dll_faq.txt

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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:
  http://www.gzip.org/zlib/zlib_faq.html


 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:
      http://www.zlib.org/

    Applications that link to ZLIB1.DLL can rely on the following
    specification:

    * 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
    earlier?

  - 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
    problem.

    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,