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$(D_S Writing Win32 DLLs in D,
$(P DLLs (Dynamic Link Libraries) are one of the foundations
of system programming for Windows. The D programming
language enables the creation of several different types of
DLLs.
)
$(P For background information on what DLLs are and how they work
Chapter 11 of Jeffrey Richter's book
$(LINK2 http://www.amazon.com/exec/obidos/ASIN/1572315482/classicempire,
Advanced Windows) is indispensible.
)
$(P This guide will show how to create DLLs of various types with D.)
$(UL
$(LI <a href="#Cinterface">DLLs with a C interface</a>)
$(LI <a href="#com">DLLs that are COM servers</a>)
$(LI <a href="#Dcode">D code calling D code in DLLs</a>)
)
<h2><a name="Cinterface">DLLs with a C Interface</a></h2>
$(P A DLL presenting a C interface can connect to any other code
in a language that supports calling C functions in a DLL.
)
$(P DLLs can be created in D in roughly the same way as in C.
A $(D DllMain())
is required, looking like:
)
--------------------------------
import std.c.windows.windows;
import core.sys.windows.dll;
__gshared HINSTANCE g_hInst;
extern (Windows)
BOOL DllMain(HINSTANCE hInstance, ULONG ulReason, LPVOID pvReserved)
{
switch (ulReason)
{
case DLL_PROCESS_ATTACH:
g_hInst = hInstance;
dll_process_attach( hInstance, true );
break;
case DLL_PROCESS_DETACH:
dll_process_detach( hInstance, true );
break;
case DLL_THREAD_ATTACH:
dll_thread_attach( true, true );
break;
case DLL_THREAD_DETACH:
dll_thread_detach( true, true );
break;
}
return true;
}
-------------------------------
$(P Notes:)
$(UL
$(LI DllMain simply forwards to the appropriate helper functions. These setup
the runtime, create thread objects for interaction with the garbage collector
and initialize thread local storage data.)
$(LI The DLL does not share its runtime or memory with other DLLs.)
$(LI The first boolean argument to the dll-helper functions specify whether all threads
should be controlled by the garbage collector. You might need more control over
this behaviour if there are threads in the process that must not be suspended.
In this case pass false to disable the automatic handling of all threads.)
$(LI The presence of $(D DllMain()) is recognized by the compiler
causing it to emit a reference to
$(LINK2 http://www.digitalmars.com/ctg/acrtused.html, __acrtused_dll)
and the $(TT phobos.lib) runtime library.)
)
Link with a .def
($(LINK2 http://www.digitalmars.com/ctg/ctgDefFiles.html, Module Definition File))
along the lines of:
$(MODDEFFILE
LIBRARY MYDLL
DESCRIPTION 'My DLL written in D'
EXETYPE NT
CODE PRELOAD DISCARDABLE
DATA WRITE
EXPORTS
DllGetClassObject @2
DllCanUnloadNow @3
DllRegisterServer @4
DllUnregisterServer @5
)
$(P The functions in the EXPORTS list are for illustration.
Replace them with the actual exported functions from MYDLL.
Alternatively, use
$(LINK2 http://www.digitalmars.com/ctg/implib.html, implib).
Here's an example of a simple DLL with a function print()
which prints a string:
)
<h4>mydll.d:</h4>
-------------------------------
module mydll;
import std.c.stdio;
export void dllprint() { printf("hello dll world\n"); }
-------------------------------
$(P Note: We use $(CODE printf)s in these examples
instead of $(CODE writefln)
to make the examples as
simple as possible.)
<h4>mydll.def:</h4>
$(MODDEFFILE
LIBRARY "mydll.dll"
EXETYPE NT
SUBSYSTEM WINDOWS
CODE SHARED EXECUTE
DATA WRITE
)
$(P Put the code above that contains $(CODE DllMain()) into a file
$(TT dll.d).
Compile and link the dll with the following command:
)
$(CONSOLE
C:>dmd -ofmydll.dll -L/IMPLIB mydll.d dll.d mydll.def
C:>
)
$(P which will create mydll.dll and mydll.lib.
