Call a function pointer

[pron]

Is there a way to invoke a function pointer received from C code?

[headius]

Directly? Not really. But I think you have at least two options:

• Bind syscall directly. I have no idea the correct way to call it from there because I've never used it.
• Use jffi to invoke the function. jffi contains the low-level native bits that jnr-ffi uses for function calls. See Foreign, which has "invoke" endpoints, and look at code that uses them.

Do you think this is a feature we should lift up to jnr-ffi?

[pron]

Do you think this is a feature we should lift up to jnr-ffi?

Oh, absolutely! This is the scenario I'm facing: I'm trying to implement an SPI for a filesystem in Java. The Java method receives a C callback -- a visitor -- which it should then invoke for each file in a directory.

AFAIK, JNA has this feature, too.

[SerCeMan]

+1, very important feature. Also, function pointer and structures as closure parameters would be very useful.

[wks]

+1. I am developing a virtual machine which exposes a JNI-like interface, where the C native program receives a struct (like JNIEnv) and each API function is a function pointer in that struct. If a Java programmer wants to control a native component that exposes such an interface (imagine controlling another JVM from one JVM), it will be very common to call many functions via their function pointers.

The desired JNR interface should be similar to Closure, albeit it is Java calling C rather than C calling back to Java. The callee is a stateless C function rather than an object-bound Java method, so it should be easier to implement.

[headius]

I don't think we've made any progress on this, but it's probably still useful. Anyone want to bring this issue back to life and try to come up with a good API for this?

[gembin]

I think it's useful, How to implement this kind of callback?
What' the equivalent of com.sun.jna.Callback ?

https://curl.haxx.se/libcurl/c/curl_easy_setopt.html

CURLOPT_HEADERFUNCTION

Callback for writing received headers. See CURLOPT_HEADERFUNCTION

 setopt(CurlOption.CURLOPT_HEADERFUNCTION, headerHandler);
 setopt(CurlOption.CURLOPT_WRITEFUNCTION, contentHandler);

[headius]

It seems unlikely that you can't already do this in jnr-ffi via some combination of APIs. Are we simply missing a nice endpoint that says "give me a callable wrapper around this C function pointer?

[pron]

I think someone has made such an API here.

[atoulme]

I'm interested by this functionality as well. Is there a better way to accomplish this now? If not, I'll look at Foreign and jffi as @headius mentioned.