kiosk-hangup-sensor/main.c

#include <assert.h>
#include <stdio.h>
#include <stdint.h>
#include <unistd.h>
#include <fcntl.h>

#define TTY_PATH "/dev/ttyUSB0"
#define STATUS_PATH "/run/hangup-sensor/status"
#define INTERVAL_MSEC 100
#define CHECK_VALUE '\xff'

#define S(a) a,(sizeof(a)-1)

enum status {
	UNKNOWN,
	CONNECTED,
	DISCONNECTED,
};

int main()
{
	int tty_fd;
	assert ( ( tty_fd = open ( TTY_PATH, O_RDWR ) ) != -1 );
	assert ( fcntl ( tty_fd, F_SETFL, fcntl ( tty_fd, F_GETFL ) | O_NONBLOCK ) != -1 );
	enum status status = UNKNOWN;
	static const uint8_t sendChar = CHECK_VALUE;

	while ( 1 ) {
		uint8_t recvChar;
		enum status newStatus;
		assert ( write ( tty_fd, &sendChar, 1 ) != -1 );
		usleep ( INTERVAL_MSEC * 1000 );
		recvChar = 0;
		ssize_t rsize = read ( tty_fd, &recvChar, 1 );
		newStatus = ( rsize == 1 && recvChar == sendChar ) ? CONNECTED : DISCONNECTED;

		if ( newStatus != status ) {
			int statusFile_fd;
			assert ( ( statusFile_fd = open ( STATUS_PATH, O_WRONLY | O_CREAT | O_TRUNC, 0640 ) ) != -1  );

			switch ( newStatus ) {
				case CONNECTED:
					assert ( write ( statusFile_fd, S ( "connected\n" ) ) != -1 );
					break;
				case DISCONNECTED:
					assert ( write ( statusFile_fd, S ( "disconnected\n" ) ) != -1 );
					break;
				default:
					fprintf ( stderr, "This shouldn't happened" );
					return -1;
			}

			close ( statusFile_fd );
		}

		status = newStatus;
	}

	return 0;
}