Difference between revisions of "Win32 DLLs in D"

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A DLL presenting a C interface can connect to any other code in a language that supports calling C functions in a DLL.
 
A DLL presenting a C interface can connect to any other code in a language that supports calling C functions in a DLL.
  
DLLs can be created in D in roughly the same way as in C. A DllMain() is required, looking like:
+
DLLs can be created in D in roughly the same way as in C. A DllMain() is required, but you can use the mixin template SimpleDllMain to insert it:
  
 
<syntaxhighlight lang="D">
 
<syntaxhighlight lang="D">
import std.c.windows.windows;
+
import core.sys.windows.windows;
 
import core.sys.windows.dll;
 
import core.sys.windows.dll;
  
__gshared HINSTANCE g_hInst;
+
mixin SimpleDllMain;
  
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;
 
 
        default:
 
    }
 
    return true;
 
}
 
 
</syntaxhighlight>
 
</syntaxhighlight>
  
 
Notes:
 
Notes:
  
* 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.
+
* Under the hood, this DllMain simply forwards to the appropriate helper functions found in core.sys.windows.dll. These setup the runtime, create thread objects for interaction with the garbage collector and initialize thread local storage data.
 
* The DLL does not share its runtime or memory with other DLLs.
 
* The DLL does not share its runtime or memory with other DLLs.
* 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.
 
 
* The presence of '''DllMain()''' is recognized by the compiler causing it to emit a reference to [http://www.digitalmars.com/ctg/acrtused.html __acrtused_dll] and the '''phobos.lib''' runtime library.
 
* The presence of '''DllMain()''' is recognized by the compiler causing it to emit a reference to [http://www.digitalmars.com/ctg/acrtused.html __acrtused_dll] and the '''phobos.lib''' runtime library.
  
Link with a .def ([http://www.digitalmars.com/ctg/ctgDefFiles.html Module Definition File]) along the lines of:
+
You must export any functions you want the user to access. Do this by using the '''export''' keyword in your code.
 +
 
 +
Alternatively, you can link with a .def ([http://www.digitalmars.com/ctg/ctgDefFiles.html Module Definition File]) along the lines of:
  
 
<pre>
 
<pre>
Line 77: Line 52:
 
<syntaxhighlight lang="D">
 
<syntaxhighlight lang="D">
 
module mydll;
 
module mydll;
import std.c.stdio;
+
import core.stdc.stdio;
 
export void dllprint() { printf("hello dll world\n"); }
 
export void dllprint() { printf("hello dll world\n"); }
 
</syntaxhighlight>
 
</syntaxhighlight>
Line 144: Line 119:
 
* Notify the GC about external references to a memory block by calling GC.addRange.
 
* Notify the GC about external references to a memory block by calling GC.addRange.
 
* Use operating system primitives like VirtualAlloc() to allocate memory to be transferred between DLLs.
 
* Use operating system primitives like VirtualAlloc() to allocate memory to be transferred between DLLs.
* 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.
+
* Use core.stdcc.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.
  
 
== COM Programming ==
 
== COM Programming ==
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().
 
  
For understanding COM, Kraig Brockshmidt's [http://www.amazon.com/exec/obidos/ASIN/1556158432/classicempire Inside OLE] is an indispensible resource.
+
COM interfaces are all derived from '''core.sys.windows.com.IUnknown'''.
  
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.
+
See [[COM Programming]] for more details.
 
 
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().
 
 
 
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:
 
 
 
<syntaxhighlight lang="D">
 
import std.c.windows.com;
 
 
 
class MyCOMobject : ComObject
 
{
 
    extern (Windows):
 
...
 
}
 
</syntaxhighlight>
 
 
 
The sample code includes an example COM client program and server DLL.
 
  
 
== D code calling D code in DLLs ==
 
== D code calling D code in DLLs ==
 
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.
 
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.
  
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.
+
The underlying difficulty is what to do about garbage collection (gc). Each EXE and DLL will have their own gc instance. When dynamically loading, it is important to load the dll with '''Runtime.loadLibrary''' instead of the system `LoadLibrary` call to ensure the druntime functions are called and the dll must use the druntime dll helper functions (e.g. mixin SimpleDllMain), the same as the C interface example.
 +
 
 +
=== Shared D Runtime(LDC) ===
 +
When using the LDC compiler, the D Runtime can share the GC by compiling both the executable and the DLL with the flag '''-link-defaultlib-shared'''. Sharing the runtime is very important for being able to handle '''Error/Exception'''. If they are not shared, throwing from the DLL will simply crash the program instead of giving a useful message, this is also true for '''assert''' and other errors the runtime checks for you, for example, range errors. If this flag is not passed at compilation, the GC allocated memory sent from the executable to the DLL and vice versa will need to have its lifetime tracked manually.  
  
