+++ /dev/null
-
- 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.net/
-
- 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 (i.e. it's neither in the PATH, nor in the SYSTEM or
- SYSTEM32 directories). 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>
projects / ossec-hids.git / blobdiff