Frequently Asked Questions about zlib | |
If your question is not there, please check the zlib home page | |
http://www.zlib.org which may have more recent information. | |
The lastest zlib FAQ is at http://www.gzip.org/zlib/zlib_faq.html | |
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 | |
http://www.zlib.org. | |
3. Where can I get a Visual Basic interface to zlib? | |
See | |
* http://www.dogma.net/markn/articles/zlibtool/zlibtool.htm | |
* 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 (zlib@gzip.org). 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 zlib@gzip.org . 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 | |
distribution. | |
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 | |
make | |
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, jhainley@myndkryme.com. | |
16. Can zlib decode Flate data in an Adobe PDF file? | |
Yes. See http://www.fastio.com/ (ClibPDF), or http://www.pdflib.com/ . | |
To modify PDF forms, see http://sourceforge.net/projects/acroformtool/ . | |
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: | |
ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so: | |
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 | |
http://www.sunfreeware.com 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 "1.2.3.4", 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 zlib@gzip.org | |
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: | |
http://www.gzip.org/#faq11 | |
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: | |
http://www.ijs.si/software/snprintf/ | |
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 java.util.zip package. If you really want | |
a version of zlib written in the Java language, look on the zlib home | |
page for links: http://www.zlib.org/ | |
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 | |
works. | |
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, http://www.gnupg.org/ , which already includes zlib compression. | |
For PKZIP compatible "encryption", look at http://www.info-zip.org/ | |
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. |