I am not sure I'd call what Graal is doing "interoperability", because you essentially pull everything into that VM, instead of interoperating with it where it is.
So they "interoperate" with C by making a C compiler that creates a Graal-compatible AST, and as far as I can tell, the only way of interoperating at this point is creating a new Graal compiler.
See: http://www.graalvm.org/docs/reference-manual/languages/llvm/ for example. Basically you generate bitcode and can run the bitcode on graal. However you can still load native libraries into Graal and access them, you can't probably access them directly in java without a wrapper (I'm not sure about that yet).
So basically you compile your C/C++/Rust Code to BitCode instead of Object Files/Binaries and run that BitCode on Graal.
I.e. with java:
Context polyglot = Context.newBuilder() .allowAllAccess(true) .option("llvm.libraries", "/usr/lib/libcurl.dylib") .build();
Source source = Source.newBuilder("llvm", new File("hello.bc")).build();
Value cpart = polyglot.eval(source);
cpart.getMember("main").execute();
int main() { CURL *curl = curl_easy_init(); if(curl) { CURLcode res; curl_easy_setopt(curl, CURLOPT_URL, "http://example.com"); res = curl_easy_perform(curl); curl_easy_cleanup(curl); }
printf("Hello from GraalVM!\n");
return 0;
}