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2 Frequently Asked Questions about ZLIB1.DLL
5 This document describes the design, the rationale, and the usage
6 of the official DLL build of zlib, named ZLIB1.DLL. If you have
7 general questions about zlib, you should see the file "FAQ" found
8 in the zlib distribution, or at the following location:
9 http://www.gzip.org/zlib/zlib_faq.html
12 1. What is ZLIB1.DLL, and how can I get it?
14 - ZLIB1.DLL is the official build of zlib as a DLL.
15 (Please remark the character '1' in the name.)
17 Pointers to a precompiled ZLIB1.DLL can be found in the zlib
18 web site at:
19 http://www.zlib.net/
21 Applications that link to ZLIB1.DLL can rely on the following
22 specification:
24 * The exported symbols are exclusively defined in the source
25 files "zlib.h" and "zlib.def", found in an official zlib
26 source distribution.
27 * The symbols are exported by name, not by ordinal.
28 * The exported names are undecorated.
29 * The calling convention of functions is "C" (CDECL).
30 * The ZLIB1.DLL binary is linked to MSVCRT.DLL.
32 The archive in which ZLIB1.DLL is bundled contains compiled
33 test programs that must run with a valid build of ZLIB1.DLL.
34 It is recommended to download the prebuilt DLL from the zlib
35 web site, instead of building it yourself, to avoid potential
36 incompatibilities that could be introduced by your compiler
37 and build settings. If you do build the DLL yourself, please
38 make sure that it complies with all the above requirements,
39 and it runs with the precompiled test programs, bundled with
40 the original ZLIB1.DLL distribution.
42 If, for any reason, you need to build an incompatible DLL,
43 please use a different file name.
46 2. Why did you change the name of the DLL to ZLIB1.DLL?
47 What happened to the old ZLIB.DLL?
49 - The old ZLIB.DLL, built from zlib-1.1.4 or earlier, required
50 compilation settings that were incompatible to those used by
51 a static build. The DLL settings were supposed to be enabled
52 by defining the macro ZLIB_DLL, before including "zlib.h".
53 Incorrect handling of this macro was silently accepted at
54 build time, resulting in two major problems:
56 * ZLIB_DLL was missing from the old makefile. When building
57 the DLL, not all people added it to the build options. In
58 consequence, incompatible incarnations of ZLIB.DLL started
59 to circulate around the net.
61 * When switching from using the static library to using the
62 DLL, applications had to define the ZLIB_DLL macro and
63 to recompile all the sources that contained calls to zlib
64 functions. Failure to do so resulted in creating binaries
65 that were unable to run with the official ZLIB.DLL build.
67 The only possible solution that we could foresee was to make
68 a binary-incompatible change in the DLL interface, in order to
69 remove the dependency on the ZLIB_DLL macro, and to release
70 the new DLL under a different name.
72 We chose the name ZLIB1.DLL, where '1' indicates the major
73 zlib version number. We hope that we will not have to break
74 the binary compatibility again, at least not as long as the
75 zlib-1.x series will last.
77 There is still a ZLIB_DLL macro, that can trigger a more
78 efficient build and use of the DLL, but compatibility no
79 longer dependents on it.
82 3. Can I build ZLIB.DLL from the new zlib sources, and replace
83 an old ZLIB.DLL, that was built from zlib-1.1.4 or earlier?
85 - In principle, you can do it by assigning calling convention
86 keywords to the macros ZEXPORT and ZEXPORTVA. In practice,
87 it depends on what you mean by "an old ZLIB.DLL", because the
88 old DLL exists in several mutually-incompatible versions.
89 You have to find out first what kind of calling convention is
90 being used in your particular ZLIB.DLL build, and to use the
91 same one in the new build. If you don't know what this is all
92 about, you might be better off if you would just leave the old
93 DLL intact.
96 4. Can I compile my application using the new zlib interface, and
97 link it to an old ZLIB.DLL, that was built from zlib-1.1.4 or
98 earlier?
100 - The official answer is "no"; the real answer depends again on
101 what kind of ZLIB.DLL you have. Even if you are lucky, this
102 course of action is unreliable.
104 If you rebuild your application and you intend to use a newer
105 version of zlib (post- 1.1.4), it is strongly recommended to
106 link it to the new ZLIB1.DLL.
