Frequently Asked Questions about ZLIB1.DLL
This document describes the design, the rationale, and the usage
of the official DLL build of zlib, named ZLIB1.DLL. If you have
general questions about zlib, you should see the file "FAQ" found
in the zlib distribution, or at the following location:
1. What is ZLIB1.DLL, and how can I get it?
- ZLIB1.DLL is the official build of zlib as a DLL.
(Please remark the character '1' in the name.)
Pointers to a precompiled ZLIB1.DLL can be found in the zlib
web site at:
Applications that link to ZLIB1.DLL can rely on the following
* The exported symbols are exclusively defined in the source
files "zlib.h" and "zlib.def", found in an official zlib
* The symbols are exported by name, not by ordinal.
* The exported names are undecorated.
* The calling convention of functions is "C" (CDECL).
* The ZLIB1.DLL binary is linked to MSVCRT.DLL.
The archive in which ZLIB1.DLL is bundled contains compiled
test programs that must run with a valid build of ZLIB1.DLL.
It is recommended to download the prebuilt DLL from the zlib
web site, instead of building it yourself, to avoid potential
incompatibilities that could be introduced by your compiler
and build settings. If you do build the DLL yourself, please
make sure that it complies with all the above requirements,
and it runs with the precompiled test programs, bundled with
the original ZLIB1.DLL distribution.
If, for any reason, you need to build an incompatible DLL,
please use a different file name.
2. Why did you change the name of the DLL to ZLIB1.DLL?
What happened to the old ZLIB.DLL?
- The old ZLIB.DLL, built from zlib-1.1.4 or earlier, required
compilation settings that were incompatible to those used by
a static build. The DLL settings were supposed to be enabled
by defining the macro ZLIB_DLL, before including "zlib.h".
Incorrect handling of this macro was silently accepted at
build time, resulting in two major problems:
* ZLIB_DLL was missing from the old makefile. When building
the DLL, not all people added it to the build options. In
consequence, incompatible incarnations of ZLIB.DLL started
to circulate around the net.
* When switching from using the static library to using the
DLL, applications had to define the ZLIB_DLL macro and
to recompile all the sources that contained calls to zlib
functions. Failure to do so resulted in creating binaries
that were unable to run with the official ZLIB.DLL build.
The only possible solution that we could foresee was to make
a binary-incompatible change in the DLL interface, in order to
remove the dependency on the ZLIB_DLL macro, and to release
the new DLL under a different name.
We chose the name ZLIB1.DLL, where '1' indicates the major
zlib version number. We hope that we will not have to break
the binary compatibility again, at least not as long as the
zlib-1.x series will last.
There is still a ZLIB_DLL macro, that can trigger a more
efficient build and use of the DLL, but compatibility no
longer dependents on it.
3. Can I build ZLIB.DLL from the new zlib sources, and replace
an old ZLIB.DLL, that was built from zlib-1.1.4 or earlier?
- In principle, you can do it by assigning calling convention
keywords to the macros ZEXPORT and ZEXPORTVA. In practice,
it depends on what you mean by "an old ZLIB.DLL", because the
old DLL exists in several mutually-incompatible versions.
You have to find out first what kind of calling convention is
being used in your particular ZLIB.DLL build, and to use the
same one in the new build. If you don't know what this is all
about, you might be better off if you would just leave the old
4. Can I compile my application using the new zlib interface, and
link it to an old ZLIB.DLL, that was built from zlib-1.1.4 or
- The official answer is "no"; the real answer depends again on
what kind of ZLIB.DLL you have. Even if you are lucky, this
course of action is unreliable.
If you rebuild your application and you intend to use a newer
version of zlib (post- 1.1.4), it is strongly recommended to
link it to the new ZLIB1.DLL.
5. Why are the zlib symbols exported by name, and not by ordinal?
- Although exporting symbols by ordinal is a little faster, it
is risky. Any single glitch in the maintenance or use of the
DEF file that contains the ordinals can result in incompatible
builds and frustrating crashes. Simply put, the benefits of
exporting symbols by ordinal do not justify the risks.
Technically, it should be possible to maintain ordinals in
the DEF file, and still export the symbols by name. Ordinals
exist in every DLL, and even if the dynamic linking performed
at the DLL startup is searching for names, ordinals serve as
hints, for a faster name lookup. However, if the DEF file
contains ordinals, the Microsoft linker automatically builds
an implib that will cause the executables linked to it to use
those ordinals, and not the names. It is interesting to
notice that the GNU linker for Win32 does not suffer from this
It is possible to avoid the DEF file if the exported symbols
are accompanied by a "__declspec(dllexport)" attribute in the
source files. You can do this in zlib by predefining the
6. I see that the ZLIB1.DLL functions use the "C" (CDECL) calling
convention. Why not use the STDCALL convention?
STDCALL is the standard convention in Win32, and I need it in
my Visual Basic project!
(For readability, we use CDECL to refer to the convention
triggered by the "__cdecl" keyword, STDCALL to refer to
the convention triggered by "__stdcall", and FASTCALL to
refer to the convention triggered by "__fastcall".)
