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diff --git a/zlib/win32/DLL_FAQ.txt b/zlib/win32/DLL_FAQ.txt new file mode 100644 index 000000000..fb18e0711 --- /dev/null +++ b/zlib/win32/DLL_FAQ.txt @@ -0,0 +1,397 @@ + + 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> |