One-shot TCP server

Challenge

Implement int single_echo_server(int port) that:

1. binds to 127.0.0.1:port,
2. listens with backlog 1,
3. accepts one client,
4. reads up to 256 bytes, writes them back to the client,
5. closes everything and returns 0.

The grader connects a client thread to port and asserts the round-trip.

Starter code

#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>

int single_echo_server(int port) {
    /* TODO */
    return -1;
}