Platform Invoke F# callback functions

问题:

I am using F# on a Raspberry Pi 2 (ARM 7 & thus mono). I am currently trying to use the WiringPi library, written in C. I have successfully managed to use some of the functions using P/Invoke.

Now I am trying to use interrupts (see http://wiringpi.com/reference/priority-interrupts-and-threads/) but I am stumped by this function with the following C signature

int wiringPiISR (int pin, int edgeType,  void (*function)(void));

Which has been translated (see https://github.com/danriches/WiringPi.Net/blob/master/WiringPi/WrapperClass.cs) by Daniel Riches into a C# library like this

//This is the C# equivelant to "void (*function)(void))" required by wiringPi to define a callback method 
public delegate void ISRCallback(); 

[DllImport("libwiringPi.so", EntryPoint = "wiringPiISR")] 
public static extern int wiringPiISR(int pin, int mode, ISRCallback method); 

How on earth would I do this in F#? I guess DllImport line looks like this ("method" is reserved in F#)

[<DllImport("libwiringPi.so", EntryPoint = "wiringPiISR")>] 
  extern int wiringPiISR(int pin, int mode, ISRCallback callBack);

What does the type definition for ISRCallback look like?

Note: this is not just "a" function pointer but a void one, with void arguments.

回答1:

The delegate definition would look something like this:

type ISRCallback = delegate of unit -> unit

And the platform invoke signature would look like this:

[<DllImport("libwiringPi.so", EntryPoint = "wiringPiISR")>] 
extern int wiringPiISR(int pin, int mode, [<MarshalAs(UnmanagedType.FunctionPtr)>]ISRCallback callBack);

An example usage of the function:

let callback : ISRCallback = ISRCallback(fun () -> (*do something interesting here*) ())
let result = wiringPiISR(1, 1, callback)

You should keep in mind that delegates in .NET are subject to garbage collection. It is the responsibility of the developer to ensure that the delegate doesn't "go out of scope" until your native library no longer needs the function callback.