IBM AS/400 manual How to, Across, Share Data

Multithreading

Considerations

THREAD(*SERIALIZE)

will

protect

most

of your variables and all your intern

control

structures from being accessed improperly

by

multiple

threads.

safe module will be locked when

a

procedure

in

the

module is

entere

unlocked

when no procedure in the

module

is

still

running.

This

serializ

ensures

that

only

one

thread

is

active

in

any one

module,

within

an

a

group,

at any one time. However,

it

is

still

up

to

the

programmer

to

safety

for storage that is shared

across

modules.

This

is

done

by

the application to synchronize access to the storage. For example, sha

exported

and

imported

storage,

and storage accessed by the addres

eter

may

be

shared

across

modules

from

multiple

threads.

To

synchroni

to

this type

of

storage,

you

can

do

one

or

both

of

the

following:

¹

Structure the application such that the shared resources are not a

simultaneously

from

multiple

threads.

¹

If

you are going to access resources simultaneously from separate

synchronize access using facilities such as semaphores or mutexes.

information, refer to the Multithreaded Applications document

under

th

gramming topic at the following URL:

http://www.as400.ibm.com/infocenter/

How to

Share Data

Across

More

Than

One

Module

Serializing access to modules using the THREAD(*SERIALIZE) control

specific

keyword

ensures sequential access

to

global

data

within

each

module,

not ensure sequential access to

shared data across modules. It is

grammer

to ensure that only one

thread can access shared data

at

o

Two

or

more

modules

can

access

the

same

data

if:

¹

EXPORT/IMPORT

keywords

are

used

on

the

definition

specifications

¹

Files

are shared

across

modules

¹

Data

is based on a pointer

where

the

pointer

is

available

to

more

module

For

example,

procedure

A

in

module

A

passes

a

pointer to

procedure

B

B,

and

procedure

B

saves

the

pointer

in

a

static

variable.

Now

both

access

to

the

based

storage

at the same time as the thread runnin

accessing the storage. Once procedure B returns, another thread coul

cedure

in module B and access

the based storage. Serialization of

storage

within modules A and B

would not prevent simultaneous access

of

same storage in each module. The following is an

example

of

two

module

access

the

same

data.

158 ILE RPG for AS/400 Programmer's Guide