The example will show both how to statically load a DLL, and to dynamically load/unload it.
+
==== Warning for Dub users ====
 +
If you're using the '''dependencies''' section in Dub, passing this flag will get you a linking error, because this is a global flag, all the dependencies must know about it. The safest way to make that is setting the environment variable '''DFLAGS=-link-defaultlib-shared''' before calling '''dub'''. This is a very important step for dub users that must be took with care.
  
Starting with the code for the DLL, mydll.d:
+
== Using a D class from a DLL ==
 +
Even classes can be loaded using `'''Runtime.loadLibrary'''`. Though those classes must be instantiated via a `factory` function. That means, a function which will return a new instance.
 +
For loading D classes, one must '''import a D interface file(*.di)''' an mark the class as `extern`:
  
 +
=== class_from_dll.d ===
 
<syntaxhighlight lang="D">
 
<syntaxhighlight lang="D">
module mydll;
+
module class_from_dll;
 
+
//This code is not needed for ldc2
/*
+
version(Windows)
* 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;
 
 
 
import myclass;
 
 
 
HINSTANCE g_hInst;
 
 
 
extern (C)
 
 
{
 
{
     void gc_setProxy(void* p);
+
     import core.sys.windows.dll;
     void gc_clrProxy();
+
     mixin SimpleDllMain;
 
}
 
}
  
extern (Windows) BOOL DllMain(HINSTANCE hInstance, ULONG ulReason, LPVOID pvReserved)
+
export class ClassFromDll
 
{
 
{
     switch (ulReason)
+
     void print()
 
     {
 
     {
         case DLL_PROCESS_ATTACH:
+
         import std.stdio;
            printf("DLL_PROCESS_ATTACH\n");
+
         writeln("Hello from exported Class!");
            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;
 
 
 
        default:
 
 
     }
 
     }
    g_hInst = hInstance;
 
    return true;
 
}
 
 
export void MyDLL_Initialize(void* gc)
 
{
 
    printf("MyDLL_Initialize()\n");
 
    gc_setProxy(gc);
 
 
}
 
}
  
export void MyDLL_Terminate()
+
export ClassFromDll factory(){return new ClassFromDll;}
{
 
    printf("MyDLL_Terminate()\n");
 
    gc_clrProxy();
 
}
 
 
 
static this()
 
{
 
    printf("static this for mydll\n");
 
}
 
 
 
static ~this()
 
{
 
    printf("static ~this for mydll\n");
 
}
 
 
</syntaxhighlight>
 
</syntaxhighlight>
  
 
+
=== class_from_dll.di ===
 
<syntaxhighlight lang="D">
 
<syntaxhighlight lang="D">
module myclass;
+
module class_from_dll;
 
+
extern class ClassFromDll
class MyClass
 
{
 
    string concat(string a, string b)
 
    {
 
        return a ~ " " ~ b;
 
    }
 
}
 
 
 
export MyClass getMyClass()
 
 
{
 
{
     return new MyClass();
+
     void print();
 
}
 
}
 +
extern ClassFromDll factory();
 
</syntaxhighlight>
 
</syntaxhighlight>
  
'''DllMain'''
+
=== app.d ===
 +
<syntaxhighlight lang="D">
 +
module app;
  
This is the main entry point for any D DLL. It gets called by the C startup code (for DMC++, the source is \dm\src\win32\dllstart.c). The 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.
 
 
 
'''MyDLL_Initialize'''
 
 
When the DLL is dynamically linked via 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 gc_setProxy(). The function is exported as that is how a function is made visible outside of a DLL.
 
 
 
'''MyDLL_Terminate'''
 
 
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 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.
 
 
 
'''static this, static ~this'''
 
 
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.
 
 
 
'''MyClass'''
 
 
This is an example of a class that can be exported from and used by the caller of a DLL. The '''concat''' member function allocates some gc memory, and free frees gc memory.
 
 
 
'''getMyClass'''
 
 
An exported factory that allocates an instance of '''MyClass''' and returns a reference to it.
 
 
To build the '''mydll.dll''' DLL:
 
 
1. <code>dmd -c mydll myclass -g</code>
 
 
Compiles mydll.d into mydll.obj and myclass.d into myclass.obj. -g turns on debug info generation.
 
   
 
 
2. <code>dmd mydll.obj myclass.obj mydll.def -g -map</code>
 
 
Links mydll.obj and myclass.obj into a DLL named mydll.dll. mydll.def is the [http://www.digitalmars.com/ctg/ctgDefFiles.html Module Definition File], and has the contents:
 
 
<pre>
 
LIBRARY        MYDLL
 
DESCRIPTION    'MyDll demonstration DLL'
 
EXETYPE NT
 
CODE            PRELOAD DISCARDABLE
 
DATA            PRELOAD MULTIPLE
 
</pre>
 
 
 
'''-g''' turns on debug info generation, and '''-map''' generates a map file '''mydll.map'''.
 
