Day45 - Learning Log

📌 Summary

Implemented a TCP server using:

• blocking socket
• poll
• epoll
• non-blocking I/O

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📌 Key Learnings

1. poll vs epoll

• poll scans all fds
• epoll only returns ready fds

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2. epoll is NOT non-blocking

• epoll = event notification
• non-blocking = fd behavior

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3. accept must drain queue

while (accept()):
    handle client

Otherwise connections may be missed.

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4. recv() behavior

• n > 0 → data
• n == 0 → client closed
• n < 0 → error

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5. EAGAIN is not an error

EAGAIN → no more data now

Used to terminate read loop.

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6. send() is not guaranteed to send all

• partial write possible
• may return EAGAIN

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7. Non-blocking design

• never block inside event loop
• always return control to loop

8. TX queue design insight

TX queue is:

buffer + offset

NOT:

FIFO packet queue

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9. EPOLLOUT must be controlled

Wrong:

always enable EPOLLOUT
→ busy loop

Correct:

enable only when needed

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📌 Observations

• epoll handles burst connections efficiently
• connection lifecycle becomes clear with logs
• testing tools (nc) behavior affects results

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📌 Next Improvements

• TX queue + EPOLLOUT
• integrate input subsystem (Day44)
• support protocol parsing