Day49 - signalfd + epoll (Graceful Shutdown)

Overview

In this lab, we integrate Linux signalfd into the epoll-based TCP server.

This allows the server to:

• Handle signals (SIGINT / SIGTERM) as file descriptor events
• Avoid traditional signal handlers
• Perform graceful shutdown inside the main event loop

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Why signalfd?

Traditional signal handling:

signal(SIGINT, handler);

Problems:

• asynchronous execution context
• limited safe APIs
• race conditions
• difficult to integrate with event-driven design

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Core Concept

signal → file descriptor → epoll event

Instead of:

signal → interrupt → handler

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Key APIs

1. sigset_t

sigset_t mask;
sigemptyset(&mask);
sigaddset(&mask, SIGINT);
sigaddset(&mask, SIGTERM);

Defines which signals we want to handle.

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2. sigprocmask()

sigprocmask(SIG_BLOCK, &mask, NULL);

Block signals so they are not handled by default behavior.

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3. signalfd()

fd = signalfd(-1, &mask, SFD_NONBLOCK | SFD_CLOEXEC);

Creates a file descriptor that receives signal events.

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4. read()

read(signal_fd, &siginfo, sizeof(siginfo));

Reads signal events.

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Important Behavior

• Signals must be blocked (sigprocmask) before using signalfd
• signalfd becomes readable when a signal is pending
• read() must be called to clear the event
• otherwise epoll will continuously trigger

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Integration with epoll

We convert all event sources into epoll_item:

EPOLL_ITEM_SERVER
EPOLL_ITEM_CLIENT
EPOLL_ITEM_TIMER
EPOLL_ITEM_SIGNAL

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Event Flow

Ctrl+C (SIGINT)
    ↓
kernel queues signal
    ↓
signalfd becomes readable
    ↓
epoll_wait()
    ↓
read(signalfd)
    ↓
graceful shutdown

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Graceful Shutdown Design

if (siginfo.ssi_signo == SIGINT ||
    siginfo.ssi_signo == SIGTERM) {
    return 0; // exit event loop
}

Main loop exits:

server_run() → return
main() → server_cleanup()

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Resource Cleanup

All resources are released in order:

• client list
• epoll items
• timerfd
• signalfd
• epoll fd
• server socket

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Comparison with timerfd

Feature	timerfd	signalfd
Source	time	signal
Trigger	periodic	external event
read()	expiration count	signal info

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Summary

This lab completes the event-driven model:

• network I/O → socket
• timer → timerfd
• signal → signalfd

All unified under epoll.