IBM Version 4 manual Signal APIs, Semaphore, Are identifie, Shmget

sends one, or that a message exist on the queue before a process receive one.

A semaphore	is a synchronization mechanism similar	to a mutex or a mac
interface	(MI) lock. It can be used to control	access to shared r
to notify	other processes of the availability	of resources.

Processes can communicate directly with one another by sharing parts memory space and then reading and writing the datashared stored in the

memory . Synchronization of shared memory is the responsibility of the program. Semaphores can be used to synchronize shared memory use acr processes. Mutexes or condition variables can be used to synchronize memory use across threads.

Although each IPC service provides a specific type of interprocess the three services share many similarities. Each service defines a

through	which	its communications take place. For	message	queues, that
anism is a message queue; for semaphores, it			is a semaphore set; a
memory,	it is	a shared memory segment. These	mechanisms	are identifie

unique positive integer, called, respectively, a messagemsqid),queuea identi

semaphore identifiersemid), (and

a shared memory identifiershmid).

(

Associated with

each

identifier

is a

data

structure

that contains

the IPC

mechanism, as

well

as

ownership

and

permissions

information. This

ture is similar to a file permissions

structure,

and

is initialized

b

creates

the IPC

mechanism. It

is

then

checked

by

all

subsequent

IP

to determine if the requesting process has the required permissions operation.

To get	an	identifier,	a process must			either		create a		new IPC	mech
an existing mechanism. This is done throughmsgget()							,thesemget() ,		and
shmget()	functions. Each		get operation takeskeyasparameterinput a and
returns	an	identifier.	Each	get	operationflag alsoparametertakes. Thisaflag
parameter		contains the	IPC permissions for the mechanism as well as b
determine whether or not a				new	mechanism		is	created.
When a	message queue,		semaphore		set,	or	shared		memory	segment	is c

the process that creates it determines how it can be accessed. Su

operations	do a	permission test for the calling process before allow
to perform	the	requested operation.

Signal APIs

An X/Open**	specification	1	as “a mechanism by which a process
		definessignal
may be notified of, or		affected by, an event occurring in the sy
signal is	also used to	refer to the event itself.

A signal is said to be whgeneratedthe event that causes the signal first Examples of such events include the following:

ŸSystem-detected errors

ŸTimer expiration

1X/Open CAE Specification System Interface Definitions Issue 4, Number 2, Glossary, page 27. X/Open Company Kingdom, 1994.

Chapter 8. Use of OS/400 8APIs-23