlib usage are in the files test/example.c
    and test/minigzip.c, with more in examples/ .
M
Mark Adler 已提交
49

M
Mark Adler 已提交
50
 7. Why don't you use GNU autoconf or libtool or ...?
M
Mark Adler 已提交
51

M
Mark Adler 已提交
52 53
    Because we would like to keep zlib as a very small and simple package.
    zlib is rather portable and doesn't need much configuration.
M
Mark Adler 已提交
54

M
Mark Adler 已提交
55
 8. I found a bug in zlib.
M
Mark Adler 已提交
56

M
Mark Adler 已提交
57 58 59 60
    Most of the time, such problems are due to an incorrect usage of zlib.
    Please try to reproduce the problem with a small program and send the
    corresponding source to us at zlib@gzip.org .  Do not send multi-megabyte
    data files without prior agreement.
M
Mark Adler 已提交
61

M
Mark Adler 已提交
62
 9. Why do I get "undefined reference to gzputc"?
M
Mark Adler 已提交
63

M
Mark Adler 已提交
64
    If "make test" produces something like
M
Mark Adler 已提交
65

M
Mark Adler 已提交
66
       example.o(.text+0x154): undefined reference to `gzputc'
M
Mark Adler 已提交
67

M
Mark Adler 已提交
68 69
    check that you don't have old files libz.* in /usr/lib, /usr/local/lib or
    /usr/X11R6/lib. Remove any old versions, then do "make install".
M
Mark Adler 已提交
70

M
Mark Adler 已提交
71
10. I need a Delphi interface to zlib.
M
Mark Adler 已提交
72

M
Mark Adler 已提交
73
    See the contrib/delphi directory in the zlib distribution.
M
Mark Adler 已提交
74

M
Mark Adler 已提交
75
11. Can zlib handle .zip archives?
M
Mark Adler 已提交
76

M
Mark Adler 已提交
77
    Not by itself, no.  See the directory contrib/minizip in the zlib
M
Mark Adler 已提交
78
    distribution.
M
Mark Adler 已提交
79

M
Mark Adler 已提交
80
12. Can zlib handle .Z files?
M
Mark Adler 已提交
81

M
Mark Adler 已提交
82
    No, sorry.  You have to spawn an uncompress or gunzip subprocess, or adapt
M
Mark Adler 已提交
83
    the code of uncompress on your own.
M
Mark Adler 已提交
84

M
Mark Adler 已提交
85 86
13. How can I make a Unix shared library?

M
Mark Adler 已提交
87 88 89 90
    By default a shared (and a static) library is built for Unix.  So:

    make distclean
    ./configure
M
Mark Adler 已提交
91 92
    make

M
Mark Adler 已提交
93
14. How do I install a shared zlib library on Unix?
M
Mark Adler 已提交
94

M
Mark Adler 已提交
95 96
    After the above, then:

M
Mark Adler 已提交
97 98 99 100
    make install

    However, many flavors of Unix come with a shared zlib already installed.
    Before going to the trouble of compiling a shared version of zlib and
M
Mark Adler 已提交
101 102 103 104
    trying to install it, you may want to check if it's already there!  If you
    can #include <zlib.h>, it's there.  The -lz option will probably link to
    it.  You can check the version at the top of zlib.h or with the
    ZLIB_VERSION symbol defined in zlib.h .
M
Mark Adler 已提交
105

M
Mark Adler 已提交
106
15. I have a question about OttoPDF.
M
Mark Adler 已提交
107 108

    We are not the authors of OttoPDF. The real author is on the OttoPDF web
M
Mark Adler 已提交
109 110 111 112
    site: Joel Hainley, jhainley@myndkryme.com.

