16.3. Operations

16.3.1. Establishing an ASR

The rtems_signal_catch directive establishes an ASR for the calling task. The address of the ASR and its execution mode are specified to this directive. The ASR’s mode is distinct from the task’s mode. For example, the task may allow preemption, while that task’s ASR may have preemption disabled. Until a task calls rtems_signal_catch the first time, its ASR is invalid, and no signal sets can be sent to the task.

A task may invalidate its ASR and discard all pending signals by calling rtems_signal_catch with a value of NULL for the ASR’s address. When a task’s ASR is invalid, new signal sets sent to this task are discarded.

A task may disable ASR processing (RTEMS_NO_ASR) via the task_mode directive. When a task’s ASR is disabled, the signals sent to it are left pending to be processed later when the ASR is enabled.

Any directive that can be called from a task can also be called from an ASR. A task is only allowed one active ASR. Thus, each call to rtems_signal_catch replaces the previous one.

Normally, signal processing is disabled for the ASR’s execution mode, but if signal processing is enabled for the ASR, the ASR must be reentrant.

16.3.2. Sending a Signal Set

The rtems_signal_send directive allows both tasks and ISRs to send signals to a target task. The target task and a set of signals are specified to the rtems_signal_send directive. The sending of a signal to a task has no effect on the execution state of that task. If the task is not the currently running task, then the signals are left pending and processed by the task’s ASR the next time the task is dispatched to run. The ASR is executed immediately before the task is dispatched. If the currently running task sends a signal to itself or is sent a signal from an ISR, its ASR is immediately dispatched to run provided signal processing is enabled.

If an ASR with signals enabled is preempted by another task or an ISR and a new signal set is sent, then a new copy of the ASR will be invoked, nesting the preempted ASR. Upon completion of processing the new signal set, control will return to the preempted ASR. In this situation, the ASR must be reentrant.

Like events, identical signals sent to a task are not queued. In other words, sending the same signal multiple times to a task (without any intermediate signal processing occurring for the task), has the same result as sending that signal to that task once.

16.3.3. Processing an ASR

Asynchronous signals were designed to provide the capability to generate software interrupts. The processing of software interrupts parallels that of hardware interrupts. As a result, the differences between the formats of ASRs and ISRs is limited to the meaning of the single argument passed to an ASR. The ASR should have the following calling sequence and adhere to C calling conventions:

rtems_asr user_routine(
    rtems_signal_set signals
);

When the ASR returns to RTEMS the mode and execution path of the interrupted task (or ASR) is restored to the context prior to entering the ASR.