This post is a duplicate of the Github Issue here.
dart --version
Dart SDK version: 2.15.0-116.0.dev (dev) (Thu Sep 16 09:47:01 2021 -0700) on "linux_x64"
I've been looking up examples for callbacks and I have tried to get callbacks working for me in FFI.
My current situation
I have a function in my library which expects a pointer to a function. The bindings for the same generated by ffigen seem correct to me.
int SetCallback(
CallbackType callback,
) {
return _SetCallback(
callback,
);
}
late final _SetCallbackPtr =
_lookup<NativeFunction<Int32 Function(CallbackType)>>(
'SetCallback');
late final _SetCallback =
_SetCallbackPtr.asFunction<int Function(CallbackType)>();
where, typedef CallbackType = Pointer<NativeFunction<Void Function(Uint32)>>;.
What I want to do here is to setup this callback in Dart, pass it to the FFI, essentially using it as my callback as I would have in C. In my API which abstracts away from FFI code (which means I have a class MyLibrary full of static functions that the user will call directly, which in turn calls functions from an object _nativeLibrary of the class MyNativeLibrary I have created), I have:
static int SetCallback({required CallbackFuncDart callback}) {
Pointer<NativeFunction<CallbackFunc>> pointer = Pointer.fromFunction(callback);
int status = _nativeLibrary.SetCallback(
pointer,
);
if (STATUS_OK != status) {
throw LibLexemeException(status);
}
return status;
}
typedef CallbackFunc = Void Function(Uint32);
typedef CallbackFuncDart = void Function(int);
While the sqlite ffi example states here that
Features which dart:ffi does not support yet:
Callbacks from C back into Dart.
I believe the docs haven't been updated to reflect the changes at the samples here. The samples haven't been very clear due to them not having any C/C++ files, or an idea of how the C functions work. Even so, I think this example contains a segment(last code block) where a Dart function is being passed as a callback which I have replicated in my program. It is not clear to me how this will work but upon trying to compile my program I get:
ERROR: ../lib/library_lexeme.dart:180:74: Error: fromFunction expects a static function as parameter. dart:ffi only supports calling static Dart functions from native code. Closures and tear-offs are not supported because they can capture context.
ERROR: Pointer<NativeFunction<CallbackFunc>> pointer = Pointer.fromFunction(callback);
The short version is that you can't pass your callnback as an argument:
static int SetCallback({required CallbackFuncDart callback}) {
Pointer<NativeFunction<CallbackFunc>> pointer = Pointer.fromFunction(callback); // <-- this isn't considered a static function
It's quite annoying but you must use a static function defined ahead of time for your dart callbacks to be called from C.
Apparently for now only static functions can be passed via ffi. But if you have to access an instance's data and you're sure that the instance exists you can use my workaround. I use a static list to the instances. This is stupid and ugly but it works for me:
class CallbackClass {
static Int8 classCallback(int id) {
final instance = instanceList[id];
return instance.instanceCallback();
}
Int8 instanceCallback() { return instanceId; }
static List<CallbackClass> instanceList = <CallbackClass>[];
late final int instanceId;
CallbackClass {
instanceId = instanceList.length;
instanceList.insert(instanceId, this);
myFFImapping.passCallback(instanceId, Pointer.fromFunction<>(classCallback);)
}
}
I omitted the necessary c code, FFI mapping and casting to correct types for clarity, so it obviously won't compile like this.
Related
I have a class where I want to call a static method to initialize some things. I tried calling it from the global scope, like in this simplified example:
class MyClass{
static bool initialized = false;
static void init(){
initialized = true;
}
}
MyClass.init();
void main() async {
// Do something useful
}
This results in the following errors:
Functions must have an explicit list of parameters
The name 'MyClass' is already defined
What I find a bit odd is that if I make the init() method return something and assign that result to a variable, it works:
class MyClass{
static bool initialized = false;
static bool init(){
initialized = true;
return true;
}
}
bool _dummy = MyClass.init();
void main() async {
// Do something useful
}
Why is this? And is there a better workaround to call a static void method from the global scope?
The reason is that static variables are lazy evaluated in Dart, so they will first get a value the first time they are accessed.
That means that MyClass.init() in:
bool _dummy = MyClass.init();
Is only being executed when something are trying to access the variable _dummy. It also means that the following in global scope:
MyClass.init();
Does not make any sense in Dart since this code are never going to be executed since there are no reference to the code.
