1
  2            Frequently Asked Questions about ZLIB1.DLL
  3
  4
  5This document describes the design, the rationale, and the usage
  6of the official DLL build of zlib, named ZLIB1.DLL.  If you have
  7general questions about zlib, you should see the file "FAQ" found
  8in the zlib distribution, or at the following location:
  9  http://www.gzip.org/zlib/zlib_faq.html
 10
 11
 12 1. What is ZLIB1.DLL, and how can I get it?
 13
 14  - ZLIB1.DLL is the official build of zlib as a DLL.
 15    (Please remark the character '1' in the name.)
 16
 17    Pointers to a precompiled ZLIB1.DLL can be found in the zlib
 18    web site at:
 19      http://www.zlib.net/
 20
 21    Applications that link to ZLIB1.DLL can rely on the following
 22    specification:
 23
 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.
 31
 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.
 41
 42    If, for any reason, you need to build an incompatible DLL,
 43    please use a different file name.
 44
 45
 46 2. Why did you change the name of the DLL to ZLIB1.DLL?
 47    What happened to the old ZLIB.DLL?
 48
 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:
 55
 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.
 60
 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.
 66
 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.
 71
 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.
 76
 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.
 80
 81
 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?
 84
 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.
 94
 95
 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?
 99
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.
103
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.
107
108
109 5. Why are the zlib symbols exported by name, and not by ordinal?
110
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.
116
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.
127
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.
132
133
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!
138
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".)
143
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.
155
156    The functionality offered by zlib is not in the category of
157    "Windows functionality", but is more like "C functionality".
158
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.
167
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.
173
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.
181
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.
188
189
190 7. I need a DLL for my Visual Basic project.  What can I do?
191
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.
198
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.
204
205
206 8. I need to use zlib in my Microsoft .NET project.  What can I
207    do?
208
209  - Henrik Ravn has contributed a .NET wrapper around zlib.  Look
210    into contrib/dotzlib/, inside the zlib distribution.
211
212
213 9. If my application uses ZLIB1.DLL, should I link it to
214    MSVCRT.DLL?  Why?
215
216  - It is not required, but it is recommended to link your
217    application to MSVCRT.DLL, if it uses ZLIB1.DLL.
218
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.
226
227
22810. 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.
233
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.
238
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.
249
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.
255
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.
259
260    Also note that linking ZLIB1.DLL to non-Microsoft CRTs, such
261    as those provided by Borland, raises similar problems.
262
263
26411. Why are you linking ZLIB1.DLL to MSVCRT.DLL?
265
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.
272
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.
279
280
28112. Why are you not linking ZLIB1.DLL to
282    <<my favorite C run-time library>> ?
283
284  - We considered and abandoned the following alternatives:
285
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.
291
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.
298
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.
311
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.
318
319
32013. 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?
323
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.
331
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.
336
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.
343
344
34514. I need to link my own DLL build to a CRT different than
346    MSVCRT.DLL.  What can I do?
347
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.
356
357    For example, in Cygwin, zlib is linked to the Cygwin runtime
358    CYGWIN1.DLL, and it is distributed under the name CYGZ.DLL.
359
360
36115. May I include additional pieces of code that I find useful,
362    link them in ZLIB1.DLL, and export them?
363
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.
368
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.
372
373
37416. May I remove some functionality out of ZLIB1.DLL, by enabling
375    macros like NO_GZCOMPRESS or NO_GZIP at compile time?
376
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.
381
382
38317. I made my own ZLIB1.DLL build.  Can I test it for compliance?
384
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.
388
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.
393
394**
395
396This document is written and maintained by
397Cosmin Truta <cosmint@cs.ubbcluj.ro>