16. Can zlib decode Flate data in an Adobe PDF file?

M
Mark Adler 已提交
113 114
    Yes. See http://www.pdflib.com/ . To modify PDF forms, see
    http://sourceforge.net/projects/acroformtool/ .
M
Mark Adler 已提交
115 116 117 118 119 120

17. Why am I getting this "register_frame_info not found" error on Solaris?

    After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib
    generates an error such as:

M
Mark Adler 已提交
121
        ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so:
M
Mark Adler 已提交
122 123 124
        symbol __register_frame_info: referenced symbol not found

    The symbol __register_frame_info is not part of zlib, it is generated by
M
Mark Adler 已提交
125 126
    the C compiler (cc or gcc).  You must recompile applications using zlib
    which have this problem.  This problem is specific to Solaris.  See
M
Mark Adler 已提交
127 128
    http://www.sunfreeware.com for Solaris versions of zlib and applications
    using zlib.
M
Mark Adler 已提交
129

M
Mark Adler 已提交
130
18. Why does gzip give an error on a file I make with compress/deflate?
M
Mark Adler 已提交
131 132

    The compress and deflate functions produce data in the zlib format, which
M
Mark Adler 已提交
133 134 135 136
    is different and incompatible with the gzip format.  The gz* functions in
    zlib on the other hand use the gzip format.  Both the zlib and gzip formats
    use the same compressed data format internally, but have different headers
    and trailers around the compressed data.
M
Mark Adler 已提交
137

M
Mark Adler 已提交
138
19. Ok, so why are there two different formats?
M
Mark Adler 已提交
139

M
Mark Adler 已提交
140 141 142 143 144
    The gzip format was designed to retain the directory information about a
    single file, such as the name and last modification date.  The zlib format
    on the other hand was designed for in-memory and communication channel
    applications, and has a much more compact header and trailer and uses a
    faster integrity check than gzip.
M
Mark Adler 已提交
145

M
Mark Adler 已提交
146
20. Well that's nice, but how do I make a gzip file in memory?
M
Mark Adler 已提交
147

M
Mark Adler 已提交
148
    You can request that deflate write the gzip format instead of the zlib
M
Mark Adler 已提交
149 150
    format using deflateInit2().  You can also request that inflate decode the
    gzip format using inflateInit2().  Read zlib.h for more details.
M
Mark Adler 已提交
151

M
Mark Adler 已提交
152
21. Is zlib thread-safe?
M
Mark Adler 已提交
153

M
Mark Adler 已提交
154 155
    Yes.  However any library routines that zlib uses and any application-
    provided memory allocation routines must also be thread-safe.  zlib's gz*
M
Mark Adler 已提交
156
    functions use stdio library routines, and most of zlib's functions use the
M
Mark Adler 已提交
157 158
    library memory allocation routines by default.  zlib's *Init* functions
    allow for the application to provide custom memory allocation routines.
M
Mark Adler 已提交
159 160 161

    Of course, you should only operate on any given zlib or gzip stream from a
    single thread at a time.
M
Mark Adler 已提交
162

M
Mark Adler 已提交
163
22. Can I use zlib in my commercial application?
M
Mark Adler 已提交
164

M
Mark Adler 已提交
165
    Yes.  Please read the license in zlib.h.
M
Mark Adler 已提交
166

M
Mark Adler 已提交
167
23. Is zlib under the GNU license?
M
Mark Adler 已提交
168

M
Mark Adler 已提交
169
    No.  Please read the license in zlib.h.
M
Mark Adler 已提交
170

M
Mark Adler 已提交
171
24. The license says that altered source versions must be "plainly marked". So
M
Mark Adler 已提交
172 173
    what exactly do I need to do to meet that requirement?

