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Frequently Asked Questions about ZLIB1.DLL
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This document describes the design, the rationale, and the usage
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of the official DLL build of zlib, named ZLIB1.DLL. If you have
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general questions about zlib, you should see the file "FAQ" found
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in the zlib distribution, or at the following location:
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http://www.gzip.org/zlib/zlib_faq.html
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1. What is ZLIB1.DLL, and how can I get it?
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- ZLIB1.DLL is the official build of zlib as a DLL.
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(Please remark the character '1' in the name.)
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Pointers to a precompiled ZLIB1.DLL can be found in the zlib
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Applications that link to ZLIB1.DLL can rely on the following
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* The exported symbols are exclusively defined in the source
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files "zlib.h" and "zlib.def", found in an official zlib
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* The symbols are exported by name, not by ordinal.
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* The exported names are undecorated.
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* The calling convention of functions is "C" (CDECL).
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* The ZLIB1.DLL binary is linked to MSVCRT.DLL.
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The archive in which ZLIB1.DLL is bundled contains compiled
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test programs that must run with a valid build of ZLIB1.DLL.
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It is recommended to download the prebuilt DLL from the zlib
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web site, instead of building it yourself, to avoid potential
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incompatibilities that could be introduced by your compiler
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and build settings. If you do build the DLL yourself, please
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make sure that it complies with all the above requirements,
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and it runs with the precompiled test programs, bundled with
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the original ZLIB1.DLL distribution.
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If, for any reason, you need to build an incompatible DLL,
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please use a different file name.
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2. Why did you change the name of the DLL to ZLIB1.DLL?
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What happened to the old ZLIB.DLL?
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- The old ZLIB.DLL, built from zlib-1.1.4 or earlier, required
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compilation settings that were incompatible to those used by
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a static build. The DLL settings were supposed to be enabled
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by defining the macro ZLIB_DLL, before including "zlib.h".
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Incorrect handling of this macro was silently accepted at
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build time, resulting in two major problems:
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* ZLIB_DLL was missing from the old makefile. When building
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the DLL, not all people added it to the build options. In
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consequence, incompatible incarnations of ZLIB.DLL started
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to circulate around the net.
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* When switching from using the static library to using the
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DLL, applications had to define the ZLIB_DLL macro and
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to recompile all the sources that contained calls to zlib
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functions. Failure to do so resulted in creating binaries
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that were unable to run with the official ZLIB.DLL build.
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The only possible solution that we could foresee was to make
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a binary-incompatible change in the DLL interface, in order to
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remove the dependency on the ZLIB_DLL macro, and to release
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the new DLL under a different name.
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We chose the name ZLIB1.DLL, where '1' indicates the major
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zlib version number. We hope that we will not have to break
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the binary compatibility again, at least not as long as the
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zlib-1.x series will last.
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There is still a ZLIB_DLL macro, that can trigger a more
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efficient build and use of the DLL, but compatibility no
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longer dependents on it.
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3. Can I build ZLIB.DLL from the new zlib sources, and replace
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an old ZLIB.DLL, that was built from zlib-1.1.4 or earlier?
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- In principle, you can do it by assigning calling convention
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keywords to the macros ZEXPORT and ZEXPORTVA. In practice,
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it depends on what you mean by "an old ZLIB.DLL", because the
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old DLL exists in several mutually-incompatible versions.
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You have to find out first what kind of calling convention is
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being used in your particular ZLIB.DLL build, and to use the
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same one in the new build. If you don't know what this is all
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about, you might be better off if you would just leave the old
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4. Can I compile my application using the new zlib interface, and
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link it to an old ZLIB.DLL, that was built from zlib-1.1.4 or
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- The official answer is "no"; the real answer depends again on
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what kind of ZLIB.DLL you have. Even if you are lucky, this
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course of action is unreliable.
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If you rebuild your application and you intend to use a newer
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version of zlib (post- 1.1.4), it is strongly recommended to
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link it to the new ZLIB1.DLL.
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5. Why are the zlib symbols exported by name, and not by ordinal?
