Unsafe External Functions

You can declare foreign functions for access from Rust with unsafe extern. This is unsafe because the compiler has to way to reason about their behavior. Functions declared in an extern block must be marked as safe or unsafe, depending on whether they have preconditions for safe use:

use std::ffi::c_char;

unsafe extern "C" {
    // `abs` doesn't deal with pointers and doesn't have any safety requirements.
    safe fn abs(input: i32) -> i32;

    /// # Safety
    ///
    /// `s` must be a pointer to a NUL-terminated C string which is valid and
    /// not modified for the duration of this function call.
    unsafe fn strlen(s: *const c_char) -> usize;
}

fn main() {
    println!("Absolute value of -3 according to C: {}", abs(-3));

    unsafe {
        // SAFETY: We pass a pointer to a C string literal which is valid for
        // the duration of the program.
        println!("String length: {}", strlen(c"String".as_ptr()));
    }
}