 
 
3. <code>implib /noi /system mydll.lib mydll.dll</code>
 
 
Creates an [http://www.digitalmars.com/ctg/implib.html import library] '''mydll.lib''' suitable for linking in with an application that will be statically loading '''mydll.dll'''.
 
 
 
 
Here's '''test.d''', a sample application that makes use of '''mydll.dll'''. There are two versions, one statically binds to the DLL, and the other dynamically loads it.
 
 
<syntaxhighlight lang="D">
 
 
import core.runtime;
 
import core.runtime;
import std.stdio;
+
import core.sys.windows.winbase:GetProcAddress;
import core.memory;
+
void main()
 
 
import myclass;
 
 
 
//version=DYNAMIC_LOAD;
 
 
 
version (DYNAMIC_LOAD)
 
 
{
 
{
     import std.c.windows.windows;
+
     void* lib = Runtime.loadLibrary("./class_from_dll.dll");
 
+
    import class_from_dll;
    alias MyClass function() getMyClass_fp;
+
    auto factoryFunction = cast(typeof(&factory))GetProcAddress(lib, factory.mangleof);
 
+
    ClassFromDll cls = factoryFunction();
    int main()
+
    cls.print();
    {
 
        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, "D7myclass10getMyClassFZC7myclass7MyClass");
 
        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()
+
</syntaxhighlight>
    {
 
        printf("Start Static Link...\n");
 
        MyDLL_Initialize(gc_getProxy());
 
        foo(getMyClass());
 
        MyDLL_Terminate();
 
        printf("End...\n");
 
        return 0;
 
    }
 
}
 
  
void foo(MyClass c)
+
<syntaxhighlight lang="text">
{
+
dmd -shared class_from_dll.d
    string s = c.concat("Hello", "world!");
+
dmd app.d class_from_dll.di
    writeln(s);
 
}
 
 
</syntaxhighlight>
 
</syntaxhighlight>
 
Let's start with the statically linked version, which is simpler. It's compiled and linked with the command:
 
 
<pre>
 
C:>dmd test myclass mydll.lib -g
 
</pre>
 
 
Note how it is linked with '''mydll.lib''', the import library for '''mydll.dll'''. The code is straightforward, it initializes mydll.lib with a call to '''MyDLL_Initialize()''', passing the handle to test.exe's gc. Then, we can use the DLL and call its functions just as if it were part of '''test.exe'''. In '''foo()''', gc memory is allocated and freed both by '''test.exe''' and '''mydll.dll'''. When we're done using the DLL, it is terminated with '''MyDLL_Terminate()'''.
 
 
Running it looks like this:
 
 
<pre>
 
C:>test
 
DLL_PROCESS_ATTACH
 
Start Static Link...
 
MyDLL_Initialize()
 
static this for mydll
 
Hello world!
 
MyDLL_Terminate()
 
static ~this for mydll
 
End...
 
C:>
 
</pre>
 
 
The dynamically linked version is a little harder to set up. Compile and link it with the command:
 
 
<pre>
 
C:>dmd test myclass -version=DYNAMIC_LOAD -g
 
</pre>
 
 
The import library '''mydll.lib''' is not needed. The DLL is loaded with a call to '''Runtime.loadLibrary()''', and each exported function has to be retrieved via a call to '''GetProcAddress()'''. An easy way to get the decorated name to pass to '''GetProcAddress()''' is to copy and paste it from the generated '''mydll.map''' file under the '''Export''' heading. Once this is done, we can use the member functions of the DLL classes as if they were part of '''test.exe'''. When done, release the DLL with '''Runtime.unloadLibrary()'''.
 
 
Running it looks like this:
 
 
<pre>
 
C:>test
 
Start Dynamic Link...
 
DLL_PROCESS_ATTACH
 
static this for mydll
 
Hello world!
 
static ~this for mydll
 
DLL_PROCESS_DETACH
 
End...
 
C:>
 
</pre>
 
 
  
 
----
 
----

Latest revision as of 19:59, 27 December 2023

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.

For background information on what DLLs are and how they work Chapter 11 of Jeffrey Richter's book Advanced Windows is indispensible.

This guide will show how to create DLLs of various types with D.

Compiling a DLL

Use the -shared switch to tell the compiler that the generated code is to be put into a DLL. Code compiled for an EXE file will use the optimization assumption that _tls_index==0. Such code in a DLL will crash.

DLLs with a C Interface

A DLL presenting a C interface can connect to any other code in a language that supports calling C functions in a DLL.

