Wrapping srand(3) and rand(3)

use libc::{rand, srand};

// unsafe extern "C" {
//     /// Seed the rng
//     fn srand(seed: std::ffi::c_uint);

//     fn rand() -> std::ffi::c_int;
// }

fn main() {
    unsafe { srand(12345) };

    let a = unsafe { rand() as i32 };
    println!("{a:?}");
}
This slide should take about 15 minutes.

This slide attempts to demonstrate that it is very easy for wrappers to trigger undefined behavior if they are written incorrectly. We’ll see how easy it is to trigger type safety problems.

Explain that rand and srand functions are provided by the C standard library (libc).

Explain that the functions are exported by the libc crate, but we can also write an FFI wrapper for them manually.

Show calling the functions from the exported.

Code compiles because libc is linked to Rust programs by default.

Explain that Rust will trust you if you use the wrong type(s).

Modify fn rand() -> std::ffi::c_int; to return char.

Avoiding type safety issues is a reason for using tools for generating wrappers, rather than doing it by hand.