The reason for this design can be found described in the Dart Language Specification:
Static variable declarations with an initializing expression are initialized lazily.
The lazy semantics are given because we do not want a language where one tends to define expensive initialization computations, causing long application startup times. This is especially crucial for Dart, which must support the coding of client applications.
https://dart.dev/guides/language/specifications/DartLangSpec-v2.10.pdf
I have a file fancy_button.dart for a custom Flutter widget FancyButton which is like:
class FancyButton extends StatefulWidget {
// ...
}
class _FancyButtonState extends State<FancyButton> {
// ...
}
// Declaration outside any class:
Map<_FancyButtonState, Color> _buttonColors = {};
final _random = Random();
int next(int min, int max) => min + _random.nextInt(max - min);
// ...
The application works just fine. Notice that I declare and use some variables outside any class. Now my question is: how is it even possible? Shouldn't everything be inside a class in Dart, like Java?
No, Dart supports variables and functions defined in global space. You can see this with the main() method which are declared outside any class.
Also, global variables (and static class variables) are lazy evaluated so the value are first defined when you are trying to use them. So your runtime are not going to slow down even if there are a bunch of global variables there are not used.
Are you coming from Java before touching Dart?
Basically, Dart is not single-class-single-file like how Java works. Yes, it does support Object Oriented Programming (in kinda different way). The behavior of constructor is different. There is no public, private, and protected keywords. Please just refer to the official docs.
Anyway, you don't need a complex public static void main(). The real entry point is main(). Unless you define that function, you won't be able to run a file in command line.
I have a class in c++ that I'm wrapping into python with pybind11. That class has a std::function, and I'd like to control how the arguments to that function are dealt with (like return value policies). I just can't find the syntax or examples to do this...
class N {
public:
using CallbackType = std::function<void(const OtherClass*)>;
N(CallbackType callback): callback(callback) { }
CallbackType callback;
void doit() {
OtherClass * o = new OtherClass();
callback(o);
}
}
wrapped with
py::class_<OtherClass>(...standard stuff...);
py::class_<N>(m, "N")
.def(py::init<N::CallbackType>(),
py::arg("callback"));
I all works: I can do this in python:
def callback(o):
dosomethingwith(o)
k = N(callback)
, but I'd like to be able to control what happens when callback(o); is called - whether python then will take ownership of the wrapped o variable or not, basically.
I put a printout in the destructor of OtherClass, and as far as I can tell, it never gets called.
OK, I think I figured it out:
Instead of std::function, use a pybind11::function:
using CallbackType = pybind11::function
and then
void doit(const OtherClass &input) {
if (<I want to copy it>) {
callback(pybind11::cast(input, pybind11::return_value_policy::copy));
} else {
callback(pybind11::cast(input, pybind11::return_value_policy::reference));
}
}
I see nothing in pybind11/functional that allows you to change the ownership of the parameters at the point of call, as the struct func_wrapper used is function local, so can not be specialized. You could provide another wrapper yourself, but in the code you can't know whether the callback is a Python function or a bound C++ function (well, technically you can if that bound C++ function is bound by pybind11, but you can't know in general). If the function is C++, then changing Python ownership in the wrapper would be the wrong thing to do, as the temporary proxy may destroy the object even as its payload is stored by the C++ callback.