109 5. Why are the zlib symbols exported by name, and not by ordinal?
111 - Although exporting symbols by ordinal is a little faster, it
112 is risky. Any single glitch in the maintenance or use of the
113 DEF file that contains the ordinals can result in incompatible
114 builds and frustrating crashes. Simply put, the benefits of
115 exporting symbols by ordinal do not justify the risks.
117 Technically, it should be possible to maintain ordinals in
118 the DEF file, and still export the symbols by name. Ordinals
119 exist in every DLL, and even if the dynamic linking performed
120 at the DLL startup is searching for names, ordinals serve as
121 hints, for a faster name lookup. However, if the DEF file
122 contains ordinals, the Microsoft linker automatically builds
123 an implib that will cause the executables linked to it to use
124 those ordinals, and not the names. It is interesting to
125 notice that the GNU linker for Win32 does not suffer from this
126 problem.
128 It is possible to avoid the DEF file if the exported symbols
129 are accompanied by a "__declspec(dllexport)" attribute in the
130 source files. You can do this in zlib by predefining the
131 ZLIB_DLL macro.
134 6. I see that the ZLIB1.DLL functions use the "C" (CDECL) calling
135 convention. Why not use the STDCALL convention?
136 STDCALL is the standard convention in Win32, and I need it in
137 my Visual Basic project!
139 (For readability, we use CDECL to refer to the convention
140 triggered by the "__cdecl" keyword, STDCALL to refer to
141 the convention triggered by "__stdcall", and FASTCALL to
142 refer to the convention triggered by "__fastcall".)
144 - Most of the native Windows API functions (without varargs) use
145 indeed the WINAPI convention (which translates to STDCALL in
146 Win32), but the standard C functions use CDECL. If a user
147 application is intrinsically tied to the Windows API (e.g.
148 it calls native Windows API functions such as CreateFile()),
149 sometimes it makes sense to decorate its own functions with
150 WINAPI. But if ANSI C or POSIX portability is a goal (e.g.
151 it calls standard C functions such as fopen()), it is not a
152 sound decision to request the inclusion of <windows.h>, or to
153 use non-ANSI constructs, for the sole purpose to make the user
154 functions STDCALL-able.
156 The functionality offered by zlib is not in the category of
157 "Windows functionality", but is more like "C functionality".
159 Technically, STDCALL is not bad; in fact, it is slightly
160 faster than CDECL, and it works with variable-argument
161 functions, just like CDECL. It is unfortunate that, in spite
162 of using STDCALL in the Windows API, it is not the default
163 convention used by the C compilers that run under Windows.
164 The roots of the problem reside deep inside the unsafety of
165 the K&R-style function prototypes, where the argument types
166 are not specified; but that is another story for another day.
168 The remaining fact is that CDECL is the default convention.
169 Even if an explicit convention is hard-coded into the function
170 prototypes inside C headers, problems may appear. The
171 necessity to expose the convention in users' callbacks is one
172 of these problems.
174 The calling convention issues are also important when using
175 zlib in other programming languages. Some of them, like Ada
176 (GNAT) and Fortran (GNU G77), have C bindings implemented
177 initially on Unix, and relying on the C calling convention.
178 On the other hand, the pre- .NET versions of Microsoft Visual
179 Basic require STDCALL, while Borland Delphi prefers, although
180 it does not require, FASTCALL.
182 In fairness to all possible uses of zlib outside the C
183 programming language, we choose the default "C" convention.
184 Anyone interested in different bindings or conventions is
185 encouraged to maintain specialized projects. The "contrib/"
186 directory from the zlib distribution already holds a couple
187 of foreign bindings, such as Ada, C++, and Delphi.
190 7. I need a DLL for my Visual Basic project. What can I do?
192 - Define the ZLIB_WINAPI macro before including "zlib.h", when
193 building both the DLL and the user application (except that
194 you don't need to define anything when using the DLL in Visual
195 Basic). The ZLIB_WINAPI macro will switch on the WINAPI
196 (STDCALL) convention. The name of this DLL must be different
197 than the official ZLIB1.DLL.
199 Gilles Vollant has contributed a build named ZLIBWAPI.DLL,
200 with the ZLIB_WINAPI macro turned on, and with the minizip
201 functionality built in. For more information, please read
202 the notes inside "contrib/vstudio/readme.txt", found in the
203 zlib distribution.
