Day53 - UNIX Domain Socket IPC

Objective

Build a local IPC middleware daemon using:

• AF_UNIX socket
• epoll
• multi-client dispatcher
• /dev/mypoll
• kernel event broadcast

---

Environment

Platform

• Raspberry Pi 5
• Debian Linux
• UNIX domain socket

---

Directory Structure

day53-unix-ipc/
├── Makefile
├── server.c
├── client.c
├── log.h
└── mypoll_timer.c

---

Build Userspace Applications

Native build

make

Cross compile for Raspberry Pi 5

make CROSS=1

---

Start Server

./server

Expected:

[INFO] Server listening on /tmp/mypoll.sock

---

Connect Using Client

./client client-1

Expected:

[INFO] Connected to /tmp/mypoll.sock

---

Connect Using netcat

nc -U /tmp/mypoll.sock

Test commands:

echo hello
trigger
start_timer
stop_timer

---

Multi-Client Test

Open multiple clients

for i in 1 2 3 4 5; do
    ./client client-$i &
done

wait

Expected:

• multiple client connections
• independent event handling
• no dispatcher blocking

---

Broadcast Test

Terminal 1

nc -U /tmp/mypoll.sock

Terminal 2

nc -U /tmp/mypoll.sock

Terminal 3

echo "trigger" | nc -U /tmp/mypoll.sock

Expected:

• all connected clients receive event

---

Backpressure Test

Modify client to continuously send commands:

while (1) {
    client_send_message(...);
}

Expected:

• TX queue full
• EPOLLIN disabled temporarily
• dispatcher remains alive
• no server crash

---

Verify /dev/mypoll Integration

Start timer

echo "start_timer" | nc -U /tmp/mypoll.sock

Expected:

OK start timer

Then periodic events should appear.

---

Event Flow Verification

Expected architecture:

client command
    ↓
server parser
    ↓
write(/dev/mypoll)
    ↓
kernel event
    ↓
epoll dispatcher
    ↓
broadcast
    ↓
all clients

---

Cleanup

Remove socket file:

rm -f /tmp/mypoll.sock