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- Although exporting symbols by ordinal is a little faster, it
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is risky. Any single glitch in the maintenance or use of the
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DEF file that contains the ordinals can result in incompatible
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builds and frustrating crashes. Simply put, the benefits of
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exporting symbols by ordinal do not justify the risks.
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Technically, it should be possible to maintain ordinals in
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the DEF file, and still export the symbols by name. Ordinals
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exist in every DLL, and even if the dynamic linking performed
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at the DLL startup is searching for names, ordinals serve as
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hints, for a faster name lookup. However, if the DEF file
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contains ordinals, the Microsoft linker automatically builds
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an implib that will cause the executables linked to it to use
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those ordinals, and not the names. It is interesting to
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notice that the GNU linker for Win32 does not suffer from this
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It is possible to avoid the DEF file if the exported symbols
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are accompanied by a "__declspec(dllexport)" attribute in the
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source files. You can do this in zlib by predefining the
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6. I see that the ZLIB1.DLL functions use the "C" (CDECL) calling
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convention. Why not use the STDCALL convention?
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STDCALL is the standard convention in Win32, and I need it in
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my Visual Basic project!
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(For readability, we use CDECL to refer to the convention
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triggered by the "__cdecl" keyword, STDCALL to refer to
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the convention triggered by "__stdcall", and FASTCALL to
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refer to the convention triggered by "__fastcall".)
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- Most of the native Windows API functions (without varargs) use
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indeed the WINAPI convention (which translates to STDCALL in
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Win32), but the standard C functions use CDECL. If a user
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application is intrinsically tied to the Windows API (e.g.
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it calls native Windows API functions such as CreateFile()),
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sometimes it makes sense to decorate its own functions with
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WINAPI. But if ANSI C or POSIX portability is a goal (e.g.
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it calls standard C functions such as fopen()), it is not a
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sound decision to request the inclusion of <windows.h>, or to
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use non-ANSI constructs, for the sole purpose to make the user
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functions STDCALL-able.
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The functionality offered by zlib is not in the category of
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"Windows functionality", but is more like "C functionality".
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Technically, STDCALL is not bad; in fact, it is slightly
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faster than CDECL, and it works with variable-argument
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functions, just like CDECL. It is unfortunate that, in spite
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of using STDCALL in the Windows API, it is not the default
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convention used by the C compilers that run under Windows.
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The roots of the problem reside deep inside the unsafety of
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the K&R-style function prototypes, where the argument types
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are not specified; but that is another story for another day.
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The remaining fact is that CDECL is the default convention.
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Even if an explicit convention is hard-coded into the function
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prototypes inside C headers, problems may appear. The
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necessity to expose the convention in users' callbacks is one
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The calling convention issues are also important when using
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zlib in other programming languages. Some of them, like Ada
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(GNAT) and Fortran (GNU G77), have C bindings implemented
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initially on Unix, and relying on the C calling convention.
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On the other hand, the pre- .NET versions of Microsoft Visual
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Basic require STDCALL, while Borland Delphi prefers, although
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it does not require, FASTCALL.
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In fairness to all possible uses of zlib outside the C
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programming language, we choose the default "C" convention.
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Anyone interested in different bindings or conventions is
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encouraged to maintain specialized projects. The "contrib/"
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directory from the zlib distribution already holds a couple
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of foreign bindings, such as Ada, C++, and Delphi.
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7. I need a DLL for my Visual Basic project. What can I do?
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- Define the ZLIB_WINAPI macro before including "zlib.h", when
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building both the DLL and the user application (except that
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you don't need to define anything when using the DLL in Visual
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Basic). The ZLIB_WINAPI macro will switch on the WINAPI
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(STDCALL) convention. The name of this DLL must be different
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than the official ZLIB1.DLL.
