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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 symbol '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 and available at the zlib
+    web site.
+
+    If, for any reason, you need to build an incompatible DLL,
+    please use a different 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.x and 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 interfacing, 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.
+
+    If you have a compiled application that works with a certain
+    ZLIB.DLL without any known security issues, there is hardly
+    a need to rebuild the DLL from new sources only to link it to
+    the old app binary.  But if you really want to do it, you have
+    to find out first what kind of calling convention uses 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 forget it.
+
+
+ 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 fact that remains is that CDECL is the default convention.
+    Even if an explicit convention (such as STDCALL or FASTCALL)
+    is hard-coded into the function prototypes inside C headers,
+    problems may appear.  One problem, for example, deals with the
+    necessity to expose the convention in users' callbacks.
+
+    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. 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.
+
+
+ 9. Why are you saying that ZLIB1.DLL and my application must 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.
+
+
+10. 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.  The number of Windows 95 installations
+    is rapidly decreasing, Microsoft stopped supporting it 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.
+
+    There is also the fact that the mainstream C compilers for
+    Windows are Microsoft Visual C++ 6.0, and gcc/MinGW.  Both
+    are producing executables that link to MSVCRT.DLL by default,
+    without offering other dynamic CRTs as alternatives easy to
+    select by users.
+
+
+11. 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 very appealing,
+      because CRTDLL.DLL is present on every Win32 installation.
+      Unfortunately, it has a series of problems: it raises
+      difficulties when using it with C++ code, it does not work
+      with 64-bit file offsets, (and so on...), and Microsoft
+      discontinued its support a long time ago.
+
+    * Linking ZLIB1.DLL to MSVCR70.DLL, supplied with the
+      Microsoft .NET platform and Visual C++ 7.0 or newer, is not
+      a good option.  Although it is available for free download
+      and distribution, its presence is scarce on today's Win32
+      installations.  If it will ever become more popular than
+      MSVCRT.DLL and will be pre-installed on the future Win32
+      systems, we will probably think again about it.
+
+    * Linking ZLIB1.DLL to NTDLL.DLL is not possible.
+      NTDLL.DLL exports only a part of the C library, and only on
+      Windows NT systems.
+
+
+12. 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.
+
+
+13. 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 sources.  But
+    you can make your own private build, and give it a different
+    name, as suggested in the previous answer.
+
+    For example, in Borland Delphi and C++ Builder, zlib is a part
+    of the standard VCL library.  If an application links to VCL
+    dynamically, the name of the distributable binary (VCLxx.DLL)
+    does not posess any danger of clashing with a legitimate but
+    incompatible ZLIB1.DLL.
+
+
+14. 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.
+