CSCE 313 Lecture 15

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Inter-Process Communication (IPC)

Processes in system can be independent or cooperating. Cooperating processes can be affected (modifying shared data).

Fundamental communication models:

1. Process A and Process B have shared memory between them.
2. Each process has its own memory space ("mailbox"), and they access each other's memory through the kernel

Message Passing

sending and receiving

In order for processes to communicate, they need to establish a link

Link Implementations:

• Physical shared hardware (memory or bus)
• logical properties

Direct Communication involves specifying the process to send to or receive from:

• send message to process P
• receive message from process Q

Indirect communication relies on mailboxes (ports), each of which has a unique ID.

• A link is established only if processes share a common mailbox
• Therefore a link can be associated with multiple processes
• Who gets the message if multiple processes linked?

Synchronization

Blocking vs. Non-blocking message

POSIX IPC

These commands are available in sys/msg.h

primitive	POSIX Function	Description
message queues	msgget(key_t key, int msgflg);	create or access
	msgctl	control
	msgsnd/msgrcv	send/receive message
semaphores	semget	create or access
	semctl	control
	semop	wait or post operation
shared memory	shmget	create or access
	shmctl	control
	shmat/shmdt	attach to / detach from process

IPC status can be observed using ipcs command from prompt:

IPC status from <running system> as of Thu Mar  8 10:39:58 CST 2012
T     ID     KEY        MODE       OWNER    GROUP
Message Queues:

T     ID     KEY        MODE       OWNER    GROUP
Shared Memory:

T     ID     KEY        MODE       OWNER    GROUP
Semaphores:

Naming xxGET

Condition variables and mutex locks

based on a memory variable and does not work across processes

Pipes

Uses file descriptors

Works across processes, but not for unrelated process (relies on inherited file descriptors)

Named Pipes

Uses file system as name space for pipe

Works across any processes

file system (disk) overhead

IPC Objects

system-wide integer keys