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15.3.8 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 ‘foo.h’:

#ifndef FOO_H
#define FOO_H

int one (void);
int two (void);
extern int three;

#endif /* FOO_H */

And the implementation could be something like this:

Example ‘foo.c’:

#include "foo.h"

int one (void)
{
  return 1;
}

int two (void)
{
  return three - one ();
}

int three = 3;

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 __declspec(dllimport) and __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 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 __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 ‘-export-symbols’ or ‘-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 ‘-DDLL_EXPORT’ which is automatically provided by Libtool when libtool builds the copy of the object that is destined for the DLL. The second define is ‘-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 ‘--enable-auto-import’ option for some corner cases when it does not (see Options specific to i386 PE targets in Using ld, 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 (‘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 ‘-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 ‘foo.h’:

#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 */

When the targets are limited to contemporary GNU tools and Microsoft tools, the above can be simplified to the following:

Simplified ‘foo.h’:

#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 */

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 FlexDLL and edll.


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