How can I call a variadic C function with a list of arguments pulled from a rust Vec?

Question

I am trying to call the JNI function CallStaticVoidMethod from rust. The jni-sys wrapper declares it as

pub CallStaticVoidMethod:
    Option<unsafe extern "C" fn(env: *mut JNIEnv, cls: jclass, methodID: jmethodID, ...)>,

I would like to wrap this call in something safer, so I have created this method on my wrapper object:

pub fn call_static_void_method(&mut self, cls: jclass, method: jmethod, args: & Vec<jobject>)
{
    let csvm;
    unsafe {
        csvm = (**self.env_ptr).CallStaticVoidMethod.expect("no implementation of CallStaticVoidMethod");
    }
    unsafe {
        return csvm(self.env_ptr, cls, method, /*what here?*/);
    }
}

How can I convert args:Vec<jobject> so that I can provide the final arguments to the CallStaticVoidMethod JNI function?

harmic · Accepted Answer · 2019-06-21T01:08:01.810

You can call variadic C functions from rust in the normal way, however calling them with an argument list that is only known at runtime is another matter. From what I understand, it is not even really possible in C (at least not in a standards compliant manner).

Luckily, though, it looks as though you don't need to do that. The JNI provides an alternative method, CallStaticVoidMethodA, which is defined as:

NativeType CallStaticVoidMethodA(JNIEnv *env, jclass clazz, jmethodID methodID, jvalue *args);

where args points to an array containing the arguments, in the form of jvalue.

You are receiving a Vec<jobject>; a jvalue is a union of various kinds of value, so you will need to convert your Vec<jobject> into an array of jvalues. Alternatively it would be more flexible if your function could accept Vec<jvalue> as you would be able to call methods that accept various argument types - in that case you could just pass args.as_ptr() like so:

pub fn call_static_void_method(&mut self, cls: jclass, method: jmethod, args: & Vec<jvalue>)
{
    let csvm;
    unsafe {
        csvm = (**self.env_ptr).CallStaticVoidMethodA.expect("no implementation of CallStaticVoidMethodA");
    }
    unsafe {
        return csvm(self.env_ptr, cls, method, args.as_ptr());
    }
}

There are variations on this theme for all of the JNI functions for calling methods.

How can I call a variadic C function with a list of arguments pulled from a rust Vec?

1 Answers1