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Managing with Local RINs
A RIN used to manage a resource being shared by related processes is
called a local RIN. You use local RINs to prevent simultaneous access to
a resource by two or more processes in the same job/session. Each local
RIN is a positive integer that is unique within your job/session.
Local RINs are assigned with the GETLOCRIN intrinsic, managed with the
LOCKLOCRIN and UNLOCKLOCRIN intrinsics, and released with the FREELOCRIN
intrinsic.
Acquiring Local RINs
You must acquire local RINs with the GETLOCRIN intrinsic before you can
use them within your job/session. The following intrinsic call,
GETLOCRIN (6);
acquires six local RINs, RIN numbers 1 through 6, that can be used by the
calling process as well as other processes in the same job/session.
All local RINs you are planning to use in your program must be acquired
in just one call to GETLOCRIN. If your program requires additional local
RINs after the initial GETLOCRIN call, you must first release all local
RINs then acquire the new number of local RINs with another GETLOCRIN
call.
Locking and Unlocking Local RINs
You can lock a local RIN using the LOCKLOCRIN intrinsic. Once your
process has successfully locked the RIN, no other process can lock the
same local RIN until your process unlocks it with the UNLOCKLOCRIN
intrinsic.
While you have the local RIN locked, exclusive access is guaranteed only
if other processes first attempt to lock the local RIN prior to accessing
the resource associated with the locked RIN.
A local RIN, acquired by your program, can be locked and unlocked by any
process in your program's process structure.
Following is an example of a LOCKLOCRIN intrinsic call from a program
that has previously acquired local RINs 1 through 4:
.
.
.
RINNUM := 4;
LOCK := 1;
LOCKLOCRIN (RINNUM,LOCK);
.
.
.
The parameters specified in the example are described below.
RINNUM | Passes the local RIN number
| associated with the resource you
| wish exclusive access to. The
| value 4 is a local RIN returned by
| the GETLOCRIN intrinsic.
LOCKCOND | LOCKCOND passes a value indicating
| the following: if the local RIN is
| currently locked by another
| process, control does not return to
| your program from the LOCKLOCRIN
| call until the local RIN is again
| available and successfully locked
| by your process.
You use the UNLOCKLOCRIN intrinsic to unlock a local RIN that has been
previously locked by the calling process.
Example 2-4 illustrates how the LOCKLOCRIN and UNLOCKLOCRIN intrinsics
can be used by two processes executing in the same job/session. Assume
that both the parent process (PARENT) and the child process (CHILD) are
executing concurrently, line by line.
PARENT PROCESS CHILD PROCESS
{ PARENT BEGINS EXECUTION }
.
.
.
HPFOPEN (LP,STATUS,... );
GETLOCRIN (3);
LPRIN := 1;
FWRITE (LP,... );
CREATE (PROGNAME,,CHILD... );
LOCKLOCRIN (LPRIN,1);
ACTIVATE (CHILD); { CHILD BEGINS EXECUTION }
FWRITE (LP,... ); .
. .
. LPRIN := 1;
. HPFOPEN (LP,STATUS,... );
. LOCKLOCRIN (LPRIN,1);
.
. CHILD IS BLOCKED WHEN IT
. ATTEMPTS TO LOCK RIN.
.
. EXECUTION CONTINUES WHEN RIN
. IS UNLOCKED BY PARENT AND
. LOCKED BY CHILD.
UNLOCKLOCRIN (LPRIN);
. .
LOCKLOCRIN (LPRIN,1); .
.
PARENT IS BLOCKED WHEN IT FWRITE (LP,... );
ATTEMPTS TO LOCK RIN. .
.
EXECUTION CONTINUES WHEN RIN .
IS UNLOCKED BY CHILD AND .
LOCKED BY PARENT. .
UNLOCKLOCRIN (LPRIN);
FWRITE (LP,... ); .
. .
. .
. .
Figure 2.4
Example 2-4. Locking and Unlocking Local RINs.
In Example 2-4, both processes have agreed to RIN management, associating
RIN 1 (designated in the program as LPRIN) with a line printer
(designated as LP). When PARENT first accesses LP, CHILD has not been
created, and so RIN management is not yet required.
To guarantee exclusive access to LP first, PARENT locks LPRIN before
CHILD is activated. When PARENT finishes with LP, it unlocks LPRIN, thus
making LPRIN available to be locked. In this case, CHILD has been
blocked and is waiting for LPRIN to become available.
