embassy-socket 0.1.0

tcp client/server callback
Documentation 
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use core::cell::{Cell, UnsafeCell};
use core::mem::MaybeUninit;
use core::ptr::NonNull;

/// socket pool
pub struct Pool<T, const N: usize> {
    /// is this memory already used?
    used: [Cell<bool>; N],
    /// memory data
    data: [UnsafeCell<MaybeUninit<T>>; N],
}

/// custom method
impl<T, const N: usize> Pool<T, N> {
    const VALUE: Cell<bool> = Cell::new(false);
    const UNINIT: UnsafeCell<MaybeUninit<T>> = UnsafeCell::new(MaybeUninit::uninit());

    pub const fn new() -> Self {
        Self {
            used: [Self::VALUE; N],
            data: [Self::UNINIT; N],
        }
    }

    pub fn alloc(&self) -> Option<NonNull<T>> {
        for n in 0..N {
            // this can't race because Pool is not Sync.
            if !self.used[n].get() {
                self.used[n].set(true);
                let p = self.data[n].get() as *mut T;
                return Some(unsafe { NonNull::new_unchecked(p) });
            }
        }
        None
    }

    /// safety: p must be a pointer obtained from self.alloc that hasn't been freed yet.
    #[allow(unsafe_op_in_unsafe_fn)]
    pub unsafe fn free(&self, p: NonNull<T>) {
        let origin = self.data.as_ptr() as *mut T;
        let n = p.as_ptr().offset_from(origin);
        assert!(n >= 0);
        assert!((n as usize) < N);
        self.used[n as usize].set(false);
    }
}