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Binding D to C Part Five

Posted by Aldacron, in D, Binding to C 31 December 2012 · 1,888 views

This is the fifth and final part of a series on creating bindings to C libraries for the D Programming Language.

In part one, I introduced the difference between dynamic and static bindings and some of the things to consider when choosing which kind to implement. In part two, I talked about the different linkage attributes to be aware of when declaring external C functions in D. In part three, I showed how to translate C types to D. In part four, I wrapped up the discussion of type translation with a look at structs. Here in part five, I'm going to revisit the static vs. dynamic binding issue, this time looking at implementation differences.

Terminology

Back in part one, I gave this definition of static vs. dynamic bindings.

By static, I mean a binding that allows you to link with C libraries or object files directly. By dynamic, I mean a binding that does not allow linking, but instead loads a shared library (DLL/so/dylib/framework) at runtime.

Those two unfortunate sentences did not clearly express my meaning. Only one person demonstrated a misunderstanding of the above in a comment on the post, but in the intervening 12 months I've found a few more people using the two terms in the wrong way. Before explaining more clearly what it is I actually mean, let me make emphatically clear what I don't.

When I talk of a static binding, I am not referring to static linking. While the two terms are loosely related, they are not the same at all. A static binding can certainly be used to link with static libraries, but, and this is what I failed to express clearly in part one, they can also be linked with dynamic libraries at compile time. In the C or C++ world, it is quite common when using shared libraries to link them at compile time. On Windows, this is done via an import library. Given an application "Foo" that makes use of the DLL "Bar", when "Foo" is compiled it will be linked with an import library named Bar.lib. This will cause the DLL to be loaded automatically by the operating system when the application is executed. The same thing can be accomplished on Posix systems by linking directly with the shared object file (extending the example, that would be libBar.so in this case). So with a static binding in D, a program can be linked at compile time with the static library Bar.lib (Windows) or libBar.a (Posix) for static linkage, or the import library Bar.lib (Windows) or libBar.so (Posix) for dynamic linkage.

A dynamic binding can not be linked to anything at compile time. No static libraries, no import libraries, no shared objects. It is designed explicitly for loading a shared library manually at run time. In the C and C++ world, this technique is often used to implement plugin systems, or to implement hot swapping of different application subsystems (for example, switching between an OpenGL and D3D renderer) among other things. The approach used here is to declare exported shared library symbols as pointers, call into the OS API for loading shared libraries, then manually extract the exported symbols and assign them to the pointers. This is exactly what a dynamic binding does. It sacrifices the convenience of letting the OS load the shared library for more control over when and what is loaded.

So to reiterate, a static binding can be used with either static libraries or shared libraries that are linked at compile time. Both cases are subject to the issue with object file formats that I outlined in part one (though the very-soon-to-be-released DMD 2.061 alleviates this a good deal, as the 64-bit version knows how to work with the Visual Studio linker). A dynamic binding cannot be linked to the bound library at compile time, but must provide a mechanism to manually load the library at run time.

Now that I've hopefully gotten that point across, it's time to examine the only difference in implementing the two types of bindings. In part two, I foreshadowed this discussion with the following.

Although I'm not going to specifically talk about static bindings in this post, the following examples use function declarations as you would in a static binding. For dynamic bindings, you'll use function pointers instead.

Static Bindings

In D, we generally do not have to declare a function before using it. The implementation is the declaration. And it doesn't matter if it's declared before or after the point at which its called. As long as it is in the currently visible namespace, it's callable. However, when linking with a C library, we don't have access to any function implementations (nor, actually, to the declarations--hence the binding). They are external to the application. In order to call into that library, the D compiler needs to be made aware of the existence of the functions that need to be called so that, at link time, it can match up the proper address offsets to make the call. This is the only case I can think of in D where a function declaration isn't just useful, but required.

I explained linkage attributes in part two. The examples I gave there, coupled with the details in part three regarding type translation, are all you need to know to implement a function declaration for a static D binding to a C library. But I'll give an example anyway.
// In C, foo.h
extern int foo(float f);
extern void bar(void);

// In D
extern( C )
{

    int foo(float);

    void bar();
}
Please be sure to read parts 2 - 4 completely. With all of that, and the info in this post up to this point, you have almost everything you need to know to implement a static binding in D, barring any corner cases that I've failed to consider or am yet to encounter myself. Oh, and function pointers. But that's coming in the next section.