- Most of the native Windows API functions (without varargs) use
indeed the WINAPI convention (which translates to STDCALL in
Win32), but the standard C functions use CDECL. If a user
application is intrinsically tied to the Windows API (e.g.
it calls native Windows API functions such as CreateFile()),
sometimes it makes sense to decorate its own functions with
WINAPI. But if ANSI C or POSIX portability is a goal (e.g.
it calls standard C functions such as fopen()), it is not a
sound decision to request the inclusion of <windows.h>, or to
use non-ANSI constructs, for the sole purpose to make the user
The functionality offered by zlib is not in the category of
"Windows functionality", but is more like "C functionality".
Technically, STDCALL is not bad; in fact, it is slightly
faster than CDECL, and it works with variable-argument
functions, just like CDECL. It is unfortunate that, in spite
of using STDCALL in the Windows API, it is not the default
convention used by the C compilers that run under Windows.
The roots of the problem reside deep inside the unsafety of
the K&R-style function prototypes, where the argument types
are not specified; but that is another story for another day.
The remaining fact is that CDECL is the default convention.
Even if an explicit convention is hard-coded into the function
prototypes inside C headers, problems may appear. The
necessity to expose the convention in users' callbacks is one
of these problems.
The calling convention issues are also important when using
zlib in other programming languages. Some of them, like Ada
(GNAT) and Fortran (GNU G77), have C bindings implemented
initially on Unix, and relying on the C calling convention.
On the other hand, the pre- .NET versions of Microsoft Visual
Basic require STDCALL, while Borland Delphi prefers, although
it does not require, FASTCALL.
In fairness to all possible uses of zlib outside the C
programming language, we choose the default "C" convention.
Anyone interested in different bindings or conventions is
encouraged to maintain specialized projects. The "contrib/"
directory from the zlib distribution already holds a couple
of foreign bindings, such as Ada, C++, and Delphi.
7. I need a DLL for my Visual Basic project. What can I do?
- Define the ZLIB_WINAPI macro before including "zlib.h", when
building both the DLL and the user application (except that
you don't need to define anything when using the DLL in Visual
Basic). The ZLIB_WINAPI macro will switch on the WINAPI
(STDCALL) convention. The name of this DLL must be different
than the official ZLIB1.DLL.
Gilles Vollant has contributed a build named ZLIBWAPI.DLL,
with the ZLIB_WINAPI macro turned on, and with the minizip
functionality built in. For more information, please read
the notes inside "contrib/vstudio/readme.txt", found in the
8. I need to use zlib in my Microsoft .NET project. What can I
- Henrik Ravn has contributed a .NET wrapper around zlib. Look
into contrib/dotzlib/, inside the zlib distribution.
9. If my application uses ZLIB1.DLL, should I link it to
- It is not required, but it is recommended to link your
application to MSVCRT.DLL, if it uses ZLIB1.DLL.
The executables (.EXE, .DLL, etc.) that are involved in the
same process and are using the C run-time library (i.e. they
are calling standard C functions), must link to the same
library. There are several libraries in the Win32 system:
CRTDLL.DLL, MSVCRT.DLL, the static C libraries, etc.
Since ZLIB1.DLL is linked to MSVCRT.DLL, the executables that
depend on it should also be linked to MSVCRT.DLL.
10. Why are you saying that ZLIB1.DLL and my application should
be linked to the same C run-time (CRT) library? I linked my
application and my DLLs to different C libraries (e.g. my
application to a static library, and my DLLs to MSVCRT.DLL),
and everything works fine.
- If a user library invokes only pure Win32 API (accessible via
<windows.h> and the related headers), its DLL build will work
in any context. But if this library invokes standard C API,
things get more complicated.
There is a single Win32 library in a Win32 system. Every
function in this library resides in a single DLL module, that
is safe to call from anywhere. On the other hand, there are
multiple versions of the C library, and each of them has its
own separate internal state. Standalone executables and user
DLLs that call standard C functions must link to a C run-time
(CRT) library, be it static or shared (DLL). Intermixing
occurs when an executable (not necessarily standalone) and a
DLL are linked to different CRTs, and both are running in the
Intermixing multiple CRTs is possible, as long as their
internal states are kept intact. The Microsoft Knowledge Base
articles KB94248 "HOWTO: Use the C Run-Time" and KB140584
"HOWTO: Link with the Correct C Run-Time (CRT) Library"
mention the potential problems raised by intermixing.
If intermixing works for you, it's because your application
and DLLs are avoiding the corruption of each of the CRTs'
internal states, maybe by careful design, or maybe by fortune.
Also note that linking ZLIB1.DLL to non-Microsoft CRTs, such
as those provided by Borland, raises similar problems.
11. Why are you linking ZLIB1.DLL to MSVCRT.DLL?
- MSVCRT.DLL exists on every Windows 95 with a new service pack
installed, or with Microsoft Internet Explorer 4 or later, and
on all other Windows 4.x or later (Windows 98, Windows NT 4,
or later). It is freely distributable; if not present in the
system, it can be downloaded from Microsoft or from other
software provider for free.
