Suppose you must write code that remains busy while checking to see whether a key has been pressed. The program repeats a loop, performing various tasks, but eager for that key press. When a key is pressed, the code fetches the key. Two things stand in your way to make this happen.
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\nFirst, standard input is streaming. True, input can be buffered or unbuffered, but it all comes in the stream not directly from the keyboard.<\/p>\n
Second, C library functions that read standard input are blocking calls. They wait for input, pausing the entire program.<\/p>\n
The process I’m referencing is called polling<\/em>. Activity takes place, such as in a loop, and certain conditions are checked. If one of these conditions is whether a key has been pressed on the keyboard, how can you check it without pausing the entire program?<\/p>\n While it may seem impossible to perform a “just checking” routine on standard input, it can be done. But first I must present a silly example.<\/p>\n This code builds a program that continuously outputs uppercase letters A through Z. As it doesn’t do anything else, it runs fast, spewing the alphabet all over the terminal window. Press Ctrl+C to terminate it.<\/p>\n The program’s endless while<\/em> loop should have a terminating condition, but it’s anticipated that the loop terminates when the user presses a key. The problem is how to implement this condition without halting the program?<\/p>\n To prove that it can’t work (at least for now), the following update to the code calls the getchar()<\/em> function. This function is one way to check on the keyboard, but it halts the entire program:<\/p>\n When the getchar()<\/em> function is encountered, the loop stops to wait for keyboard input. In fact, the loop runs only once. This code is a poor example of polling.<\/p>\n One solution is to check standard input at a low level. For next week’s Lesson<\/a>, I show how to access the standard input device in this manner. But other solutions exist as well. For example, network programming involves a lot of polling. I cover these other solutions as I continue to explore checking the keyboard for input.<\/p>\n","protected":false},"excerpt":{"rendered":" Sometimes a program needs to check to see whether a key has been pressed without reading that key. Continue reading 2024_03_23-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n char a;\r\n\r\n a = 'A';\r\n while(1)\r\n {\r\n printf(\" %c\",a++);\r\n if( a=='Z' )\r\n a = 'A';\r\n }\r\n\r\n return 0;\r\n}<\/pre>\n2024_03_23-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n char a;\r\n\r\n a = 'A';\r\n while(1)\r\n {\r\n printf(\" %c\",a++);\r\n if( a=='Z' )\r\n a = 'A';\r\n \/* terminate on key *\/<\/span>\r\n if(getchar())\r\n break;\r\n }\r\n\r\n return 0;\r\n}<\/pre>\n