Slides: Sysprog: POSIX IPC

  • Unix IPC: mechanisms to communicate between unrelated* processes

    • Semaphores

    • Shared memory

    • Message queues

    • No sockets

History: Two IPC Variants

  • System V IPC

    • Cumbersome, unnecessarily complex API

    • Older -> more portable between Unixen

  • POSIX IPC

    • Easy to use

    • Much of it implemented in userspace (through memory mapped files)

    • Optional feature in POSIX (fully supported in Linux though)

Object Names

  • Communication between unrelated processes

    • Related: none is a descendant of the other (fork())

    • Cannot inherit object

    • Must be able to locate them

  • System-wide visibility via names

    • Just like files

    • In fact, most IPC mechanisms are files (at least in Linux)

    • Consistently with leading slash: /some-object-name

File Semantics

  • Objects look just like files. For example,

    • shm_open(), using the same flags as good ol’ open()

    • shm_unlink() to remove a shared memory segment, just like good ol’ unlink() removes a file.

  • Handles are reference counted

    • Shared memory: int - a true file descriptor

    • Message queue: mqd_t (typedef int)

    • Incremented across fork(), dup() etc.

Semaphores

Communication and synchronization device

  • Bag of N elements

  • N items can be consumed without waiting

  • (N+1)st consumer has to wait until an item is goiven back

Creation parameter

  • Initial value N

man -s 7 sem_overview

Open/Create: sem_open()

sem_t *sem_open(const char *name, int oflag);
sem_t *sem_open(const char *name, int oflag,
                mode_t mode, unsigned int value);

  • Again: file semantics

  • Like open(), to calling “signatures”

    • create

    • open

  • value: initial value N (creation only)

man -s 3 sem_open

Communication: sem_wait(), sem_post()

int sem_wait(sem_t *sem);
int sem_trywait(sem_t *sem);
int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout);
int sem_post(sem_t *sem);

  • wait: consume element; blocks if count is zero

  • post: give element back

man -s 3 sem_wait

man -s 3 sem_post

Semaphores are Files

$ ls -l /dev/shm/
total 1604
-rw------- ... sem.my-semaphore

  • /dev/shm is a tmpfs instance

  • sem.my-semaphore is a regulare file in it

    • Contains a flat structure, the semaphore

  • sem_t encapsulates open file descriptor and void* (the mapped memory)

Shared Memory (1)

int shm_open(const char *name, int oflag, mode_t mode);
int shm_unlink(const char *name);

POSIX shared memory is almost non-existing …

  • Small wrapper around existing system calls

  • shm_open() only dictates the object name (/some-name)

    • Explicitly returns a file descriptor

  • shm_close() does not exist. Use close().

man -s 7 shm_overview

Shared Memory (2)

Workflow

  • After creating (shm_open()), size is zero

  • ftruncate(), to adjust the size

  • mmap(), to create the mapping

As simple as it can get!

Exercise: POSIX Message Queues

Add a POSIX message queue to our server like follows

  • The client (to be written) opens an existing message queue, sends a message, and closes the queue afterwards.

  • The server

    • creates the message queue in the startup phase

    • receives (file descriptor based) notifications in the main loop, and reads and outputs messages just like the others

    • closes and removes the queue in the shutdown phase