Day47 - Multi-message TX Queue & EPOLLOUT Pipeline

Key Concepts

• TCP is stream-based, not message-based.
• send() may send only partial data.
• Each pending TX message needs its own offset.
• TX queue stores multiple pending responses.
• EPOLLOUT should be enabled only when TX queue is not empty.

TX Queue Model

struct tx_msg {
    unsigned char *buf;
    size_t len;
    size_t offset;
};

struct tx_queue {
    struct tx_msg msgs[MAX_TXQ_MSG];
    size_t head;
    size_t tail;
    size_t count;
};

Flow

EPOLLIN:
    recv raw bytes
    generate response
    tx_queue_push()
    enable EPOLLOUT

EPOLLOUT:
    front message
    send remaining bytes
    update offset
    if message done:
        pop and free
    if queue empty:
        disable EPOLLOUT

Important Finding

Current server treats each recv() result as one application message. This is not protocol-correct because TCP may merge or split data.

Next step: RX buffer + protocol parser.