An example of storing a Vec in a thread-local "scratchpad" static variable, to keep from repeatedly allocating and deallocating eat

thread_local_scratchpad.rs

use std::cell::UnsafeCell;

#[repr(C)]
struct Channel;

extern {
    fn foo(inputs: *mut *mut Channel);
}

fn call_foo(inputs: &mut [&mut [Channel]]) {
    thread_local! {
        static SCRATCHPAD: UnsafeCell<Vec<*mut Channel>> = UnsafeCell::new(Vec::new());
    }

    SCRATCHPAD.with(|scratchpad| {
        unsafe {
            // safe because we only access this thread local from this function
            // and there's no recursive calls inside of here AFAIK
            let scratchpad = &mut *scratchpad.get();
            
            // safe because no destructor will run on a raw pointer
            scratchpad.set_len(0);

            scratchpad.reserve(inputs.len());
            scratchpad.extend(inputs.iter_mut().map(|slice: &mut &mut [Channel]| slice.as_mut_ptr()));

            foo(scratchpad.as_mut_ptr());
        }
    });
}

fn main() {
    println!("Hello, world!");
}