M
Mark Adler 已提交
174
    You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h.  In
M
Mark Adler 已提交
175
    particular, the final version number needs to be changed to "f", and an
M
Mark Adler 已提交
176
    identification string should be appended to ZLIB_VERSION.  Version numbers
M
Mark Adler 已提交
177
    x.x.x.f are reserved for modifications to zlib by others than the zlib
M
Mark Adler 已提交
178
    maintainers.  For example, if the version of the base zlib you are altering
M
Mark Adler 已提交
179
    is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
M
Mark Adler 已提交
180
    ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3".  You can also
M
Mark Adler 已提交
181
    update the version strings in deflate.c and inftrees.c.
M
Mark Adler 已提交
182 183 184

    For altered source distributions, you should also note the origin and
    nature of the changes in zlib.h, as well as in ChangeLog and README, along
M
Mark Adler 已提交
185
    with the dates of the alterations.  The origin should include at least your
M
Mark Adler 已提交
186 187 188 189 190
    name (or your company's name), and an email address to contact for help or
    issues with the library.

    Note that distributing a compiled zlib library along with zlib.h and
    zconf.h is also a source distribution, and so you should change
M
Mark Adler 已提交
191 192
    ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes
    in zlib.h as you would for a full source distribution.
M
Mark Adler 已提交
193

M
Mark Adler 已提交
194
25. Will zlib work on a big-endian or little-endian architecture, and can I
M
Mark Adler 已提交
195 196 197 198
    exchange compressed data between them?

    Yes and yes.

M
Mark Adler 已提交
199
26. Will zlib work on a 64-bit machine?
M
Mark Adler 已提交
200

M
Mark Adler 已提交
201 202
    Yes.  It has been tested on 64-bit machines, and has no dependence on any
    data types being limited to 32-bits in length.  If you have any
M
Mark Adler 已提交
203 204
    difficulties, please provide a complete problem report to zlib@gzip.org

M
Mark Adler 已提交
205
27. Will zlib decompress data from the PKWare Data Compression Library?
M
Mark Adler 已提交
206

M
Mark Adler 已提交
207 208
    No.  The PKWare DCL uses a completely different compressed data format than
    does PKZIP and zlib.  However, you can look in zlib's contrib/blast
M
Mark Adler 已提交
209 210
    directory for a possible solution to your problem.

M
Mark Adler 已提交
211
28. Can I access data randomly in a compressed stream?
M
Mark Adler 已提交
212

M
Mark Adler 已提交
213 214 215 216 217 218 219
    No, not without some preparation.  If when compressing you periodically use
    Z_FULL_FLUSH, carefully write all the pending data at those points, and
    keep an index of those locations, then you can start decompression at those
    points.  You have to be careful to not use Z_FULL_FLUSH too often, since it
    can significantly degrade compression.  Alternatively, you can scan a
    deflate stream once to generate an index, and then use that index for
    random access.  See examples/zran.c .
M
Mark Adler 已提交
220

M
Mark Adler 已提交
221
29. Does zlib work on MVS, OS/390, CICS, etc.?
M
Mark Adler 已提交
222

M
Mark Adler 已提交
223 224 225 226
    It has in the past, but we have not heard of any recent evidence.  There
    were working ports of zlib 1.1.4 to MVS, but those links no longer work.
    If you know of recent, successful applications of zlib on these operating
    systems, please let us know.  Thanks.
M
Mark Adler 已提交
227

M
Mark Adler 已提交
228 229
30. Is there some simpler, easier to read version of inflate I can look at to
    understand the deflate format?
M
Mark Adler 已提交
230

M
Mark Adler 已提交
231
    First off, you should read RFC 1951.  Second, yes.  Look in zlib's
M
Mark Adler 已提交
232 233
    contrib/puff directory.

M
Mark Adler 已提交
234
31. Does zlib infringe on any patents?
M
Mark Adler 已提交
235

M
Mark Adler 已提交
236 237
    As far as we know, no.  In fact, that was originally the whole point behind
    zlib.  Look here for some more information:
M
Mark Adler 已提交
238 239 240

    http://www.gzip.org/#faq11

M
Mark Adler 已提交
241
32. Can zlib work with greater than 4 GB of data?
M
Mark Adler 已提交
242