Now for a program, test.d, which will use the dll:
)
<h4>test.d:</h4>
-------------------------------
import mydll;
int main()
{
mydll.dllprint();
return 0;
}
-------------------------------
$(P Create an interface file mydll.di that doesn't have the function bodies:)
<h4>mydll.di:</h4>
-------------------------------
export void dllprint();
-------------------------------
Compile and link with the command:
$(CONSOLE
C:>dmd test.d mydll.lib
C:>
)
and run:
$(CONSOLE
C:>test
hello dll world
C:>
)
<h3>Memory Allocation</h3>
$(P D DLLs use garbage collected memory management. The question is what
happens when pointers to allocated data cross DLL boundaries?
If the DLL presents a C interface, one would assume the reason
for that is to connect with code written in other languages.
Those other languages will not know anything about D's memory
management. Thus, the C interface will have to shield the
DLL's callers from needing to know anything about it.
)
$(P There are many approaches to solving this problem:)
$(UL
$(LI Do not return pointers to D gc allocated memory to the caller of
the DLL. Instead, have the caller allocate a buffer, and have the DLL
fill in that buffer.)
$(LI Retain a pointer to the data within the D DLL so the GC will not free
it. Establish a protocol where the caller informs the D DLL when it is
safe to free the data.)
$(LI Notify the GC about external references to a memory block by
calling GC.addRange.)
$(LI Use operating system primitives like VirtualAlloc() to allocate
memory to be transferred between DLLs.)
$(LI Use std.c.stdlib.malloc() (or another non-gc allocator) when
allocating data to be returned to the caller. Export a function
that will be used by the caller to free the data.)
)
<h2><a name="com">COM Programming</a></h2>
Many Windows API interfaces are in terms of COM (Common Object Model)
objects (also called OLE or ActiveX objects). A COM object is an object
who's first field is a pointer to a vtbl[], and the first 3 entries
in that vtbl[] are for QueryInterface(), AddRef(), and Release().
<p>
For understanding COM, Kraig Brockshmidt's
$(LINK2 http://www.amazon.com/exec/obidos/ASIN/1556158432/classicempire, Inside OLE)
is an indispensible resource.
<p>
COM objects are analogous to D interfaces. Any COM object can be
expressed as a D interface, and every D object with an interface X
can be exposed as a COM object X.
This means that D is compatible with COM objects implemented
in other languages.
<p>
While not strictly necessary, the Phobos library provides an Object
useful as a super class for all D COM objects, called ComObject.
ComObject provides a default implementation for
QueryInterface(), AddRef(), and Release().
<p>
Windows COM objects use the Windows calling convention, which is not
the default for D, so COM functions need to have the attribute
extern (Windows).
So, to write a COM object:
-------------------------------
import std.c.windows.com;
class MyCOMobject : ComObject
{
extern (Windows):
...
}
-------------------------------
The sample code includes an example COM client program and server DLL.
<h2><a name="Dcode">D code calling D code in DLLs</a></h2>
Having DLLs in D be able to talk to each other as if they
were statically linked together is, of course, very desirable
as code between applications can be shared, and different
DLLs can be independently developed.
<p>
The underlying difficulty is what to do about garbage collection (gc).
Each EXE and DLL will have their own gc instance. While
these gc's can coexist without stepping on each other,
it's redundant and inefficient to have multiple gc's running.
The idea explored here is to pick one gc and have the DLLs
redirect their gc's to use that one. The one gc used here will be
the one in the EXE file, although it's also possible to make a
separate DLL just for the gc.
<p>
The example will show both how to statically load a DLL, and
to dynamically load/unload it.
<p>
Starting with the code for the DLL, mydll.d:
-------------------------------
/*
* MyDll demonstration of how to write D DLLs.
