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Signal programming refers to a program that responds to signals or events generated by user input or system changes. Signals can be messages, events, or interrupts that indicate a change in state or data. Common architectures include listener/observer models and signal scheduling with queues. Concurrent processes and multi-threaded applications can cause complications, but thread synchronization methods can help prevent data synchronization issues.
“Signal programming” is a term often used to refer to a computer program that runs primarily in response to signals or events generated by some change of state or user input. Almost every part of a computer system, from a peripheral device to the operating system itself, can generate a signal that can be captured by an application and then processed. In many forms, signal programming resembles event-driven programming. Some of the more common uses for signal programming are with graphical user interfaces that respond to user input, network communications, and low-level programs that capture and manipulate specific system signals.
A signal in computer programming can be defined as a message, an event or an interrupt, depending on its origin, the architecture of the system or even the preference of the programmer. In broader terms, it is an indicator that a state or data has changed, and is sometimes accompanied by additional information about the change that has occurred. The signal can be generated by the user when a key is pressed, by hardware such as a monitor when a vertical retrace occurs, or even by the operating system when something like a page fault occurs while allocating memory.
Within an application, signal programming can take on many different types of architectures, but one of the most common is a listener or observer model. In this method, a user-defined piece of code can be passed in some way to a signal handler, usually within the operating system, which then calls that piece of code to process any signal that matches what the function is looking for. The type of signal a function can handle is often determined by a variable known as the signal mask, so some signals aren’t passed to routines not designed to handle them.
A second type of signal scheduling involves having a program implement a queue that accepts incoming signals. This queue is polled in the main program execution cycle and the program reacts when a signal enters the queue. Each signal can then be logically sorted, handled, ignored, or passed to another process.
Some difficulties may arise in signal programming when concurrent processes or multi-threaded applications are used. Complications can arise if, for example, two separate threads are listening to signals generated by two separate user input devices or by two network sockets. If two users try to make a change to some data at the same time, the program data may get out of sync and appear different for each user. Using a linear queue or implementing thread synchronization methods can help prevent this situation.
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