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            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
      source distribution.
    * 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
    DLL intact.

 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
    ZLIB_DLL macro.

 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
    functions STDCALL-able.

    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
    zlib distribution.

 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
    MSVCRT.DLL?  Why?

  - 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
    same process.

    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
    use ZLIB1.DLL.

    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 (e.g. it's not in the SYSTEM or the SYSTEM32 directory,
    and it's not in the PATH).  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 <>