M
Mark Adler 已提交
243
    Yes.  inflate() and deflate() will process any amount of data correctly.
M
Mark Adler 已提交
244 245
    Each call of inflate() or deflate() is limited to input and output chunks
    of the maximum value that can be stored in the compiler's "unsigned int"
M
Mark Adler 已提交
246 247
    type, but there is no limit to the number of chunks.  Note however that the
    strm.total_in and strm_total_out counters may be limited to 4 GB.  These
M
Mark Adler 已提交
248
    counters are provided as a convenience and are not used internally by
M
Mark Adler 已提交
249
    inflate() or deflate().  The application can easily set up its own counters
M
Mark Adler 已提交
250 251
    updated after each call of inflate() or deflate() to count beyond 4 GB.
    compress() and uncompress() may be limited to 4 GB, since they operate in a
M
Mark Adler 已提交
252 253
    single call.  gzseek() and gztell() may be limited to 4 GB depending on how
    zlib is compiled.  See the zlibCompileFlags() function in zlib.h.
M
Mark Adler 已提交
254

M
Mark Adler 已提交
255 256 257
    The word "may" appears several times above since there is a 4 GB limit only
    if the compiler's "long" type is 32 bits.  If the compiler's "long" type is
    64 bits, then the limit is 16 exabytes.
M
Mark Adler 已提交
258

M
Mark Adler 已提交
259
33. Does zlib have any security vulnerabilities?
M
Mark Adler 已提交
260

M
Mark Adler 已提交
261 262 263 264 265 266 267 268 269 270
    The only one that we are aware of is potentially in gzprintf().  If zlib is
    compiled to use sprintf() or vsprintf(), then there is no protection
    against a buffer overflow of an 8K string space (or other value as set by
    gzbuffer()), other than the caller of gzprintf() assuring that the output
    will not exceed 8K.  On the other hand, if zlib is compiled to use
    snprintf() or vsnprintf(), which should normally be the case, then there is
    no vulnerability.  The ./configure script will display warnings if an
    insecure variation of sprintf() will be used by gzprintf().  Also the
    zlibCompileFlags() function will return information on what variant of
    sprintf() is used by gzprintf().
M
Mark Adler 已提交
271 272 273 274 275

    If you don't have snprintf() or vsnprintf() and would like one, you can
    find a portable implementation here:

        http://www.ijs.si/software/snprintf/
M
Mark Adler 已提交
276

M
Mark Adler 已提交
277 278 279 280
    Note that you should be using the most recent version of zlib.  Versions
    1.1.3 and before were subject to a double-free vulnerability, and versions
    1.2.1 and 1.2.2 were subject to an access exception when decompressing
    invalid compressed data.
M
Mark Adler 已提交
281

M
Mark Adler 已提交
282
34. Is there a Java version of zlib?
M
Mark Adler 已提交
283 284

    Probably what you want is to use zlib in Java. zlib is already included
M
Mark Adler 已提交
285
    as part of the Java SDK in the java.util.zip package. If you really want
M
Mark Adler 已提交
286
    a version of zlib written in the Java language, look on the zlib home
M
Mark Adler 已提交
287
    page for links: http://zlib.net/ .
M
Mark Adler 已提交
288

M
Mark Adler 已提交
289 290
35. I get this or that compiler or source-code scanner warning when I crank it
    up to maximally-pedantic. Can't you guys write proper code?
M
Mark Adler 已提交
291 292

    Many years ago, we gave up attempting to avoid warnings on every compiler
M
Mark Adler 已提交
293 294 295
    in the universe.  It just got to be a waste of time, and some compilers
    were downright silly as well as contradicted each other.  So now, we simply
    make sure that the code always works.
M
Mark Adler 已提交
296

M
Mark Adler 已提交
297 298 299 300
36. Valgrind (or some similar memory access checker) says that deflate is
    performing a conditional jump that depends on an uninitialized value.
    Isn't that a bug?

