Signals are software interrupts that send information about an occurrence of an asynchronous event to an executing program, or process.
信号就是软件中断，可以向正在执行的程序(进程)发送有关异步事件发生的信息。
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The RTSJ includes support for handling asynchronous events triggered by a number of sources including timers, operating-system signals, missed deadlines, and other application-defined events.
Due to the asynchronous nature of signals, it is difficult to point out the bug caused when a signal-handling function triggers a non-reentrant function.