Do you have control over the implementation of class N? The reason is that by using std::shared_ptr all your ownership problems will automagically evaporate, regardless of whether the callback function is C++ or Python and whether it stores the argument or not. Would work like so, expanding on your example above:
#include <pybind11/pybind11.h>
#include <pybind11/functional.h>
namespace py = pybind11;
class OtherClass {};
class N {
public:
using CallbackType = std::function<void(const std::shared_ptr<OtherClass>&)>;
N(CallbackType callback): callback(callback) { }
CallbackType callback;
void doit() {
auto o = std::make_shared<OtherClass>();
callback(o);
}
};
PYBIND11_MODULE(example, m) {
py::class_<OtherClass, std::shared_ptr<OtherClass>>(m, "OtherClass");
py::class_<N>(m, "N")
.def(py::init<N::CallbackType>(), py::arg("callback"))
.def("doit", &N::doit);
}
In a C++/CLI assembly, I'm trying to call a managed delegate from a native callback. I followed Doc Brown's answer here, and my implementation so far looks like this:
The native callback - ignore the commented out parts for now:
static ssize_t idaapi idb_callback(void* user_data, int notification_code, va_list va)
{
switch (notification_code)
{
case idb_event::byte_patched:
{
//ea_t address = va_arg(va, ea_t);
//uint32 old_value = va_arg(va, uint32);
return IdaEvents::BytePatched(0, 0);
}
break;
}
return 0;
}
As you can see above, I call this managed delegate instantiated in a static class:
public delegate int DatabaseBytePatchedHandler(int address, int originalValue);
private ref class IdaEvents
{
static IdaEvents()
{
BytePatched = gcnew DatabaseBytePatchedHandler(&OnDatabaseBytePatched);
}
public: static DatabaseBytePatchedHandler^ BytePatched;
private: static int OnDatabaseBytePatched(int address, int originalValue)
{
return 0;
}
};
This compiles fine. But the code is incomplete - remember the commented out part in the native callback above? I actually have to retrieve the values from the va_list passed to the callback, and pass those on to my managed delegate:
ea_t address = va_arg(va, ea_t);
uint32 old_value = va_arg(va, uint32);
return IdaEvents::BytePatched(address, old_value);
But as soon as I uncomment one of the lines using va_arg, I cannot compile the project anymore and retrieve the following errors marking the line where I call the managed delegate:
C3821 'IdaEvents': managed type or function cannot be used in an unmanaged function
C3821 'IdaEvents::BytePatched': managed type or function cannot be used in an unmanaged function
C3821 'BytePatched': managed type or function cannot be used in an unmanaged function
C3821 'DatabaseBytePatchedHandler::Invoke': managed type or function cannot be used in an unmanaged function
C3642 'int DatabaseBytePatchedHandler::Invoke(int,int)': cannot call a function with __clrcall calling convention from native code
C3175 'DatabaseBytePatchedHandler::Invoke': cannot call a method of a managed type from unmanaged function 'idb_callback'
This really confuses me. Why is the compiler suddenly acting up as soon as I try to use va_arg? Even a single line without any assignment causes this error to pop up.
Am I thinking too naive here? I'm obviously missing a piece of the puzzle, and any help supporting me in finding it is greatly appreciated.
I've just started making libraries in Arduino. I've made a library named inSerialCmd. I want to call a function named delegate() that is defined in the main program file, stackedcontrol.ino, after the inSerialCmd library is included.
When I try to compile, one error is thrown:
...\Arduino\libraries\inSerialCmd\inSerialCmd.cpp: In member function
'void inSerialCmd::serialListen()':
...\Arduino\libraries\inSerialCmd\inSerialCmd.cpp:32: error:
'delegate' has not been declared
After doing a bit of searching, it seemed that adding the scope resolution operator might do the trick. So I added the "::" before delegate(), now "::delegate()", but the same error is thrown.
Now I'm stumped.
You cannot and should not directly call a function in a program from a library. Keep in mind a key aspect that makes a library into a library:
A library does not depend on the specific application. A library can be fully compiled and packaged into the .a file without the existence of a program.
So there is a one way dependency, a program depends on a library. This at first glance may seem to prevent you from achieving what you want. You can achieve the functionality you are asking about through what is sometimes referred to as a callback. The main program would provide to the library at runtime a pointer to the function to execute.
// in program somwehere
int myDelegate(int a, int b);
// you set this to the library
setDelegate( myDelegate );
You see this in the arduino if you look at how interrupt handlers are installed. This same concept exists in many environments - event listeners, action adapters - all with the same goal of allowing a program to define the specific action that a library cannot know.
The library would store and call the function via the function pointer. Here is a rough sketch of what this looks like:
// in the main program
int someAction(int t1, int t2) {
return 1;
}
/* in library
this is the delegate function pointer
a function that takes two int's and returns an int */
int (*fpAction)(int, int) = 0;
/* in library
this is how an application registers its action */
void setDelegate( int (*fp)(int,int) ) {
fpAction = fp;
}
/* in libary
this is how the library can safely execute the action */
int doAction(int t1, int t2) {
int r;
if( 0 != fpAction ) {
r = (*fpAction)(t1,t2);
}
else {
// some error or default action here
r = 0;
}
return r;
}
/* in program
The main program installs its delegate, likely in setup() */
void setup () {
...
setDelegate(someAction);
...