206 8. I need to use zlib in my Microsoft .NET project. What can I
207 do?
209 - Henrik Ravn has contributed a .NET wrapper around zlib. Look
210 into contrib/dotzlib/, inside the zlib distribution.
213 9. If my application uses ZLIB1.DLL, should I link it to
214 MSVCRT.DLL? Why?
216 - It is not required, but it is recommended to link your
217 application to MSVCRT.DLL, if it uses ZLIB1.DLL.
219 The executables (.EXE, .DLL, etc.) that are involved in the
220 same process and are using the C run-time library (i.e. they
221 are calling standard C functions), must link to the same
222 library. There are several libraries in the Win32 system:
223 CRTDLL.DLL, MSVCRT.DLL, the static C libraries, etc.
224 Since ZLIB1.DLL is linked to MSVCRT.DLL, the executables that
225 depend on it should also be linked to MSVCRT.DLL.
228 10. Why are you saying that ZLIB1.DLL and my application should
229 be linked to the same C run-time (CRT) library? I linked my
230 application and my DLLs to different C libraries (e.g. my
231 application to a static library, and my DLLs to MSVCRT.DLL),
232 and everything works fine.
234 - If a user library invokes only pure Win32 API (accessible via
235 <windows.h> and the related headers), its DLL build will work
236 in any context. But if this library invokes standard C API,
237 things get more complicated.
239 There is a single Win32 library in a Win32 system. Every
240 function in this library resides in a single DLL module, that
241 is safe to call from anywhere. On the other hand, there are
242 multiple versions of the C library, and each of them has its
243 own separate internal state. Standalone executables and user
244 DLLs that call standard C functions must link to a C run-time
245 (CRT) library, be it static or shared (DLL). Intermixing
246 occurs when an executable (not necessarily standalone) and a
247 DLL are linked to different CRTs, and both are running in the
248 same process.
250 Intermixing multiple CRTs is possible, as long as their
251 internal states are kept intact. The Microsoft Knowledge Base
252 articles KB94248 "HOWTO: Use the C Run-Time" and KB140584
253 "HOWTO: Link with the Correct C Run-Time (CRT) Library"
254 mention the potential problems raised by intermixing.
256 If intermixing works for you, it's because your application
257 and DLLs are avoiding the corruption of each of the CRTs'
258 internal states, maybe by careful design, or maybe by fortune.
260 Also note that linking ZLIB1.DLL to non-Microsoft CRTs, such
261 as those provided by Borland, raises similar problems.
264 11. Why are you linking ZLIB1.DLL to MSVCRT.DLL?
266 - MSVCRT.DLL exists on every Windows 95 with a new service pack
267 installed, or with Microsoft Internet Explorer 4 or later, and
268 on all other Windows 4.x or later (Windows 98, Windows NT 4,
269 or later). It is freely distributable; if not present in the
270 system, it can be downloaded from Microsoft or from other
271 software provider for free.
273 The fact that MSVCRT.DLL does not exist on a virgin Windows 95
274 is not so problematic. Windows 95 is scarcely found nowadays,
275 Microsoft ended its support a long time ago, and many recent
276 applications from various vendors, including Microsoft, do not
277 even run on it. Furthermore, no serious user should run
278 Windows 95 without a proper update installed.
281 12. Why are you not linking ZLIB1.DLL to
282 <<my favorite C run-time library>> ?
284 - We considered and abandoned the following alternatives:
286 * Linking ZLIB1.DLL to a static C library (LIBC.LIB, or
287 LIBCMT.LIB) is not a good option. People are using the DLL
288 mainly to save disk space. If you are linking your program
289 to a static C library, you may as well consider linking zlib
290 in statically, too.
292 * Linking ZLIB1.DLL to CRTDLL.DLL looks appealing, because
293 CRTDLL.DLL is present on every Win32 installation.
294 Unfortunately, it has a series of problems: it does not
295 work properly with Microsoft's C++ libraries, it does not
296 provide support for 64-bit file offsets, (and so on...),
297 and Microsoft discontinued its support a long time ago.