When CHILD attempts to lock LPRIN, CHILD passes a value to LOCKLOCRIN
indicating that execution should be blocked until LOCKLOCRIN can
successfully return the locked RIN. The blocking occurs because PARENT
currently has LPRIN locked. Execution continues only when PARENT unlocks
LPRIN, thus making RIN 1 available to be locked by CHILD. While CHILD has
LPRIN locked, PARENT is unable to lock LPRIN until it is unlocked by
CHILD.
Releasing Local RINs
You can use the FREELOCRIN intrinsic to release all local RINs your
program previously acquired with GETLOCRIN. Following is an example of a
call to FREELOCRIN:
FREELOCRIN;
Any process in your program's process structure can release local RINs.
If you do not use FREELOCRIN to release local RINs, they are released to
MPE XL when your program terminates.
Identifying a Local RIN Locker
The LOCRINOWNER intrinsic identifies the process in your program's
process structure that has a particular local RIN locked. If the RIN is
locked by the parent of the calling process, LOCRINOWNER returns a zero.
If the RIN is locked by any other process, LOCRINOWNER returns the
Process Identification Number (PIN) of that process.
Knowing the identity of the locking process is useful when parent and
child processes are synchronizing access to one another through calls to
the ACTIVATE and SUSPEND intrinsics.
Example 2-5 is an example of RIN management where a parent process
(PARENT) acts as a monitor for several child processes (one of whom is
identified as CHILD1). Assume that both PARENT and CHILD1 are executing
concurrently, line by line.
Note that two agreements have been made by the programmer regarding RIN
management prior to writing the code in Example 2-5.
* When a child process wishes to communicate with PARENT it must first
successfully lock local RIN 1 (designated in the program as
WHICHCHILD). This guarantees that other child processes cannot
interfere in the communication being performed while local RIN 2
(designated as SYNCHRIN) is locked.
* A child process must successfully lock local RIN 2 only after it has
successfully locked local RIN 1. PARENT locks RIN 2 to guarantee
that the child process that activated PARENT is suspended while
PARENT executes code in the WHILE loop.
PARENT PROCESS CHILD PROCESS
{ PARENT EXECUTING } { CHILD EXECUTING }
. .
GETLOCRIN (2); .
. LOCKLOCRIN (WHICHCHILD,1);
CHILDCOUNT := 0; LOCKLOCRIN (SYNCHRIN,1);
WHILE CHILDCOUNT <= MAXCOUNT DO .
BEGIN .
SUSPEND (CHILDWAIT,SYNCHRIN); .
.
PARENT IS SUSPENDED .
UNTIL ACTIVATED .
BY A CHILD .
PARENT := FATHER;
ACTIVATE (PARENT);
. SUSPEND (PARENTWAIT,SYNCHRIN);
LOCKLOCRIN (SYNCHRIN,1);
OWNER := LOCRINOWNER (WHICHCHILD); CHILD1 IS SUSPENDED
. UNTIL ACTIVATED
. BY PARENT
CHILDCOUNT := CHILDCOUNT + 1;
ACTIVATE (OWNER);
END; {WHILE LOOP} .
. UNLOCKLOCRIN (WHICHCHILD);
. .
. .
Figure 2.5
Example 2-5. Identifying a Local RIN Locker.
Both processes in Figure 2-5 share the following constants:
WHICHCHILD = 1 | All processes in the program
| example agree that WHICHCHILD is
| the local RIN used to determine the
| identity of the child process that
| activated the parent process
| PARENT.
SYNCHRIN = 2 | All processes in the program
| example agree that SYNCHRIN is the
| local RIN used to ensure that
| PARENT can execute required code
| located in the WHILE loop before
| another child process can activate
| it.
PARENTWAIT = 1 | Used by both SUSPEND and ACTIVATE
| to indicate that the suspended
| process permits only its parent
| process to activate it with an
| ACTIVATE intrinsic call.
CHILDWAIT = 2 | Used by both SUSPEND and ACTIVATE
| to indicate that the suspended
| process permits only a child
| process to activate it with an
| ACTIVATE intrinsic call.
In Example 2-5, PARENT waits in a suspended state, unable to execute
until an activation signal is received from a child process (in this
case, by CHILD1 calling ACTIVATE). Once activated, PARENT locks SYNCHRIN
to synchronize its communication with CHILD1. LOCRINOWNER determines the
identity of the process that activated PARENT (the process that locked
WHICHCHILD). PARENT then performs its required duty within the WHILE
loop.
PARENT activates the process that activated PARENT (CHILD1) then suspends
itself to again await activation by a child process. Note that SYNCHRIN
is passed as a parameter to SUSPEND. SUSPEND releases the RIN, making
SYNCHRIN available to be locked by other processes.
For details on using SUSPEND and ACTIVATE, refer to the discussion of
suspending and activating processes in Process Management Programmer's
Guide (32650-90023).