*/
import core.runtime;
import std.c.stdio;
import std.c.stdlib;
import std.string;
import std.c.windows.windows;
HINSTANCE g_hInst;
extern (C)
{
void gc_setProxy(void* p);
void gc_clrProxy();
}
extern (Windows)
BOOL $(B DllMain)(HINSTANCE hInstance, ULONG ulReason, LPVOID pvReserved)
{
switch (ulReason)
{
case DLL_PROCESS_ATTACH:
printf("DLL_PROCESS_ATTACH\n");
Runtime.initialize();
break;
case DLL_PROCESS_DETACH:
printf("DLL_PROCESS_DETACH\n");
Runtime.terminate();
break;
case DLL_THREAD_ATTACH:
printf("DLL_THREAD_ATTACH\n");
return false;
case DLL_THREAD_DETACH:
printf("DLL_THREAD_DETACH\n");
return false;
}
g_hInst = hInstance;
return true;
}
export void $(B MyDLL_Initialize)(void* gc)
{
printf("MyDLL_Initialize()\n");
gc_setProxy(gc);
}
export void $(B MyDLL_Terminate)()
{
printf("MyDLL_Terminate()\n");
gc_clrProxy();
}
$(B static this)()
{
printf("static this for mydll\n");
}
$(B static ~this)()
{
printf("static ~this for mydll\n");
}
/* --------------------------------------------------------- */
class $(B MyClass)
{
char[] $(B concat)(char[] a, char[] b)
{
return a ~ " " ~ b;
}
void $(B free)(char[] s)
{
delete s;
}
}
export MyClass $(B getMyClass)()
{
return new MyClass();
}
-------------------------------
<dl>
<dt>$(B DllMain)
<dd>This is the main entry point for any D DLL. It gets called
by the C startup code
(for DMC++, the source is $(TT \dm\src\win32\dllstart.c)).
The $(B printf)'s are placed there so one can trace how it gets
called.
Notice that the initialization and termination code seen in
the earlier DllMain sample code is in this version as well.
This is because the same DLL should be usable from both C and
D programs, so the same initialization process should work
for both.
<p>
<dt>$(B MyDLL_Initialize)
<dd>
When the DLL is dynamically linked via $(B Runtime.loadLibrary)()
the runtime makes sure that any initialization steps required
by the D program are executed after the library is loaded. If
the library is statically linked, this routine is not called by
the program, so to make sure the DLL is initialized properly we
have to do some of the work ourselves. And because the library
is being statically linked, we need a function specific to this
DLL to perform the initialization.
This function takes one argument, a handle to the
caller's gc. We'll see how that handle is obtained later.
To pass this handle to the runtime and override the DLL's built-in
gc we'll call $(B gc_setProxy)().
The function is $(B export)ed as that is how a function is made
visible outside of a DLL.
<p>
<dt>$(B MyDLL_Terminate)
<dd>Correspondingly, this function terminates the DLL, and is
called prior to unloading it.
It has only one job: informing the runtime that the DLL will
no longer be using the caller's gc via $(B gc_clrProxy)().
This is critical, as the DLL will be unmapped from memory,
and if the gc continues to scan its data areas it will cause
segment faults.
<p>
<dt>$(B static this, static ~this)
<dd>These are examples of the module's static constructor
and destructor,
here with a print in each to verify that they are running
and when.
<p>
<dt>$(B MyClass)
<dd>This is an example of a class that can be exported from
and used by the caller of a DLL. The $(B concat) member
function allocates some gc memory, and $(B free) frees gc
memory.
<p>
<dt>$(B getMyClass)
<dd>An exported factory that allocates an instance of $(B MyClass)
and returns a reference to it.
<p>
</dl>
To build the $(TT mydll.dll) DLL:
$(OL
$(LI$(B $(TT dmd -c mydll -g))
<br>Compiles $(D mydll.d) into $(TT mydll.obj).
$(B -g) turns on debug info generation.
)
$(LI $(B $(TT dmd mydll.obj mydll.def -g -L/map))
<br>Links $(TT mydll.obj) into a DLL named $(TT mydll.dll).
$(TT mydll.def) is the
$(LINK2 http://www.digitalmars.com/ctg/ctgDefFiles.html, Module Definition File),
and has the contents:
$(MODDEFFILE
LIBRARY MYDLL
DESCRIPTION 'MyDll demonstration DLL'
EXETYPE NT
CODE PRELOAD DISCARDABLE
DATA PRELOAD MULTIPLE
)
$(B -g) turns on debug info generation, and
$(B -L/map) generates a map file $(TT mydll.map).