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Gilles Vollant has contributed a build named ZLIBWAPI.DLL,
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with the ZLIB_WINAPI macro turned on, and with the minizip
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functionality built in. For more information, please read
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the notes inside "contrib/vstudio/readme.txt", found in the
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8. I need to use zlib in my Microsoft .NET project. What can I
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- Henrik Ravn has contributed a .NET wrapper around zlib. Look
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into contrib/dotzlib/, inside the zlib distribution.
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9. If my application uses ZLIB1.DLL, should I link it to
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- It is not required, but it is recommended to link your
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application to MSVCRT.DLL, if it uses ZLIB1.DLL.
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The executables (.EXE, .DLL, etc.) that are involved in the
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same process and are using the C run-time library (i.e. they
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are calling standard C functions), must link to the same
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library. There are several libraries in the Win32 system:
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CRTDLL.DLL, MSVCRT.DLL, the static C libraries, etc.
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Since ZLIB1.DLL is linked to MSVCRT.DLL, the executables that
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depend on it should also be linked to MSVCRT.DLL.
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10. Why are you saying that ZLIB1.DLL and my application should
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be linked to the same C run-time (CRT) library? I linked my
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application and my DLLs to different C libraries (e.g. my
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application to a static library, and my DLLs to MSVCRT.DLL),
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and everything works fine.
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- If a user library invokes only pure Win32 API (accessible via
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<windows.h> and the related headers), its DLL build will work
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in any context. But if this library invokes standard C API,
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things get more complicated.
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There is a single Win32 library in a Win32 system. Every
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function in this library resides in a single DLL module, that
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is safe to call from anywhere. On the other hand, there are
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multiple versions of the C library, and each of them has its
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own separate internal state. Standalone executables and user
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DLLs that call standard C functions must link to a C run-time
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(CRT) library, be it static or shared (DLL). Intermixing
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occurs when an executable (not necessarily standalone) and a
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DLL are linked to different CRTs, and both are running in the
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Intermixing multiple CRTs is possible, as long as their
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internal states are kept intact. The Microsoft Knowledge Base
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articles KB94248 "HOWTO: Use the C Run-Time" and KB140584
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"HOWTO: Link with the Correct C Run-Time (CRT) Library"
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mention the potential problems raised by intermixing.
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If intermixing works for you, it's because your application
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and DLLs are avoiding the corruption of each of the CRTs'
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internal states, maybe by careful design, or maybe by fortune.
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Also note that linking ZLIB1.DLL to non-Microsoft CRTs, such
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as those provided by Borland, raises similar problems.
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11. Why are you linking ZLIB1.DLL to MSVCRT.DLL?
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- MSVCRT.DLL exists on every Windows 95 with a new service pack
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installed, or with Microsoft Internet Explorer 4 or later, and
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on all other Windows 4.x or later (Windows 98, Windows NT 4,
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or later). It is freely distributable; if not present in the
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system, it can be downloaded from Microsoft or from other
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software provider for free.
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The fact that MSVCRT.DLL does not exist on a virgin Windows 95
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is not so problematic. Windows 95 is scarcely found nowadays,
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Microsoft ended its support a long time ago, and many recent
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applications from various vendors, including Microsoft, do not
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even run on it. Furthermore, no serious user should run
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Windows 95 without a proper update installed.
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12. Why are you not linking ZLIB1.DLL to
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<<my favorite C run-time library>> ?
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- We considered and abandoned the following alternatives:
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* Linking ZLIB1.DLL to a static C library (LIBC.LIB, or
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LIBCMT.LIB) is not a good option. People are using the DLL
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mainly to save disk space. If you are linking your program
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to a static C library, you may as well consider linking zlib
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* Linking ZLIB1.DLL to CRTDLL.DLL looks appealing, because
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CRTDLL.DLL is present on every Win32 installation.
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Unfortunately, it has a series of problems: it does not
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work properly with Microsoft's C++ libraries, it does not
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provide support for 64-bit file offsets, (and so on...),
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and Microsoft discontinued its support a long time ago.
