Intercept the U-boot sequence by casting ctrl+u programmatically as early as possible

ttykey.c

#include <errno.h>
#include <fcntl.h> 
#include <stdio.h>
#include <string.h>
#include <termios.h>
#include <unistd.h>

#define error_message(...) fprintf(stderr, __VA_ARGS__)

static int set_interface_attribs(int fd, int speed, int parity)
{
	struct termios tty;
	if (tcgetattr(fd, &tty) != 0)
	{
		error_message("error %d from tcgetattr\n", errno);
		return -1;
	}

	cfsetospeed(&tty, speed);
	cfsetispeed(&tty, speed);

	tty.c_cflag =(tty.c_cflag & ~CSIZE) | CS8;     // 8-bit chars
	// disable IGNBRK for mismatched speed tests; otherwise receive break
	// as \000 chars
	tty.c_iflag &= ~IGNBRK;	 // disable break processing
	tty.c_lflag = 0;		// no signaling chars, no echo,
					// no canonical processing
	tty.c_oflag = 0;		// no remapping, no delays
	tty.c_cc[VMIN]  = 0;	    // read doesn't block
	tty.c_cc[VTIME] = 5;	    // 0.5 seconds read timeout

	tty.c_iflag &= ~(IXON | IXOFF | IXANY); // shut off xon/xoff ctrl

	tty.c_cflag |=(CLOCAL | CREAD);// ignore modem controls,
					// enable reading
	tty.c_cflag &= ~(PARENB | PARODD);      // shut off parity
	tty.c_cflag |= parity;
	tty.c_cflag &= ~CSTOPB;
	tty.c_cflag &= ~CRTSCTS;

	if (tcsetattr(fd, TCSANOW, &tty) != 0)
	{
		error_message("error %d from tcsetattr\n", errno);
		return -1;
	}
	return 0;
}

static void set_blocking(int fd, int should_block)
{
	struct termios tty;
	memset(&tty, 0, sizeof tty);
	if (tcgetattr(fd, &tty) != 0)
	{
		error_message("error %d from tggetattr\n", errno);
		return;
	}

	tty.c_cc[VMIN]  = should_block ? 1 : 0;
	tty.c_cc[VTIME] = 5;	    // 0.5 seconds read timeout

	if (tcsetattr(fd, TCSANOW, &tty) != 0)
		error_message("error %d setting term attributes\n", errno);
}

static const char *portname = "/dev/ttyUSB0";

// The value of ctrl+u key sequence, according to
// https://en.wikipedia.org/wiki/ASCII#Control_code_chart
static const char value = 21;

int main()
{
	
	int fd = open(portname, O_RDWR | O_NOCTTY | O_SYNC);
	if (fd < 0)
	{
		error_message("error %d opening %s: %s\n", errno, portname, strerror(errno));
		return -1;
	}

	set_interface_attribs(fd, B115200, 0); // set speed to 115,200 bps, 8n1(no parity)
	set_blocking(fd, 0); // set no blocking

	while (1)
		write(fd, &value, sizeof(value));

	return 0;
}