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Frequently Asked Questions about zlib
If your question is not there, please check the zlib home page which may have more recent information.
The lastest zlib FAQ is at
1. Is zlib Y2K-compliant?
Yes. zlib doesn't handle dates.
2. Where can I get a Windows DLL version?
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
3. Where can I get a Visual Basic interface to zlib?
* contrib/visual-basic.txt in the zlib distribution
* win32/DLL_FAQ.txt in the zlib distribution
4. compress() returns Z_BUF_ERROR.
Make sure that before the call of compress, the length of the compressed
buffer is equal to the total size of the compressed buffer and not
zero. For Visual Basic, check that this parameter is passed by reference
("as any"), not by value ("as long").
5. deflate() or inflate() returns Z_BUF_ERROR.
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.
6. Where's the zlib documentation (man pages, etc.)?
It's in zlib.h for the moment, and Francis S. Lin has converted it to a
web page zlib.html. Volunteers to transform this to Unix-style man pages,
please contact us ( Examples of zlib usage are in the files
example.c and minigzip.c.
7. Why don't you use GNU autoconf or libtool or ...?
Because we would like to keep zlib as a very small and simple
package. zlib is rather portable and doesn't need much configuration.
8. I found a bug in zlib.
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 . Do not send
multi-megabyte data files without prior agreement.
9. Why do I get "undefined reference to gzputc"?
If "make test" produces something like
example.o(.text+0x154): undefined reference to `gzputc'
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".
10. I need a Delphi interface to zlib.
See the contrib/delphi directory in the zlib distribution.
11. Can zlib handle .zip archives?
Not by itself, no. See the directory contrib/minizip in the zlib
12. Can zlib handle .Z files?
No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt
the code of uncompress on your own.
13. How can I make a Unix shared library?
make clean
./configure -s
14. How do I install a shared zlib library on Unix?
After the above, then:
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
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.
15. I have a question about OttoPDF.
We are not the authors of OttoPDF. The real author is on the OttoPDF web
site: Joel Hainley,
16. Can zlib decode Flate data in an Adobe PDF file?
Yes. See (ClibPDF), or .
To modify PDF forms, see .
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: rpm: fatal: relocation error: file /usr/local/lib/
symbol __register_frame_info: referenced symbol not found
The symbol __register_frame_info is not part of zlib, it is generated by
the C compiler (cc or gcc). You must recompile applications using zlib
which have this problem. This problem is specific to Solaris. See for Solaris versions of zlib and applications
using zlib.
18. Why does gzip give an error on a file I make with compress/deflate?
The compress and deflate functions produce data in the zlib format, which
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.
19. Ok, so why are there two different formats?
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.
20. Well that's nice, but how do I make a gzip file in memory?
You can request that deflate write the gzip format instead of the zlib
format using deflateInit2(). You can also request that inflate decode
the gzip format using inflateInit2(). Read zlib.h for more details.
21. Is zlib thread-safe?
Yes. However any library routines that zlib uses and any application-
provided memory allocation routines must also be thread-safe. zlib's gz*
functions use stdio library routines, and most of zlib's functions use the
library memory allocation routines by default. zlib's Init functions allow
for the application to provide custom memory allocation routines.
Of course, you should only operate on any given zlib or gzip stream from a
single thread at a time.
22. Can I use zlib in my commercial application?
Yes. Please read the license in zlib.h.
23. Is zlib under the GNU license?
No. Please read the license in zlib.h.
24. The license says that altered source versions must be "plainly marked". So
what exactly do I need to do to meet that requirement?
You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In
particular, the final version number needs to be changed to "f", and an
identification string should be appended to ZLIB_VERSION. Version numbers
x.x.x.f are reserved for modifications to zlib by others than the zlib
maintainers. For example, if the version of the base zlib you are altering
is "", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and
ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also
update the version strings in deflate.c and inftrees.c.
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
with the dates of the alterations. The origin should include at least your
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
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.
25. Will zlib work on a big-endian or little-endian architecture, and can I
exchange compressed data between them?
Yes and yes.
26. Will zlib work on a 64-bit machine?
It should. 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
difficulties, please provide a complete problem report to
27. Will zlib decompress data from the PKWare Data Compression Library?
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
directory for a possible solution to your problem.
28. Can I access data randomly in a compressed stream?
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.
29. Does zlib work on MVS, OS/390, CICS, etc.?
We don't know for sure. We have heard occasional reports of success on
these systems. If you do use it on one of these, please provide us with
a report, instructions, and patches that we can reference when we get
these questions. Thanks.
30. Is there some simpler, easier to read version of inflate I can look at
to understand the deflate format?
First off, you should read RFC 1951. Second, yes. Look in zlib's
contrib/puff directory.
31. Does zlib infringe on any patents?
As far as we know, no. In fact, that was originally the whole point behind
zlib. Look here for some more information:
32. Can zlib work with greater than 4 GB of data?
Yes. inflate() and deflate() will process any amount of data correctly.
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"
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
counters are provided as a convenience and are not used internally by
inflate() or deflate(). The application can easily set up its own counters
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
single call. gzseek() and gztell() may be limited to 4 GB depending on how
zlib is compiled. See the zlibCompileFlags() function in zlib.h.
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.
33. Does zlib have any security vulnerabilities?
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 a 4K string space, other than the caller of
gzprintf() assuring that the output will not exceed 4K. 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().
If you don't have snprintf() or vsnprintf() and would like one, you can
find a portable implementation here:
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.
34. Is there a Java version of zlib?
Probably what you want is to use zlib in Java. zlib is already included
as part of the Java SDK in the package. If you really want
a version of zlib written in the Java language, look on the zlib home
page for links:
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?
Many years ago, we gave up attempting to avoid warnings on every compiler
in the universe. It just got to be a waste of time, and some compilers
were downright silly. So now, we simply make sure that the code always
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?
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.
37. Will zlib read the (insert any ancient or arcane format here) compressed
data format?
Probably not. Look in the comp.compression FAQ for pointers to various
formats and associated software.
38. How can I encrypt/decrypt zip files with zlib?
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, , which already includes zlib compression.
For PKZIP compatible "encryption", look at
39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings?
"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
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
specficiation in RFC 1951, most notably Microsoft. So even though the
"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.
Bottom line: use the gzip format for HTTP 1.1 encoding.
40. Does zlib support the new "Deflate64" format introduced by PKWare?
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.
41. Can you please sign these lengthy legal documents and fax them back to us
so that we can use your software in our product?
No. Go away. Shoo.