The fact that MSVCRT.DLL does not exist on a virgin Windows 95
is not so problematic. Windows 95 is scarcely found nowadays,
Microsoft ended its support a long time ago, and many recent
applications from various vendors, including Microsoft, do not
even run on it. Furthermore, no serious user should run
Windows 95 without a proper update installed.
12. Why are you not linking ZLIB1.DLL to
<<my favorite C run-time library>> ?
- We considered and abandoned the following alternatives:
* Linking ZLIB1.DLL to a static C library (LIBC.LIB, or
LIBCMT.LIB) is not a good option. People are using the DLL
mainly to save disk space. If you are linking your program
to a static C library, you may as well consider linking zlib
in statically, too.
* Linking ZLIB1.DLL to CRTDLL.DLL looks appealing, because
CRTDLL.DLL is present on every Win32 installation.
Unfortunately, it has a series of problems: it does not
work properly with Microsoft's C++ libraries, it does not
provide support for 64-bit file offsets, (and so on...),
and Microsoft discontinued its support a long time ago.
* Linking ZLIB1.DLL to MSVCR70.DLL or MSVCR71.DLL, supplied
with the Microsoft .NET platform, and Visual C++ 7.0/7.1,
raises problems related to the status of ZLIB1.DLL as a
system component. According to the Microsoft Knowledge Base
article KB326922 "INFO: Redistribution of the Shared C
Runtime Component in Visual C++ .NET", MSVCR70.DLL and
MSVCR71.DLL are not supposed to function as system DLLs,
because they may clash with MSVCRT.DLL. Instead, the
application's installer is supposed to put these DLLs
(if needed) in the application's private directory.
If ZLIB1.DLL depends on a non-system runtime, it cannot
function as a redistributable system component.
* Linking ZLIB1.DLL to non-Microsoft runtimes, such as
Borland's, or Cygwin's, raises problems related to the
reliable presence of these runtimes on Win32 systems.
It's easier to let the DLL build of zlib up to the people
who distribute these runtimes, and who may proceed as
explained in the answer to Question 14.
13. If ZLIB1.DLL cannot be linked to MSVCR70.DLL or MSVCR71.DLL,
how can I build/use ZLIB1.DLL in Microsoft Visual C++ 7.0
(Visual Studio .NET) or newer?
- Due to the problems explained in the Microsoft Knowledge Base
article KB326922 (see the previous answer), the C runtime that
comes with the VC7 environment is no longer considered a
system component. That is, it should not be assumed that this
runtime exists, or may be installed in a system directory.
Since ZLIB1.DLL is supposed to be a system component, it may
not depend on a non-system component.
In order to link ZLIB1.DLL and your application to MSVCRT.DLL
in VC7, you need the library of Visual C++ 6.0 or older. If
you don't have this library at hand, it's probably best not to
We are hoping that, in the future, Microsoft will provide a
way to build applications linked to a proper system runtime,
from the Visual C++ environment. Until then, you have a
couple of alternatives, such as linking zlib in statically.
If your application requires dynamic linking, you may proceed
as explained in the answer to Question 14.
14. I need to link my own DLL build to a CRT different than
MSVCRT.DLL. What can I do?
- Feel free to rebuild the DLL from the zlib sources, and link
it the way you want. You should, however, clearly state that
your build is unofficial. You should give it a different file
name, and/or install it in a private directory that can be
accessed by your application only, and is not visible to the
others (i.e. it's neither in the PATH, nor in the SYSTEM or
SYSTEM32 directories). Otherwise, your build may clash with
applications that link to the official build.
For example, in Cygwin, zlib is linked to the Cygwin runtime
CYGWIN1.DLL, and it is distributed under the name CYGZ.DLL.
15. May I include additional pieces of code that I find useful,
link them in ZLIB1.DLL, and export them?
- No. A legitimate build of ZLIB1.DLL must not include code
that does not originate from the official zlib source code.
But you can make your own private DLL build, under a different
file name, as suggested in the previous answer.
For example, zlib is a part of the VCL library, distributed
with Borland Delphi and C++ Builder. The DLL build of VCL
is a redistributable file, named VCLxx.DLL.
16. May I remove some functionality out of ZLIB1.DLL, by enabling
macros like NO_GZCOMPRESS or NO_GZIP at compile time?
- No. A legitimate build of ZLIB1.DLL must provide the complete
zlib functionality, as implemented in the official zlib source
code. But you can make your own private DLL build, under a
different file name, as suggested in the previous answer.
17. I made my own ZLIB1.DLL build. Can I test it for compliance?
- We prefer that you download the official DLL from the zlib
web site. If you need something peculiar from this DLL, you
can send your suggestion to the zlib mailing list.
However, in case you do rebuild the DLL yourself, you can run
it with the test programs found in the DLL distribution.
Running these test programs is not a guarantee of compliance,
but a failure can imply a detected problem.
This document is written and maintained by
Cosmin Truta <firstname.lastname@example.org>