The task management information is required only for establishing communications among tasks in the same job step. The relationship of tasks in a job step is shown in Figure 1. The horizontal lines in Figure 1. separate originating tasks and subtasks; they have no bearing on task priority. Tasks A, A1, A2, A2a, B, B1 and B1a are all subtasks of the job-step task; tasks A1, A2, and A2a are subtasks of task A. Tasks A2a and B1a are the lowest level tasks in the job step. Although task B1 is at the same level as tasks A1 and A2, it is not considered a subtask of task A.
All of the tasks in the job step compete independently for processor time; if no constraints are provided, the tasks are performed and are terminated asynchronously. However, since each task is performing a portion of the same job step, some communication and constraints between tasks are required, such as notifying each other when a subtask completes. If a predecessor task attempts to terminate before all of its subtasks are complete, those subtasks and the predecessor task are abnormally terminated.
Two parameters, the ECB and ETXR parameters, are provided in the ATTACH or ATTACHX macro to assist in communication between a subtask and the originating task. These parameters are used to indicate the normal or abnormal termination of a subtask to the originating task. If you coded the ECB or ETXR parameter, or both, in the ATTACH or ATTACHX macro, the task control block of the subtask is not removed from the system when the subtask is terminated. The originating task must remove the task control block from the system after termination of the subtask by issuing a DETACH. If you specified the ECB parameter in the ATTACH or ATTACHX macro, the ECB must be in storage addressable by the attaching task and control program so that the issuer of ATTACH can wait on it (using the WAIT macro) and the control program can post it on behalf of the terminating task. The task control blocks for all subtasks must be removed before the originating task can terminate normally.
The ETXR parameter specifies the address of an end-of-task exit routine in the originating task, which is to be given control when the subtask being created is terminated. The end-of-task routine is given control asynchronously after the subtask has terminated and must therefore be in virtual storage when it is required. After the control program terminates the subtask, the end-of-task routine specified is scheduled to be executed. It competes for CPU time using the priority of the originating task and of its address space and can receive control even though the originating task is in the wait condition. Although the DETACH does not have to be issued in the end-of-task routine, this is a good place for it.
The ECB parameter specifies the address of an event control block (discussed under "Task Synchronization"), which is posted by the control program when the subtask is terminated. After posting occurs, the event control block contains the completion code specified for the subtask.
If you specified neither the ECB nor the ETXR parameter in the ATTACH or ATTACHX macro, the task control block for the subtask is removed from the system when the subtask terminates. Its originating task does not have to issue a DETACH. A reference to the task control block in a CHAP or a DETACH macro in this case is as risky as task termination. Since the originating task is not notified of subtask termination, you may refer to a task control block that has been removed from the system, which would cause the active task to be abnormally terminated.