)
$(LI $(B $(TT implib /noi /system mydll.lib mydll.dll))
<br>Creates an
$(LINK2 http://www.digitalmars.com/ctg/implib.html, import library)
$(TT mydll.lib) suitable
for linking in with an application that will be statically
loading $(TT mydll.dll).
)
)
$(P Here's $(TT test.d), a sample application that makes use of
$(TT mydll.dll). There are two versions, one statically binds to
the DLL, and the other dynamically loads it.
)
-------------------------------
import core.runtime;
import std.stdio;
import std.gc;
import mydll;
//version=DYNAMIC_LOAD;
version (DYNAMIC_LOAD)
{
import std.c.windows.windows;
alias MyClass function() getMyClass_fp;
int main()
{ HMODULE h;
FARPROC fp;
getMyClass_fp getMyClass;
MyClass c;
printf("Start Dynamic Link...\n");
h = cast(HMODULE) Runtime.loadLibrary("mydll.dll");
if (h is null)
{
printf("error loading mydll.dll\n");
return 1;
}
fp = GetProcAddress(h, "D5mydll10getMyClassFZC5mydll7MyClass");
if (fp is null)
{ printf("error loading symbol getMyClass()\n");
return 1;
}
getMyClass = cast(getMyClass_fp) fp;
c = (*getMyClass)();
foo(c);
if (!Runtime.unloadLibrary(h))
{ printf("error freeing mydll.dll\n");
return 1;
}
printf("End...\n");
return 0;
}
}
else
{ // static link the DLL
extern (C)
{
void* gc_getProxy();
}
int main()
{
printf("Start Static Link...\n");
MyDLL_Initialize(gc_getProxy());
foo(getMyClass());
MyDLL_Terminate();
printf("End...\n");
return 0;
}
}
void foo(MyClass c)
{
char[] s;
s = c.concat("Hello", "world!");
writefln(s);
c.free(s);
delete c;
}
-------------------------------
$(P Let's start with the statically linked version, which is simpler.
It's compiled and linked with the command:
)
$(CONSOLE
C:>dmd test mydll.lib -g
)
$(P Note how it is linked with $(TT mydll.lib), the import library
for $(TT mydll.dll).
The code is straightforward, it initializes $(TT mydll.lib) with
a call to $(B MyDLL_Initialize)(), passing the handle
to $(TT test.exe)'s gc.
Then, we can use the DLL and call its functions just as if
it were part of $(TT test.exe). In $(B foo)(), gc memory
is allocated and freed both by $(TT test.exe) and $(TT mydll.dll).
When we're done using the DLL, it is terminated with
$(B MyDLL_Terminate)().
)
$(P Running it looks like this:)
$(CONSOLE
C:>test
DLL_PROCESS_ATTACH
Start Static Link...
MyDLL_Initialize()
static this for mydll
Hello world!
MyDLL_Terminate()
static ~this for mydll
End...
C:>
)
$(P The dynamically linked version is a little harder to set up.
Compile and link it with the command:
)
$(CONSOLE
C:>dmd test -version=DYNAMIC_LOAD -g
)
$(P The import library $(TT mydll.lib) is not needed.
The DLL is loaded with a call to
$(B Runtime.loadLibrary)(),
and each exported function has to be retrieved via
a call to
$(B GetProcAddress)().
An easy way to get the decorated name to pass to $(B GetProcAddress)()
is to copy and paste it from the generated $(TT mydll.map) file
under the $(B Export) heading.
Once this is done, we can use the member functions of the
DLL classes as if they were part of $(TT test.exe).
When done, release the DLL with
$(B Runtime.unloadLibrary)().
)
$(P Running it looks like this:)
$(CONSOLE
C:>test
Start Dynamic Link...
DLL_PROCESS_ATTACH
static this for mydll
Hello world!
static ~this for mydll
DLL_PROCESS_DETACH
End...
C:>
)
)
Macros:
TITLE=Writing Win32 DLLs
WIKI=DLLs
CATEGORY_HOWTOS=$0
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