source: rtems/cpukit/zlib/FAQ @ d71b3a7

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2                Frequently Asked Questions about zlib
5If your question is not there, please check the zlib home page
6 which may have more recent information.
7The lastest zlib FAQ is at
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 .
20 3. Where can I get a Visual Basic interface to zlib?
22    See
23        *
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 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 .  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".
7110. I need a Delphi interface to zlib.
73    See the contrib/delphi directory in the zlib distribution.
7511. Can zlib handle .zip archives?
77    Not by itself, no.  See the directory contrib/minizip in the zlib
78    distribution.
8012. 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.
8513. How can I make a Unix shared library?
87    make clean
88    ./configure -s
89    make
9114. How do I install a shared zlib library on Unix?
93    After the above, then:
95    make install
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 .
10415. 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,
10916. Can zlib decode Flate data in an Adobe PDF file?
111    Yes. See . To modify PDF forms, see
112 .
11417. 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 rpm: fatal: relocation error: file /usr/local/lib/
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 for Solaris versions of zlib and applications
126    using zlib.
12818. 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.
13619. 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.
14420. 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.
15021. 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.
16122. Can I use zlib in my commercial application?
163    Yes.  Please read the license in zlib.h.
16523. Is zlib under the GNU license?
167    No.  Please read the license in zlib.h.
16924. 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 "", 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.
19225. Will zlib work on a big-endian or little-endian architecture, and can I
193    exchange compressed data between them?
195    Yes and yes.
19726. 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
20327. 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.
20928. 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 .
21929. 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.
22630. 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.
23231. 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:
23932. 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.
25733. 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:
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.
28034. 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 package. If you really want
284    a version of zlib written in the Java language, look on the zlib home
285    page for links: .
28735. 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.
29536. Valgrind (or some similar memory access checker) says that deflate is
296    performing a conditional jump that depends on an uninitialized value.
297    Isn't that a bug?
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
304    checkers.
30637. Will zlib read the (insert any ancient or arcane format here) compressed
307    data format?
309    Probably not. Look in the comp.compression FAQ for pointers to various
310    formats and associated software.
31238. 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, , which already includes zlib
317    compression.  For PKZIP compatible "encryption", look at
32039. 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.
33640. 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.
34341. 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.
35142. 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
353    GNU GPL?
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.
35943. Is zlib subject to export controls?  What is its ECCN?
361    zlib is not subject to export controls, and so is classified as EAR99.
36344. 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?
366    No. Go away. Shoo.
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