Interlude -- Function Pointers

I could have covered function pointers in part three. After all, it isn't uncommon to encounter C libraries that use function pointers as typedefed callbacks (or declared inline in a function parameter list), or as struct members. And there's no difference in declaring function pointers for callbacks or for loading function symbols from a shared library. The syntax is identical. But, there are some issues that need to be considered in the different situations. And it's important to understand this before we get into dynamic bindings.

Let's look first at the syntax for declaring a function pointer in D.

int function() MyFuncPtr;
Very simple: return type->function keyword->parameter list->function pointer name. Though it's possible to use MyFuncPtr directly, it's often convenient to declare an alias:


alias int function() da_MyFuncPtr;
da_MyFuncPtr MyFuncPtr;
What's the difference? Let's see.


int foo(int i)
{
  return i;
}

void main()
{
    int function(int) fooPtr;
    fooPtr = &foo;


    alias int function(int) da_fooPtr;
    da_fooPtr fooPtr2 = &foo;


    import std.stdio;
    writeln(fooPtr(1));
    writeln(fooPtr2(2));
}
There is none! At least, not on the surface. I'll get into that later. Let's look at another example. Translating a C callback into D.

// In C, foo.h
typedef int (*MyCallback)(void);
 
// In D
extern( C ) alias int function() MyCallback;
Notice that I used the alias form here. Anytime you declare a typedefed C function pointer in D, it should be aliased so that it can be used the same way. Finally, the case of function pointers declared inline in a paramter list.

// In C, foo.h
extern void foo(int (*BarPtr)(int));

// In D.
// Option 1
extern( C ) void foo(int function(int) BarPtr);

// Option 2
extern( C ) alias int function(int) BarPtr;
extern( C ) void foo(BarPtr);
Personally, I prefer option 2. Also, I generally prefer to use extern blocks to include multiple declarations so that I don't have to type extern( C ) or extern(System) all the time (as I did in the previous example).

Now that the function pointer intro is out of the way, it's time to look at dynamic bindings.

Dynamic Bindings

At this point, I would very much like to say that you know everything you need to know about dynamic bindings. But that would be untrue. As it turns out, simply declaring function pointers is not enough. There are two issues to take into consideration. The first is function pointer initialization.

In one of the examples above (fooPtr), I showed how a function pointer can be declared and initialized. But in that example, it is obvious to the compiler that the function foo and the pointer fooPtr have the same basic signature (return type and parameter list). Now consider this example.

// This is all D.
int foo() { return 1; }

void* getPtr() { return cast(void*)&foo; }

void main()
{

    int function() fooPtr;

    fooPtr = getPtr();
}
Try to compile this and you'll see something like:

fptr.d(10): Error: cannot implicitly convert expression (getPtr()) of type void* to int function()

Now, obviously this is a contrived example. But I'm mimicking what a dynamic binding has to go through. OS API calls (like GetProcAddress or dlsym) return function pointers of void* type. So this is exactly the sort of error you will encounter if you try to directly assign the return value to a function pointer declared in this manner.

The first solution that might come to mind is to go ahead and insert an explicit cast. So, let's see what that gets us.

fooPtr = cast(fooPtr)getPtr();
The error here might be obvious to an experienced coder, but certainly not to most. I'll let the compiler explain.

fptr.d(10): Error: fooPtr is used as a type

Exactly. fooPtr is not a type, it's a variable. This is akin to declaring int i = cast(i)x; You can't do that. So the next obvious solution might be to use an aliased function pointer declaration. Then it can be used as a type. And that is, indeed, one possible solution (and, for reasons I'll explain below, the best one).

alias int function() da_fooPtr;
da_fooPtr fooPtr = cast(da_fooPtr)getPtr();
And this compiles. For the record, the 'da_' prefix is something I always use with function pointer aliases. It means 'D alias'. You can do as you please.

