Dynamic library

The embedding example is rather contrived, we rarely need to expose a function from an application that embeds Julia. It's much more likely that some Rust crate implements useful functionality that we want to expose to Julia.

Julia can load dynamic libraries at runtime.¹ Let's create a new dynamic library that exposes the add function we've defined above. We start by creating a new crate.²

cargo new julia_lib --lib

We need to change the crate type to cdylib, this can be configured in Cargo.toml:

[package]
name = "julia_lib"
version = "0.1.0"
edition = "2021"

[profile.release]
panic = "abort"

[profile.dev]
panic = "abort"

[lib]
crate-type = ["cdylib"]

[dependencies]

We don't need to add jlrs as a dependency, we'll discuss the advantages and disadvantages of using jlrs with dynamic libraries in the next chapter.

Replace the content of lib.rs with the following code:

#[no_mangle]
pub unsafe extern "C" fn add(a: f64, b: f64) -> f64 {
    a + b
}

The function is annotated with #[no_mangle] to prevent the name from being mangled. After building with cargo build we can find the library in target/debug. On Linux it will be named libjulia_lib.so, on macOS libjulia_lib.dylib, and on Windows libjulia_lib.dll. Let's use it!

Open the Julia REPL in julia_lib's root directory and evaluate the following code:

julia> using Libdl

julia> handle = dlopen("./target/debug/libjulia_lib")
Ptr{Nothing} @0x0000000000e2a620

julia> func = dlsym(handle, "add")
Ptr{Nothing} @0x000073b0dec02100

julia> ccall(func, Float64, (Float64, Float64), 1.0, 2.0)
3.0

Note that we don't have to provide the extension when opening the library.

If the library is on the library search path we don't even need to open it or acquire function pointers, but can refer to it directly:

julia> ccall((:add, "libjulia_lib"), Float64, (Float64, Float64), 1.0, 2.0)
3.0