How to call functions from a dylib using Crystal? - ffi

Question

I wish to call the various functions defined in a Rust dylib using Crystal. I've gone through the manual, but I was not able to understand it. How do I properly include and call this dylib? Am I using the CLI flags wrong?
Here's the Crystal code:
#[Link("libmy_dylib")]
lib MyDylib
# In C: double cos(double x)
# In Rust: print_number(x:i32)
fun print_number(value : Int32)
end
MyDylib.print_number(10)
I compiled a dylib using this repo. The code compiles fine to a libmy_dylib.dylib:
extern crate libc;
use libc::uint32_t;
#[no_mangle]
pub extern "C" fn hello() {
println!("Hello from Rust!");
}
#[no_mangle]
pub extern "C" fn print_number(x: i32) {
println!("x is: {}", x);
}
#[no_mangle]
pub extern "C" fn addition(a: uint32_t, b: uint32_t) -> uint32_t {
let c = a + b;
println!("Sum : {}", c);
return a + b;
}
#[allow(dead_code)]
pub extern "C" fn fix_linking_when_not_using_stdlib() {
panic!()
}

Answer 1

You need to specify the dylib with an absolute path, and pass it through ldflags. For instance, the following Rust file
extern crate libc;
#[no_mangle]
pub extern "C" fn hello() {
println!("Hello from Rust!")
}
that compiles to libmy_dylib.dylib can be linked like this:
#[Link(ldflags: "/absolute/path/to/libmy_dylib.dylib")]
lib MyDylib
fun hello : Void
end
MyDylib.hello
And the program will compile to print "Hello from Rust!"