M
Mark Adler 已提交
301 302 303 304 305 306
    No.  That is intentional for performance reasons, and the output of deflate
    is not affected.  This only started showing up recently since zlib 1.2.x
    uses malloc() by default for allocations, whereas earlier versions used
    calloc(), which zeros out the allocated memory.  Even though the code was
    correct, versions 1.2.4 and later was changed to not stimulate these
    checkers.
M
Mark Adler 已提交
307 308

37. Will zlib read the (insert any ancient or arcane format here) compressed
M
Mark Adler 已提交
309 310 311 312 313
    data format?

    Probably not. Look in the comp.compression FAQ for pointers to various
    formats and associated software.

M
Mark Adler 已提交
314
38. How can I encrypt/decrypt zip files with zlib?
M
Mark Adler 已提交
315

M
Mark Adler 已提交
316 317 318 319 320
    zlib doesn't support encryption.  The original PKZIP encryption is very
    weak and can be broken with freely available programs.  To get strong
    encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib
    compression.  For PKZIP compatible "encryption", look at
    http://www.info-zip.org/
M
Mark Adler 已提交
321

M
Mark Adler 已提交
322
39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
M
Mark Adler 已提交
323

M
Mark Adler 已提交
324 325 326
    "gzip" is the gzip format, and "deflate" is the zlib format.  They should
    probably have called the second one "zlib" instead to avoid confusion with
    the raw deflate compressed data format.  While the HTTP 1.1 RFC 2616
M
Mark Adler 已提交
327 328 329
    correctly points to the zlib specification in RFC 1950 for the "deflate"
    transfer encoding, there have been reports of servers and browsers that
    incorrectly produce or expect raw deflate data per the deflate
M
Mark Adler 已提交
330
    specification in RFC 1951, most notably Microsoft.  So even though the
M
Mark Adler 已提交
331 332 333 334 335
    "deflate" transfer encoding using the zlib format would be the more
    efficient approach (and in fact exactly what the zlib format was designed
    for), using the "gzip" transfer encoding is probably more reliable due to
    an unfortunate choice of name on the part of the HTTP 1.1 authors.

M
Mark Adler 已提交
336 337
    Bottom line: use the gzip format for HTTP 1.1 encoding.

M
Mark Adler 已提交
338
40. Does zlib support the new "Deflate64" format introduced by PKWare?
M
Mark Adler 已提交
339

M
Mark Adler 已提交
340 341 342 343
    No.  PKWare has apparently decided to keep that format proprietary, since
    they have not documented it as they have previous compression formats.  In
    any case, the compression improvements are so modest compared to other more
    modern approaches, that it's not worth the effort to implement.
M
Mark Adler 已提交
344

M
Mark Adler 已提交
345 346
41. I'm having a problem with the zip functions in zlib, can you help?

M
Mark Adler 已提交
347 348 349 350 351
    There are no zip functions in zlib.  You are probably using minizip by
    Giles Vollant, which is found in the contrib directory of zlib.  It is not
    part of zlib.  In fact none of the stuff in contrib is part of zlib.  The
    files in there are not supported by the zlib authors.  You need to contact
    the authors of the respective contribution for help.
M
Mark Adler 已提交
352 353 354 355 356

42. The match.asm code in contrib is under the GNU General Public License.
    Since it's part of zlib, doesn't that mean that all of zlib falls under the
    GNU GPL?

M
Mark Adler 已提交
357
    No.  The files in contrib are not part of zlib.  They were contributed by
M
Mark Adler 已提交
358
    other authors and are provided as a convenience to the user within the zlib
M
Mark Adler 已提交
359
    distribution.  Each item in contrib has its own license.
M
Mark Adler 已提交
360 361 362 363

43. Is zlib subject to export controls?  What is its ECCN?

    zlib is not subject to export controls, and so is classified as EAR99.
M
Mark Adler 已提交
364

M
Mark Adler 已提交
365
44. Can you please sign these lengthy legal documents and fax them back to us
M
Mark Adler 已提交
366 367
    so that we can use your software in our product?

M
Mark Adler 已提交
368
    No. Go away. Shoo.