DLLs can be created in D in roughly the same way as in C. A DllMain() is required, but you can use the mixin template SimpleDllMain to insert it:

import core.sys.windows.windows;
import core.sys.windows.dll;

mixin SimpleDllMain;

Notes:

  • Under the hood, this DllMain simply forwards to the appropriate helper functions found in core.sys.windows.dll. These setup the runtime, create thread objects for interaction with the garbage collector and initialize thread local storage data.
  • The DLL does not share its runtime or memory with other DLLs.
  • The presence of DllMain() is recognized by the compiler causing it to emit a reference to __acrtused_dll and the phobos.lib runtime library.

You must export any functions you want the user to access. Do this by using the export keyword in your code.

Alternatively, you can link with a .def (Module Definition File) along the lines of:

LIBRARY         MYDLL
DESCRIPTION     'My DLL written in D'

EXETYPE		NT
CODE            PRELOAD DISCARDABLE
DATA            WRITE

EXPORTS
		DllGetClassObject       @2
		DllCanUnloadNow         @3
		DllRegisterServer       @4
		DllUnregisterServer     @5

The functions in the EXPORTS list are for illustration. Replace them with the actual exported functions from MYDLL. Alternatively, use implib. Here's an example of a simple DLL with a function print() which prints a string:

mydll.d:

module mydll;
import core.stdc.stdio;
export void dllprint() { printf("hello dll world\n"); }

Note: We use printfs in these examples instead of writefln to make the examples as simple as possible.

mydll.def:

LIBRARY "mydll.dll"
EXETYPE NT
SUBSYSTEM WINDOWS
CODE SHARED EXECUTE
DATA WRITE

Put the code above that contains DllMain() into a file dll.d. Compile and link the dll with the following command:

C:>dmd -ofmydll.dll -L/IMPLIB mydll.d dll.d mydll.def
C:>

which will create mydll.dll and mydll.lib. Now for a program, test.d, which will use the dll:

test.d:

import mydll;

int main()
{
   mydll.dllprint();
   return 0;
}

Create an interface file mydll.di that doesn't have the function bodies.

mydll.di:

export void dllprint();

Compile and link with the command:

C:>dmd test.d mydll.lib
C:>

and run:

C:>test
hello dll world
C:>

Memory Allocation

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.

There are many approaches to solving this problem:

  • 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.
  • 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.
  • Notify the GC about external references to a memory block by calling GC.addRange.
  • Use operating system primitives like VirtualAlloc() to allocate memory to be transferred between DLLs.
  • Use core.stdcc.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.

COM Programming

COM interfaces are all derived from core.sys.windows.com.IUnknown.

See COM Programming for more details.

D code calling D code in DLLs

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.

The underlying difficulty is what to do about garbage collection (gc). Each EXE and DLL will have their own gc instance. When dynamically loading, it is important to load the dll with Runtime.loadLibrary instead of the system `LoadLibrary` call to ensure the druntime functions are called and the dll must use the druntime dll helper functions (e.g. mixin SimpleDllMain), the same as the C interface example.

Shared D Runtime(LDC)

When using the LDC compiler, the D Runtime can share the GC by compiling both the executable and the DLL with the flag -link-defaultlib-shared. Sharing the runtime is very important for being able to handle Error/Exception. If they are not shared, throwing from the DLL will simply crash the program instead of giving a useful message, this is also true for assert and other errors the runtime checks for you, for example, range errors. If this flag is not passed at compilation, the GC allocated memory sent from the executable to the DLL and vice versa will need to have its lifetime tracked manually.

Warning for Dub users

If you're using the dependencies section in Dub, passing this flag will get you a linking error, because this is a global flag, all the dependencies must know about it. The safest way to make that is setting the environment variable DFLAGS=-link-defaultlib-shared before calling dub. This is a very important step for dub users that must be took with care.

Using a D class from a DLL

Even classes can be loaded using `Runtime.loadLibrary`. Though those classes must be instantiated via a `factory` function. That means, a function which will return a new instance. For loading D classes, one must import a D interface file(*.di) an mark the class as `extern`:

class_from_dll.d

module class_from_dll;
//This code is not needed for ldc2
version(Windows)
{
    import core.sys.windows.dll;
    mixin SimpleDllMain;
}

export class ClassFromDll
{
    void print()
    {
        import std.stdio;
        writeln("Hello from exported Class!");
    }
}

export ClassFromDll factory(){return new ClassFromDll;}

class_from_dll.di

module class_from_dll;
extern class ClassFromDll
{
    void print();
}
extern ClassFromDll factory();

app.d

module app;

import core.runtime;
import core.sys.windows.winbase:GetProcAddress;
void main()
{
    void* lib = Runtime.loadLibrary("./class_from_dll.dll");
    import class_from_dll;
    auto factoryFunction = cast(typeof(&factory))GetProcAddress(lib, factory.mangleof);
    ClassFromDll cls = factoryFunction();
    cls.print();
}
dmd -shared class_from_dll.d
dmd app.d class_from_dll.di

Windows DLL know-how Development