299 * Linking ZLIB1.DLL to MSVCR70.DLL or MSVCR71.DLL, supplied
300 with the Microsoft .NET platform, and Visual C++ 7.0/7.1,
301 raises problems related to the status of ZLIB1.DLL as a
302 system component. According to the Microsoft Knowledge Base
303 article KB326922 "INFO: Redistribution of the Shared C
304 Runtime Component in Visual C++ .NET", MSVCR70.DLL and
305 MSVCR71.DLL are not supposed to function as system DLLs,
306 because they may clash with MSVCRT.DLL. Instead, the
307 application's installer is supposed to put these DLLs
308 (if needed) in the application's private directory.
309 If ZLIB1.DLL depends on a non-system runtime, it cannot
310 function as a redistributable system component.
312 * Linking ZLIB1.DLL to non-Microsoft runtimes, such as
313 Borland's, or Cygwin's, raises problems related to the
314 reliable presence of these runtimes on Win32 systems.
315 It's easier to let the DLL build of zlib up to the people
316 who distribute these runtimes, and who may proceed as
317 explained in the answer to Question 14.
320 13. If ZLIB1.DLL cannot be linked to MSVCR70.DLL or MSVCR71.DLL,
321 how can I build/use ZLIB1.DLL in Microsoft Visual C++ 7.0
322 (Visual Studio .NET) or newer?
324 - Due to the problems explained in the Microsoft Knowledge Base
325 article KB326922 (see the previous answer), the C runtime that
326 comes with the VC7 environment is no longer considered a
327 system component. That is, it should not be assumed that this
328 runtime exists, or may be installed in a system directory.
329 Since ZLIB1.DLL is supposed to be a system component, it may
330 not depend on a non-system component.
332 In order to link ZLIB1.DLL and your application to MSVCRT.DLL
333 in VC7, you need the library of Visual C++ 6.0 or older. If
334 you don't have this library at hand, it's probably best not to
335 use ZLIB1.DLL.
337 We are hoping that, in the future, Microsoft will provide a
338 way to build applications linked to a proper system runtime,
339 from the Visual C++ environment. Until then, you have a
340 couple of alternatives, such as linking zlib in statically.
341 If your application requires dynamic linking, you may proceed
342 as explained in the answer to Question 14.
345 14. I need to link my own DLL build to a CRT different than
346 MSVCRT.DLL. What can I do?
348 - Feel free to rebuild the DLL from the zlib sources, and link
349 it the way you want. You should, however, clearly state that
350 your build is unofficial. You should give it a different file
351 name, and/or install it in a private directory that can be
352 accessed by your application only, and is not visible to the
353 others (i.e. it's neither in the PATH, nor in the SYSTEM or
354 SYSTEM32 directories). Otherwise, your build may clash with
355 applications that link to the official build.
357 For example, in Cygwin, zlib is linked to the Cygwin runtime
358 CYGWIN1.DLL, and it is distributed under the name CYGZ.DLL.
361 15. May I include additional pieces of code that I find useful,
362 link them in ZLIB1.DLL, and export them?
364 - No. A legitimate build of ZLIB1.DLL must not include code
365 that does not originate from the official zlib source code.
366 But you can make your own private DLL build, under a different
367 file name, as suggested in the previous answer.
369 For example, zlib is a part of the VCL library, distributed
370 with Borland Delphi and C++ Builder. The DLL build of VCL
371 is a redistributable file, named VCLxx.DLL.
374 16. May I remove some functionality out of ZLIB1.DLL, by enabling
375 macros like NO_GZCOMPRESS or NO_GZIP at compile time?
377 - No. A legitimate build of ZLIB1.DLL must provide the complete
378 zlib functionality, as implemented in the official zlib source
379 code. But you can make your own private DLL build, under a
380 different file name, as suggested in the previous answer.
383 17. I made my own ZLIB1.DLL build. Can I test it for compliance?
385 - We prefer that you download the official DLL from the zlib
386 web site. If you need something peculiar from this DLL, you
387 can send your suggestion to the zlib mailing list.
389 However, in case you do rebuild the DLL yourself, you can run
390 it with the test programs found in the DLL distribution.
391 Running these test programs is not a guarantee of compliance,
392 but a failure can imply a detected problem.
394 **
396 This document is written and maintained by
397 Cosmin Truta <cosmint@cs.ubbcluj.ro>