How does an OS or a systems program wait for user input?

Question

I come from the world of web programming and usually the server sets a superglobal variable through the specified method (get, post, etc) that makes available the data a user inputs into a field. Another way is to use AJAX to register a callback method to an event that the AJAX XMLhttpRequest object will initiate once notified by the browser (I'm assuming...). So I guess my question would be if there is some sort of dispatch interface that a systems programmer's code must interact with vicariously to execute in response to user input or does the programmer control the "waiting" process directly? And if there is a dispatch is there a loop structure in an OS that waits for a particular event to occur?

I was prompted to ask this question here because I'm in a basic programming logic class and the professor won't answer such a "sophisticated" question as this one. My book gives a vague pseudocode example like:

    //start
    sentinel_val = 'stop';
    get user_input;
while (user_input not equal to sentinel_val)
     {
         // do something.
         get user_input;
     }
     //stop

This example leads me to believe 1) that if no input is received from the user the loop will continue to repeat the sequence "do something" with the old or no input until the new input magically appears and then it will repeat again with that or a null value. It seems the book has tried to use the example of priming and reading from a file to convey how a program would get data from event driven input, no?

I'm confused :(

Answer 1

At the lowest level, input to the computer is asynchronous-- it happens via "interrupts", which is basically something external to the CPU (a keyboard controller) sending a signal to the CPU that says "stop what you're doing and accept this data". (It's complex, but this is the general idea). So the CPU stops, grabs the keystroke, and puts it in a buffer to be read, and then continues doing what it was doing before the interrupt.

Very similar things happen with inbound network traffic, and the results of reading from a disk, etc.

At a higher level, it gets more dependent on the operating system or framework that you're using.

With keyboard input, there might be a process (application, basically) that is blocked, waiting for user input. That "block" doesn't mean the computer just sits there waiting, it lets other processes run instead. But when the keyboard result comes in, it will wake up the one who was waiting for it.

From the point of view of that waiting process, they called some function "get_next_character()" and that function returned with the character. Etc.

Frankly, how all this stuff ties together is super interesting and useful to understand. :)