1# Create a `build.rs` File 2 3We create a `build.rs` file in our crate's root. Cargo will pick up on the existence of this file, then compile and execute it before the rest of the crate is built. 4This can be used to generate code at compile time. 5And of course in our case, we will be generating Rust FFI 6bindings to `bzip2` at compile time. The resulting bindings will be written to 7`$OUT_DIR/bindings.rs` where `$OUT_DIR` is chosen by `cargo` and is something 8like `./target/debug/build/bindgen-tutorial-bzip2-sys-afc7747d7eafd720/out/`. 9 10Note that the associated shared object to `bz2` is `libbz2.so`. In general, a `lib<name>.so` should be referenced in the build file by `<name>`. 11 12```rust,ignore 13use std::env; 14use std::path::PathBuf; 15 16fn main() { 17 // Tell cargo to look for shared libraries in the specified directory 18 println!("cargo:rustc-link-search=/path/to/lib"); 19 20 // Tell cargo to tell rustc to link the system bzip2 21 // shared library. 22 println!("cargo:rustc-link-lib=bz2"); 23 24 // The bindgen::Builder is the main entry point 25 // to bindgen, and lets you build up options for 26 // the resulting bindings. 27 let bindings = bindgen::Builder::default() 28 // The input header we would like to generate 29 // bindings for. 30 .header("wrapper.h") 31 // Tell cargo to invalidate the built crate whenever any of the 32 // included header files changed. 33 .parse_callbacks(Box::new(bindgen::CargoCallbacks::new())) 34 // Finish the builder and generate the bindings. 35 .generate() 36 // Unwrap the Result and panic on failure. 37 .expect("Unable to generate bindings"); 38 39 // Write the bindings to the $OUT_DIR/bindings.rs file. 40 let out_path = PathBuf::from(env::var("OUT_DIR").unwrap()); 41 bindings 42 .write_to_file(out_path.join("bindings.rs")) 43 .expect("Couldn't write bindings!"); 44} 45``` 46 47Now, when we run `cargo build`, our bindings to `bzip2` are generated on the 48fly! 49 50[There's more info about `build.rs` files in the Cargo documentation.][build-rs] 51 52[build-rs]: https://doc.rust-lang.org/cargo/reference/build-scripts.html 53