Unlike most Unices, on [Windows] you need to explicitly specify 
which functions are publically visible when creating
a shared library.

One way to do this is to write a .DEF file listing the
symbols to be exported, and pass that to the linker
when building the .DLL.  
(See the linker documentation for details if you're 
interested; but the next way is easier).

The easier way is to use an MSVC language extension
to declare publically visible functions in the source code:
======c
extern int __declspec(dllexport) Foo_Init(Tcl_Interp *);
...
int __declspec(dllexport) Foo_Init(Tcl_Interp *interp)
{
    ...
}
======
Most other compilers for Windows (including the MinGW and
CYGWIN versions of gcc) have adopted this extension.

Naturally, the `__declspec(dllexport)` bit should be hidden
inside a preprocessor macro for cross-platform portability.
Tcl.h defines such a macro: the symbol `DLLEXPORT` expands
to the appropriate Windows voodoo if the compilation environment
supports it, and to the empty string otherwise.
======c
int DLLEXPORT Foo_Init(Tcl_Interp *interp) { ... }
======
----

That should be sufficient for Tcl extensions' `_Init()`
and `_SafeInit()` routines.  In the general case, though,
more preprocessor trickery is needed.

Windows also requires that functions appearing in an external
shared library be declared with the "`__declspec(dllimport)`"
storage class.  Instead of creating two copies of each header
file — one with `__declspec(dllimport)` for users of the
library and one with `__declspec(dllexport)` for the library
itself — the usual idiom is to say:
======c
#ifdef BUILD_FOO
#    define FOOAPI __declspec(dllexport)
#else
#    define FOOAPI __declspec(dllimport)
#endif
...
extern int FOOAPI foo_fiddle();
extern int FOOAPI foo_faddle();
extern int FOOAPI foo_twaddle();
======
in the relevant header files, then compile the library
with `-DBUILD_FOO`.  Tcl does this, too; the macro
`TCL_STORAGE_CLASS` expands to `__declspec(dllexport)`
when building Tcl on Windows, `__declspec(dllimport)`
when linking directly against the Tcl shared library,
and to the empty string when linking with the Stubs
library and on non-Windows platforms.

Many Tcl extensions use preprocessor contortions like:
======c
#undef TCL_STORAGE_CLASS
#ifdef BUILD_sampleextension
#  define TCL_STORAGE_CLASS DLLEXPORT
#endif
...
EXTERN int Sample_Init();
…
======
This is overly complex and error-prone; I recommend not
doing this.  Basically, this is hijacking Tcl's preprocessor
voodoo for reuse by other packages.  It's simpler, clearer,
and safer for extensions to just define their own voodoo.


<<categories>> Porting | Windows