Книга: Real-Time Concepts for Embedded Systems
8.4.3 Typical Uses of Signals
8.4.3 Typical Uses of Signals
Some signals are associated with hardware events and thus are usually sent by hardware ISRs. The ISR is responsible for immediately responding to these events. The ISR, however, might also send a signal so that tasks affected by these hardware events can conduct further, task-specific processing.
As depicted in Figure 8.10, signals can also be used for synchronization between tasks. Signals, however, should be used sparingly for the following reasons:
· Using signals can be expensive due to the complexity of the signal facility when used for inter-task synchronization. A signal alters the execution state of its destination task. Because signals occur asynchronously, the receiving task becomes nondeterministic, which can be undesirable in a real-time system.
· Many implementations do not support queuing or counting of signals. In these implementations, multiple occurrences of the same signal overwrite each other. For example, a signal delivered to a task multiple times before its handler is invoked has the same effect as a single delivery. The task has no way to determine if a signal has arrived multiple times.
· Many implementations do not support signal delivery that carries information, so data cannot be attached to a signal during its generation.
· Many implementations do not support a signal delivery order, and signals of various types are treated as having equal priority, which is not ideal. For example, a signal triggered by a page fault is obviously more important than a signal generated by a task indicating it is about to exit. On an equal-priority system, the page fault might not be handled first.
· Many implementations do not guarantee when an unblocked pending signal will be delivered to the destination task.
Some kernels do implement real-time extensions to traditional signal handling, which allows
· for the prioritized delivery of a signal based on the signal number,
· each signal to carry additional information, and
· multiple occurrences of the same signal to be queued.
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- 8.4.2 Typical Signal Operations
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