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* Linking ZLIB1.DLL to MSVCR70.DLL or MSVCR71.DLL, supplied
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with the Microsoft .NET platform, and Visual C++ 7.0/7.1,
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raises problems related to the status of ZLIB1.DLL as a
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system component. According to the Microsoft Knowledge Base
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article KB326922 "INFO: Redistribution of the Shared C
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Runtime Component in Visual C++ .NET", MSVCR70.DLL and
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MSVCR71.DLL are not supposed to function as system DLLs,
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because they may clash with MSVCRT.DLL. Instead, the
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application's installer is supposed to put these DLLs
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(if needed) in the application's private directory.
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If ZLIB1.DLL depends on a non-system runtime, it cannot
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function as a redistributable system component.
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* Linking ZLIB1.DLL to non-Microsoft runtimes, such as
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Borland's, or Cygwin's, raises problems related to the
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reliable presence of these runtimes on Win32 systems.
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It's easier to let the DLL build of zlib up to the people
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who distribute these runtimes, and who may proceed as
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explained in the answer to Question 14.
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13. If ZLIB1.DLL cannot be linked to MSVCR70.DLL or MSVCR71.DLL,
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how can I build/use ZLIB1.DLL in Microsoft Visual C++ 7.0
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(Visual Studio .NET) or newer?
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- Due to the problems explained in the Microsoft Knowledge Base
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article KB326922 (see the previous answer), the C runtime that
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comes with the VC7 environment is no longer considered a
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system component. That is, it should not be assumed that this
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runtime exists, or may be installed in a system directory.
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Since ZLIB1.DLL is supposed to be a system component, it may
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not depend on a non-system component.
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In order to link ZLIB1.DLL and your application to MSVCRT.DLL
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in VC7, you need the library of Visual C++ 6.0 or older. If
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you don't have this library at hand, it's probably best not to
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We are hoping that, in the future, Microsoft will provide a
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way to build applications linked to a proper system runtime,
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from the Visual C++ environment. Until then, you have a
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couple of alternatives, such as linking zlib in statically.
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If your application requires dynamic linking, you may proceed
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as explained in the answer to Question 14.
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14. I need to link my own DLL build to a CRT different than
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MSVCRT.DLL. What can I do?
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- Feel free to rebuild the DLL from the zlib sources, and link
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it the way you want. You should, however, clearly state that
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your build is unofficial. You should give it a different file
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name, and/or install it in a private directory that can be
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accessed by your application only, and is not visible to the
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others (i.e. it's neither in the PATH, nor in the SYSTEM or
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SYSTEM32 directories). Otherwise, your build may clash with
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applications that link to the official build.
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For example, in Cygwin, zlib is linked to the Cygwin runtime
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CYGWIN1.DLL, and it is distributed under the name CYGZ.DLL.
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15. May I include additional pieces of code that I find useful,
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link them in ZLIB1.DLL, and export them?
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- No. A legitimate build of ZLIB1.DLL must not include code
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that does not originate from the official zlib source code.
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But you can make your own private DLL build, under a different
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file name, as suggested in the previous answer.
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For example, zlib is a part of the VCL library, distributed
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with Borland Delphi and C++ Builder. The DLL build of VCL
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is a redistributable file, named VCLxx.DLL.
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16. May I remove some functionality out of ZLIB1.DLL, by enabling
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macros like NO_GZCOMPRESS or NO_GZIP at compile time?
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- No. A legitimate build of ZLIB1.DLL must provide the complete
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zlib functionality, as implemented in the official zlib source
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code. But you can make your own private DLL build, under a
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different file name, as suggested in the previous answer.
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17. I made my own ZLIB1.DLL build. Can I test it for compliance?
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- We prefer that you download the official DLL from the zlib
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web site. If you need something peculiar from this DLL, you
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can send your suggestion to the zlib mailing list.
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However, in case you do rebuild the DLL yourself, you can run
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it with the test programs found in the DLL distribution.
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Running these test programs is not a guarantee of compliance,
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but a failure can imply a detected problem.
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This document is written and maintained by
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Cosmin Truta <cosmint@cs.ubbcluj.ro>
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zlib-1.2.6/win32/DLL_FAQ.txt
