# Mailbox

**[Mailbox](https://en.wikipedia.org/wiki/Message_passing)** is a bounded, addressed message queue: one process sends into it, another consumes from it in FIFO order. Unlike [[sync-csp|CSP]] channels (anonymous rendezvous) or [[sync-linda|Linda]] (addressed by content), mailboxes are owned by a specific recipient and provide natural backpressure when full.

Mailboxes are standard in RTOSes (FreeRTOS, Zephyr) and language-level communication in Erlang/Elixir.

## Example

This example shows mailbox send/receive with backpressure:

```c
// compile: (pseudocode for typical RTOS API)
// description: bounded mailbox prevents unbounded queue growth

mbox_t inbox;
mbox_create(&inbox, 16);  // bounded to 16 messages

void producer(void) {
    struct msg m = {42};
    mbox_send(&inbox, &m);  // blocks if mailbox full (backpressure)
}

void consumer(void) {
    struct msg m;
    mbox_recv(&inbox, &m);  // blocks if mailbox empty
}
```