Day56 - POSIX Shared Memory IPC

Objective

This lab demonstrates Linux POSIX shared memory IPC architecture.

Topics covered:

• shm_open()
• ftruncate()
• mmap()
• independent shared memory processes
• process-shared pthread mutex
• eventfd notification
• AF_UNIX control socket
• SCM_RIGHTS fd passing
• epoll-driven IPC

---

Directory Structure

d56-posix-shm/
├── fd_passing.h
├── epoll_helper.h
├── unix_socket_helper.h
├── shm_common.h
├── shm_writer.c
├── shm_reader.c
└── Makefile

---

Build

make

or:

gcc -Wall -Wextra -pthread -o shm_writer shm_writer.c
gcc -Wall -Wextra -pthread -o shm_reader shm_reader.c

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Shared Memory Structure

struct shm_data {
    pthread_mutex_t lock;
    bool activation;
    int counter;
    char message[SHM_MSG_SIZE];
};

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Writer Responsibilities

The writer process owns:

• shared memory lifecycle
• mutex initialization
• eventfd creation
• control socket server

Writer flow:

shm_open()
ftruncate()
mmap()
pthread_mutex_init()
eventfd()
UNIX socket listen
accept reader
send eventfd
event loop

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Reader Responsibilities

The reader process:

• connects to writer
• receives eventfd
• attaches shared memory
• waits for event notification

Reader flow:

connect()
recvmsg(SCM_RIGHTS)
shm_open()
mmap()
epoll_wait()

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Step 1 - Start Writer

Terminal A:

./shm_writer

Expected output:

[INFO] shm_open success
[INFO] shared memory mapped
[INFO] socket created
[INFO] socket listen success

---

Step 2 - Start Reader

Terminal B:

./shm_reader

Expected output:

[INFO] connected to writer
[INFO] received eventfd
[INFO] epoll wait start

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Step 3 - Shared Memory Update

Writer command:

-w hello shared memory

Expected reader output:

[INFO] shm_data:
counter=2
message=hello shared memory

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Step 4 - Multiple Updates

Writer:

-w first
-w second
-w third

Expected:

• reader wakes on each eventfd notification
• counter increments correctly
• message updates correctly

---

Step 5 - Shared Memory Display

Writer:

-s

Expected:

counter=...
message=...

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Step 6 - Graceful Shutdown

Writer:

-t

Writer behavior:

activation = false
eventfd notify
cleanup

Reader behavior:

wake up
detect activation == false
exit cleanly

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Step 7 - Verify Cleanup

Verify shared memory removal:

ls -l /dev/shm

Verify socket removal:

ls -l /tmp/day56_shm_ctrl.sock

Objects should no longer exist after shutdown.

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Key Learning Points

Shared Memory

• shared pages across processes
• explicit synchronization required

---

pthread Mutex

• process-shared synchronization
• mutex object stored in shared memory

---

eventfd

• lightweight event notification
• epoll integration

---

UNIX Domain Socket

• control channel IPC
• fd passing

---

Important Notes

Reader mmap Permission

Even reader-only processes require:

PROT_READ | PROT_WRITE

because pthread mutex modifies internal state.

---

eventfd Semantics

eventfd is:

notification primitive

not:

message protocol

Shared memory stores the actual state/data.

---

eventfd Notification Order

Correct order:

lock shm
update data
unlock shm
write(eventfd)

---

Suggested Experiments

• remove mutex protection
• add multiple readers
• convert writer to full epoll loop
• use SOCK_SEQPACKET instead of SOCK_STREAM
• switch to abstract UNIX socket
• add shared ring buffer