How to Configure cdylib in Cargo#

• To configure your Rust library to compile as a cdylib, add the crate-type array to your Cargo.toml file under the [lib] section:

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

[lib]
# "cdylib" produces the C-compatible dynamic library.
# "rlib" is optionally included so you can still use it as a normal Rust dependency.
crate-type = ["cdylib", "rlib"]

Basic Implementation Example#

• When writing code for a cdylib, you generally expose functions using the C calling convention (extern "C") and prevent Rust from mangling the function names (#[no_mangle]):

• src/lib.rs :

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

Building the Project#

• Compile the library using the standard Cargo command:

cargo build --release

• After compilation, your compiled shared library file will be available in the target directory:
  • • target/release/libmy_ffi_library.so (Linux)
  • • target/release/libmy_ffi_library.dylib (macOS)
  • • target/release/my_ffi_library.dll (Windows)

cdylib vs dylib#

Advanced: Passing Linker Arguments#

• If your dynamic library requires custom linker flags (such as setting an explicit soname on Linux or managing symbols), you can target cdylib targets specifically inside a build.rs script using the cargo::rustc-link-arg-cdylib instruction:
  • https://doc.rust-lang.org/cargo/reference/build-scripts.html

// build.rs
fn main() {
    // This flag is only applied when building the cdylib artifact
    println!("cargo::rustc-link-arg-cdylib=-Wl,-rpath,@loader_path");
}

• If you are trying to implement something specific, let me know:
  • • What language or tool will be consuming this library?
  • • Do you need assistance with complex data types (like strings or structs) across the FFI boundary?