From: Peter Rosin Date: Mon, 1 Nov 2010 09:10:36 +0000 (+0100) Subject: docs: Windows DLLs and headers. X-Git-Tag: v2.4.2~37 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=7a6ca6e6942ddad9f0dc95e8c6d32e062c9cedbc;p=thirdparty%2Flibtool.git docs: Windows DLLs and headers. * doc/libtool.texi (Platform quirks): Add new subsection 'Windows DLLs'. Signed-off-by: Peter Rosin --- diff --git a/ChangeLog b/ChangeLog index d3ecba7d8..5d1ec7cb4 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,5 +1,9 @@ 2010-11-01 Peter Rosin + docs: Windows DLLs and headers. + * doc/libtool.texi (Platform quirks): Add new subsection + 'Windows DLLs'. + * doc/libtool.texi (Platform quirks): Fix typo. 2010-10-30 Ralf Wildenhues diff --git a/doc/libtool.texi b/doc/libtool.texi index 152d49107..2f48a09be 100644 --- a/doc/libtool.texi +++ b/doc/libtool.texi @@ -225,6 +225,7 @@ Platform quirks * Archivers:: Programs that create static archives. * Cross compiling:: Issues that arise when cross compiling. * File name conversion:: Converting file names between platforms. +* Windows DLLs:: Windows header defines. @end detailmenu @end menu @@ -5775,6 +5776,7 @@ write your own. * Archivers:: Programs that create static archives. * Cross compiling:: Issues that arise when cross compiling. * File name conversion:: Converting file names between platforms. +* Windows DLLs:: Windows header defines. @end menu @node References @@ -6328,6 +6330,199 @@ the source or build directory trees, and all @option{-M*} options to This is quite a fragile setup, but it has been in historical use, and so is documented here. +@node Windows DLLs +@subsection Windows DLLs +@cindex Windows DLLs + +This topic describes a couple of ways to portably create Windows Dynamic +Link Libraries (DLLs). Libtool knows how to create DLLs using GNU tools +and using Microsoft tools. + +A typical library has a ``hidden'' implementation with an interface +described in a header file. On just about every system, the interface +could be something like this: + +Example @file{foo.h}: + +@example +#ifndef FOO_H +#define FOO_H + +int one (void); +int two (void); +extern int three; + +#endif /* FOO_H */ +@end example + +@noindent +And the implementation could be something like this: + +Example @file{foo.c}: + +@example +#include "foo.h" + +int one (void) +@{ + return 1; +@} + +int two (void) +@{ + return three - one (); +@} + +int three = 3; +@end example + +When using contemporary GNU tools to create the Windows DLL, the above +code will work there too, thanks to its auto-import/auto-export +features. But that is not the case when using older GNU tools or perhaps +more interestingly when using proprietary tools. In those cases the code +will need additional decorations on the interface symbols with +@code{__declspec(dllimport)} and @code{__declspec(dllexport)} depending +on whether the library is built or it's consumed and how it's built and +consumed. However, it should be noted that it would have worked also +with Microsoft tools, if only the variable @code{three} hadn't been +there, due to the fact the Microsoft tools will automatically import +functions (but sadly not variables) and Libtool will automatically export +non-static symbols as described next. + +With Microsoft tools, Libtool digs through the object files that make up +the library, looking for non-static symbols to automatically export. +I.e., Libtool with Microsoft tools tries to mimic the auto-export feature +of contemporary GNU tools. It should be noted that the GNU auto-export +feature is turned off when an explicit @code{__declspec(dllexport)} is +seen. The GNU tools do this to not make more symbols visible for projects +that have already taken the trouble to decorate symbols. There is no +similar way to limit which symbols are visible in the code when Libtool +is using Microsoft tools. In order to limit symbol visibility in that +case you need to use one of the options @option{-export-symbols} or +@option{-export-symbols-regex}. + +No matching help with auto-import is provided by Libtool, which is why +variables must be decorated to import them from a DLL for everything but +contemporary GNU tools. As stated above, functions are automatically +imported by both contemporary GNU tools and Microsoft tools, but for +other proprietary tools the auto-import status of functions is unknown. + +When the objects that form the library are built, there are generally +two copies built for each object. One copy is used when linking the DLL +and