I implied above that there was more than one possible solution. Here's the second one.

int foo() { return 1; }void* getPtr() { return cast(void*)&foo; }void bindFunc(void** func) { *func = getPtr(); }void main()
{
    int function() fooPtr;
    bindFunc(cast(void**)&fooPtr);
}
Here, the address of fooPtr is being taken (giving us, essentially, a foo**) and cast to void**. Then bind func is able to dereference the pointer and assign it the void* value without a cast. When I first implemented Derelict, I used the alias approach. In Derelict 2, Tomasz Stachowiak implemented a new loader using the void** technique. That worked well. And, as a bonus, it eliminated a great many alias declarations from the codebase. Until something happened that, while a good thing for many users of D on Linux, turned out to be a big headache for me.

For several years, DMD did not provide a stack trace when exceptions were thrown. Then, some time ago, a release was made that implemented stack traces on Linux. The downside was that it was done in a way that broke Derelict 2 completely on that platform. To make a long story short, the DMD configuration files were preconfigured to export all symbols when compiling any binaries, be they shared objects or executables. Without this, the stack trace implementation wouldn't work. This caused every function pointer in Derelict to clash with every function exported by the bound libraries. In other words. the function pointer glClear in Derelict 2 suddenly started to conflict with the actual glClear function in the shared library, even though the library was loaded manually (which, given my Windows background, makes absolutely no sense to me whatsoever). So, I had to go back to the aliased function pointers. Aliased function pointers and variables declared of their type aren't exported. If you are going to make a publicly available dynamic binding, this is something you definitely need to keep in mind.

I still use the void** style to load function pointers, despite having switched back to aliases. It was less work than converting everything to a direct load. And when I implemented Derelict 3, I kept it that way. So if you look at the Derelict loaders...

// Instead of seeing this
foo = cast(da_Foo)getSymbol("foo");

// You'll see this
foo = bindFunc(cast(void**)&foo, "foo");
I don't particularly advocate one over the other when implementing a binding with the aid of a script. But if you're doing it by hand, the latter is much more amenable to quick copy-pasting.

There's one more important issue to discuss. Given that a dynamic binding uses function pointers, the pointers are subject to D's rules for variable storage. And by default, all variables in D are stashed in Thread-Local Storage. What that means is that, by default, each thread gets its own copy of the variable. So if a binding just blindly declares function pointers, then they are loaded in one thread and called in another... boom! Thankfully, D's function pointers are default initialized to null, so all you get is an access violation and not a call into random memory somewhere. The solution here is to let D know that the function pointers need to be shared across all threads. We can do that using one of two keywords: shared or __gshared.

One of the goals of D is to make concurrency easier than it traditionally has been in C-like languages. The shared type qualifier is intended to work toward that goal. When using it, you are telling the compiler that a particular variable is intended to be used across threads. The compiler can then complain if you try to access it in a way that isn't thread-safe. But like D's immutable and const , shared is transitive. That means if you follow any references from a shared object, they must also be shared. There are a number of issues that have yet to be worked out, so it hasn't seen a lot of practical usage that I'm aware of. And that's where __gshared comes in.

When you tell the compiler that a piece of data is __gshared, you are saying, "Hey, Mr. Compiler, I want to share this data across threads, but I don't want you to pay any attention to how I use it, mmkay?" Essentially, it's no different from a normal variable in C or C++. If you want to share a __gshared variable across threads, it's your responsibility to make sure it's properly synchronized. The compiler isn't going to help you.

So when implementing a dynamic binding, a decision has to be made: thread-local (default), shared, or __gshared. My answer is __gshared. If we pretend that our function pointers are actual functions, which are accessible across threads anyway, then there isn't too much to worry about. Care still need be taken to ensure that the functions are loaded before any other threads try to access them and that no threads try to access them after the bound library is unloaded. In Derelict, I do this with static module constructors and destructors (which can still lead to some issues during program shutdown, but I'll cover that in a separate post). Here's an example.

extern( C )
{
    alias void function(int) da_foo;
    alias int function() da_bar;
}

__gshared
{
    da_foo foo;
    da_bar bar;
}
Finally, there's the question of how to load the library. That, I'm afraid, is an exercise for the reader. In Derelict, I implemented a utility package (DerelictUtil) that abstracts the platform APIs for loading shared libraries and fetching their symbols. The abstraction is behind a set of free functions that can be used directly or via a convenient object interface. In Derelict itself, I use the latter since it makes managing loading an entire library easier. But in external projects, I often use the free-function interface for loading one or two functions at a time (such as certain Win32 functions that aren't available in the ancient libs shipped with DMD). It also supports selective loading, which is a term I use for being able to load a library if specific functions are missing (the default behavior is to throw an exception when an expected symbol fails to load).

