# `cfg`

Configuration conditional checks are possible through two different operators:

- the `cfg` attribute: `#[cfg(...)]` in attribute position
- the `cfg!` macro: `cfg!(...)` in boolean expressions

While the former enables conditional compilation, the latter conditionally evaluates to `true` or `false` literals allowing for checks at run-time. Both utilize identical argument syntax.

    // This function only gets compiled if the target OS is linux
    #[cfg(target_os = "linux")]
    fn are_you_on_linux() {
        println!("You are running linux!");
    }

    // And this function only gets compiled if the target OS is *not* linux
    #[cfg(not(target_os = "linux"))]
    fn are_you_on_linux() {
        println!("You are *not* running linux!");
    }

    fn main() {
        are_you_on_linux();

        println!("Are you sure?");
        if cfg!(target_os = "linux") {
            println!("Yes. It's definitely linux!");
        } else {
            println!("Yes. It's definitely *not* linux!");
        }
    }

### See also:

the reference, `cfg!`, and macros.