one copy is used for the static library. On Windows systems, a pair +of defines are commonly used to discriminate how the interface symbols +should be decorated. The first define is @samp{-DDLL_EXPORT} which is +automatically provided by Libtool when @command{libtool} builds the copy +of the object that is destined for the DLL. The second define is +@samp{-DLIBFOO_BUILD} (or similar) which is often added by the package +providing the library and is used when building the library, but not +when consuming the library. + +However, the matching double compile is not performed when consuming +libraries. It is therefore not possible to reliably distinguish if the +consumer is importing from a DLL or if it is going to use a static +library. + +With contemporary GNU tools, auto-import often saves the day, but see +the GNU ld documentation and its @option{--enable-auto-import} option +for some corner cases when it does not +(@pxref{Options, @option{--enable-auto-import}, Options specific to +i386 PE targets, ld, Using ld@comma{} the GNU linker}). + +With Microsoft tools you typically get away with always compiling the +code such that variables are expected to be imported from a DLL and +functions are expected to be found in a static library. The tools will +then automatically import the function from a DLL if that is where they +are found. If the variables are not imported from a DLL as expected, but +are found in a static library that is otherwise pulled in by some +function, the linker will issue a warning (LNK4217) that a locally +defined symbol is imported, but it still works. In other words, this +scheme will not work to only consume variables from a library. There is +also a price connected to this liberal use of imports in that an extra +indirection is introduced when you are consuming the static version of +the library. That extra indirection is unavoidable when the DLL is +consumed, but it is not needed when consuming the static library. + +For older GNU tools and other proprietary tools there is no generic way +to make it possible to consume either of the DLL or the static library +without user intervention, the tools need to be told what is intended. +One common assumption is that if a DLL is being built (@samp{DLL_EXPORT} +is defined) then that DLL is going to consume any dependent libraries as +DLLs. If that assumption is made everywhere, it is possible to select +how an end-user application is consuming libraries by adding a single +flag @samp{-DDLL_EXPORT} when a DLL build is required. This is of course +an all or nothing deal, either everything as DLLs or everything as static +libraries. + +To sum up the above, the header file of the foo library needs to be +changed into something like this: + +Modified @file{foo.h}: + +@example +#ifndef FOO_H +#define FOO_H + +#if defined _WIN32 && !defined __GNUC__ +# ifdef LIBFOO_BUILD +# ifdef DLL_EXPORT +# define LIBFOO_SCOPE __declspec (dllexport) +# define LIBFOO_SCOPE_VAR extern __declspec (dllexport) +# endif +# elif defined _MSC_VER +# define LIBFOO_SCOPE +# define LIBFOO_SCOPE_VAR extern __declspec (dllimport) +# elif defined DLL_EXPORT +# define LIBFOO_SCOPE __declspec (dllimport) +# define LIBFOO_SCOPE_VAR extern __declspec (dllimport) +# endif +#endif +#ifndef LIBFOO_SCOPE +# define LIBFOO_SCOPE +# define LIBFOO_SCOPE_VAR extern +#endif + +LIBFOO_SCOPE int one (void); +LIBFOO_SCOPE int two (void); +LIBFOO_SCOPE_VAR int three; + +#endif /* FOO_H */ +@end example + +When the targets are limited to contemporary GNU tools and Microsoft +tools, the above can be simplified to the following: + +Simplified @file{foo.h}: + +@example +#ifndef FOO_H +#define FOO_H + +#if defined _WIN32 && !defined __GNUC__ && !defined LIBFOO_BUILD +# define LIBFOO_SCOPE_VAR extern __declspec (dllimport) +#else +# define LIBFOO_SCOPE_VAR extern +#endif + +int one (void); +int two (void); +LIBFOO_SCOPE_VAR int three; + +#endif /* FOO_H */ +@end example + +This last simplified version can of course only work when Libtool is +used to build the DLL, as no symbols would be exported otherwise (i.e., +when using Microsoft tools). + +It should be noted that there are various projects that attempt to relax +these requirements by various low level tricks, but they are not +discussed here. +Examples are +@uref{http://alain.frisch.fr/@/flexdll.html, FlexDLL} and +@uref{http://edll.sourceforge.net/, edll}. + + @node libtool script contents @section @code{libtool} script contents @cindex implementation of libtool