javarustjnaabi

how could a random rust ABI lib include functions which are not defined?

I wrote a simple rust file as

#[no_mangle]
pub extern "C" fn add(a: i32, b: i32) -> i32 {
    a + b
}

and tried to invoke it in Java. So I loaded it and defined 2 native methods:

    public interface CLibrary extends Library {
        CLibrary INSTANCE = Native.load("add", CLibrary.class);

        void printf(String format, Object... args);
        int add(int a, int b);
    }

Notice that I put printf here except add.

But it worked:

CLibrary.INSTANCE.printf("Hello, World!\n");
System.out.println(CLibrary.INSTANCE.add(1, 2));

These code output

3
Hello, World!

How could I can access printf which is not defined in my rust source?

Solution

  • Rust uses and links the C library to define much of std so the C standard library printf is available in builds that include std (which is every one that doesn't opt out).