OS: POSIX Sockets — TCP Server/Client, epoll, UDP, TCP_NODELAY, TIME_WAIT

// POSIX Sockets — TCP client/server in C++

// TCP Server (minimal):
int fd = socket(AF_INET, SOCK_STREAM, 0); // create socket
setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));
struct sockaddr_in addr{};
addr.sin_family = AF_INET;
addr.sin_port = htons(8080); // host-to-network-short (endian)
addr.sin_addr.s_addr = INADDR_ANY;
bind(fd, (sockaddr*)&addr, sizeof(addr));
listen(fd, 128);                           // 128 = backlog (max pending connections)
int client = accept(fd, nullptr, nullptr); // blocks until connection
write(client, "hello", 5);
close(client);
close(fd);

// TCP Client:
int fd = socket(AF_INET, SOCK_STREAM, 0);
struct sockaddr_in srv{};
srv.sin_family = AF_INET;
srv.sin_port = htons(8080);
inet_pton(AF_INET, "127.0.0.1", &srv.sin_addr);
connect(fd, (sockaddr*)&srv, sizeof(srv));
char buf[256];
read(fd, buf, sizeof(buf)); // receive
close(fd);

// Non-blocking I/O with select / epoll:
// select: O(n) per call; limited to FD_SETSIZE (1024) fds
// poll:   O(n) per call; no fd limit
// epoll:  O(1) per event; Linux only; production standard
int epfd = epoll_create1(0);
struct epoll_event ev;
ev.events = EPOLLIN;
ev.data.fd = fd;
epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev);
struct epoll_event events[64];
int n = epoll_wait(epfd, events, 64, -1); // block until events
for (int i = 0; i < n; i++) {
    if (events[i].data.fd == listen_fd) { /* new connection */
    } else {                              /* data to read on events[i].data.fd */
    }
}

// UDP (connectionless):
int fd = socket(AF_INET, SOCK_DGRAM, 0);
// no connect/listen/accept — just sendto/recvfrom
sendto(fd, data, len, 0, (sockaddr*)&dst, sizeof(dst));
recvfrom(fd, buf, sizeof(buf), 0, (sockaddr*)&src, &srclen);

// Key options:
// SO_REUSEADDR: reuse port after crash (avoids TIME_WAIT)
// TCP_NODELAY: disable Nagle algorithm (for low-latency)
// SO_RCVBUF / SO_SNDBUF: tune kernel buffer sizes
// O_NONBLOCK: non-blocking I/O (use with epoll)