Step by Step solution about How to resolve the algorithm Keyboard input/Keypress check step by step in the C programming language

The provided C code establishes a basic keyboard input handling mechanism that allows the program to capture and process keystrokes without echoing them to the console (in a manner similar to how command interpreters like Bash handle keyboard input).

Here's a breakdown of how the code works:

1. Header Includes:

   • <stdio.h>: Provides input and output functions like printf and getchar.
   • <termios.h>: Contains functions for manipulating terminal settings (tcgetattr, tcsetattr).
   • <unistd.h>: Provides functions for input/output operations, including select.
   • <fcntl.h>: Provides file control options, such as O_NONBLOCK.

2. set_mode Function:

   • This function configures the terminal settings to enable or disable raw input mode.
   • When want_key is 0 (false), it restores the terminal settings to their previous state.
   • When want_key is 1 (true), it sets the terminal to raw input mode, disabling line buffering (ICANON) and character echoing (ECHO).

3. get_key Function:

   • This function waits for a key to be pressed using the select system call.
   • It sets up a file descriptor set fs to monitor the standard input (keyboard) and sets a timeout of 0 seconds.
   • When a key is pressed, FD_ISSET will return true, and getchar is used to read the character.
   • After reading the character, it restores the terminal settings to their previous state using set_mode(0).

4. main Function:

   • The program enters an infinite loop.
   • Inside the loop:
     • It enters raw input mode by calling set_mode(1).
     • It waits for a keypress using get_key and stores the character in c.
     • It prints the keycode of the pressed key.

5. Keystroke Capture:

   • When a key is pressed, the program captures it without displaying it on the console (echoing).
   • This allows the program to respond to keystrokes immediately without waiting for the user to press enter or return.

6. Flushing Buffers:

   • To prevent previously pressed keys from being processed after exiting raw input mode, the program flushes both output and input buffers before entering raw input mode.

7. Continuous Loop:

   • The program continues waiting for keypresses until the user terminates the program (by pressing Ctrl+C or similar).

#include <stdio.h>
#include <termios.h>  /* general terminal interface: tcgetattr, tcsetattr, tcflush */
#include <unistd.h>   /* synchronous I/O multiplexing: select, FD_CLR, FD_ISSET, FD_SET, FD_ZERO */
#include <fcntl.h>

void set_mode(int want_key)
{
	static struct termios old, new;
	if (!want_key) {
		tcsetattr(STDIN_FILENO, TCSANOW, &old);
		return;
	}

	tcgetattr(STDIN_FILENO, &old);
	new = old;
	new.c_lflag &= ~(ICANON | ECHO);
	tcsetattr(STDIN_FILENO, TCSANOW, &new);
}

int get_key()
{
	int c = 0;
	struct timeval tv;
	fd_set fs;
	tv.tv_usec = tv.tv_sec = 0;

	FD_ZERO(&fs);
	FD_SET(STDIN_FILENO, &fs);
	select(STDIN_FILENO + 1, &fs, 0, 0, &tv);

	if (FD_ISSET(STDIN_FILENO, &fs)) {
		c = getchar();
		set_mode(0);
	}
	return c;
}

int main()
{
	int c;
	while(1) {
		set_mode(1);
                /* force C library buffers to be written to kernel buffers,
                   and flush pending input to avoid previously pressed keys */
                fflush(stdout);
		while (!(c = get_key())) usleep(10000);
		printf("key %d\n", c);
	}
}