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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,
+ 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
+ do?
+
+ - 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 <cosmint@cs.ubbcluj.ro>