The CICS® task control facility provides functions that synchronize task activity, or that control the use of resources.
Java and C++
The application programming interface described here is the EXEC CICS API, which is not used in Java™ programs. For information about Java programs using the JCICS classes to access Task Control services CICS, see Java programming using JCICS and the JCICS Javadoc documentation. For information about C++ programs using the CICS C++ classes, see Using the CICS C++ OO foundation classes.
CICS assigns priorities based on the value set by the CICS system programmer. Control of the processor is given to the highest-priority task that is ready to be processed, and is returned to the operating system when no further work can be done by CICS or by your application programs.
- Suspend a task (SUSPEND command) to enable tasks of higher priority to proceed. This can prevent processor-intensive tasks from monopolizing the processor. When other eligible tasks have proceeded and terminated or suspended processing, control is returned to the issuing task; that is, the task remains dispatchable.
- Schedule the use of a resource by a task (ENQ and DEQ commands).
This is sometimes useful in protecting a resource from concurrent
use by more than one task; that is, by making that resource serially
reusable. Each task that is to use the resource issues an enqueue
command (ENQ). The first task to do so has the use of the resource
immediately but, if a HANDLE CONDITION ENQBUSY command has not been
issued, subsequent ENQ commands for the resource, issued by other
tasks, result in those tasks being suspended until the resource is
available.
If the NOSUSPEND option is coded on an ENQ command, control is always returned to the next instruction in the program. By inspecting the contents of the EIBRESP field, you can see whether the ENQ command was successful or not.
Each task using a resource should issue a dequeue command (DEQ) when it has finished with it. However, when using the enqueue/dequeue mechanism, there is no way to guarantee that two or more tasks issuing ENQ and DEQ commands issue these commands in a given sequence relative to each other. For a way to control the sequence of access, see Controlling sequence of access to resources.
- Change the priority assigned to a task (CHANGE TASK PRIORITY command).
- Wait for events that post MVS™ format ECBs when they complete.
Two commands are available, WAITCICS and WAIT EXTERNAL. These commands cause the issuing task to be suspended until one of the ECBs has been posted; that is, until one of the events has occurred. The task can wait on one or more ECBs. If it waits on more than one, it is dispatchable as soon as one of the ECBs is posted. You must ensure that each ECB is cleared (set to binary zeros) no later than the earliest time it could be posted. CICS cannot do this for you. If you wait on an ECB that has been previously posted and is not subsequently cleared, your task is not suspended and continues to run as though WAITCICS or WAIT EXTERNAL had not been issued.
WAIT EXTERNAL typically has less overhead, but the associated ECBs must always be posted using the MVS POST facility or by an optimized post (using the compare and swap (CS) instruction). They must never be posted by any other method. If you are in any doubt about the method of posting, use a WAITCICS command. When dealing with ECBs passed on a WAIT EXTERNAL command, CICS extends the ECBs and uses the MVS POST exit facility. A given ECB must not be waited on by more than one task at once (or appear twice in one task's ECBLIST). Failure to follow this rule leads to an INVREQ response.
WAITCICS must be used if ECBs are to be posted by any method other than the MVS POST facility or by an optimized post. For example, if your application posts the ECB by moving a value into it, WAITCICS must be used. (The WAITCICS command can also be used for ECBs that are posted using the MVS POST facility or optimized post.) Whenever CICS goes into an MVS WAIT, it passes a list to MVS of all the ECBs being waited on by tasks that have issued a WAITCICS command. The ECBLIST passed by CICS on the MVS WAIT contains duplicate addresses, and MVS abends CICS.
If you use MVS POST, WAIT EXTERNAL, WAITCICS, ENQ, or DEQ commands, you could create inter-transaction affinities that adversely affect your ability to perform dynamic transaction routing.
To help you identify potential problems with programs that issue this command, you can use the CICS Interdependency Analyzer. See
CICS Interdependency Analyzer for more information about
this utility and Affinity for more
information about transaction affinity.