Conclusion

Overall, there's a good deal of work involved in implementing any sort of binding in D. But I think it's obvious that dynamic bindings require quite some extra effort. This is especially true given that the automated tools I've seen so far are all geared toward generating static bindings. I've only recently begun to use custom scripts myself, but they still require a bit of manual preparation because I don't want to deal with a full-on C parser. That said, I prefer dynamic bindings myself. I like having the ability to load and unload at will and to have the opportunity to present my own error message to the user when a library is missing. Others disagree with me and prefer to use static bindings. That's perfectly fine.

At this point, static and dynamic bindings exist for several popular libraries already. Deimos is a collection of the former and Derelict 3 the latter. You'll find some bindings for the same library in both and several that are in one project but not the other. Use what you need and are comfortable with. And I hope that, if the need arises, you can use the advice I've laid out in this series of posts to help fill in the holes and develop static or dynamic bindings yourself.

Given that I'm just under 4 hours from 2013 as I write this in Seoul, Korea, I want to wish you a Happy New Year. May you start and finish multiple projects in the coming year!




Here's a third solution:

fooPtr = cast(typeof(fooPtr))getPtr();

and in bindFunc, "ref void* func" could be used instead of void**, that's a little nicer imho.

 

What would be really cool is if someone could come up with a compile-time only system which allows to write one binding and compile it to dynamic/static binding depending on a version statement:

 

//All done in ctfe
string genCode()
{
      Libarary lib = Library();

      lib.addLib("linux", "libGL.so");
      lib.addLib("linux", "libGL.so.1");

      lib.useStaticVersion("DerelictGLStatic");
      lib.aliasPrefix = "da_";
      lib.setBindFunc(gl_Bind); //preferably pass by alias, not a string constant

      lib.addFunction(GLEnum, "glGetError");
      lib.addFunction(void, "glGenVertexArrays", GLsizei, GLuint*);

      return lib.genCode();
}

mixin(genCode());

Here's a third solution:

fooPtr = cast(typeof(fooPtr))getPtr();
and in bindFunc, "ref void* func" could be used instead of void**, that's a little nicer imho.


I agree. Much nicer. There are still a number of features of D I've I haven't played around with yet, typeof being one of them.
 

What would be really cool is if someone could come up with a compile-time only system which allows to write one binding and compile it to dynamic/static binding depending on a version statement:

//All done in ctfe
string genCode()
{
      Libarary lib = Library();

      lib.addLib("linux", "libGL.so");
      lib.addLib("linux", "libGL.so.1");

      lib.useStaticVersion("DerelictGLStatic");
      lib.aliasPrefix = "da_";
      lib.setBindFunc(gl_Bind); //preferably pass by alias, not a string constant

      lib.addFunction(GLEnum, "glGetError");
      lib.addFunction(void, "glGenVertexArrays", GLsizei, GLuint*);

      return lib.genCode();
}

mixin(genCode());


I've seriously considered doing something like this for Derelict. But, to be honest, I'm not at all confident in the effect on compile time. In Derelict 2, compilation for the OpenGL binding was noticeably affected by the mixins we used to support both D1 and D2. For the smaller bindings, it wasn't as noticeable, but with all the packages together... ugh.

It would need lots of string manipulation / appending and I think that's known to be slow so it might indeed not be feasible yet.

 

I hope at some time CTFE will be backed by something like LLVM JIT and a proper GC.

Actually in many cases in CTFE a region allocator could already help a lot regarding memory usage:

enum int x = ctfeFun();

allocate everything in ctfeFun in a region, free region when x has been calculated. As ctfe functions can't have side effects that should be simple.

 

But there are probably more important issues right now than these performance optimizations.

 

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