--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/frbl/test/ttypassthru.c	Thu Mar 05 22:05:01 2020 +0000
@@ -0,0 +1,109 @@
+/*
+ * This module implements the pass-thru operation mode, in which
+ * the Unix host tty is cross-connected directly to the target
+ * running some code we have just loaded.
+ */
+
+#include <sys/types.h>
+#include <sys/ioctl.h>
+#include <sys/errno.h>
+#include <termios.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <strings.h>
+
+extern int errno;
+
+extern int target_fd;
+
+static struct termios saved_termios, my_termios;
+
+static void
+safe_output(buf, cc)
+	u_char *buf;
+{
+	int i, c;
+
+	for (i = 0; i < cc; i++) {
+		c = buf[i];
+		if (c == '\r' || c == '\n' || c == '\t' || c == '\b') {
+			putchar(c);
+			continue;
+		}
+		if (c & 0x80) {
+			putchar('M');
+			putchar('-');
+			c &= 0x7F;
+		}
+		if (c < 0x20) {
+			putchar('^');
+			putchar(c + '@');
+		} else if (c == 0x7F) {
+			putchar('^');
+			putchar('?');
+		} else
+			putchar(c);
+	}
+	fflush(stdout);
+}
+
+static void
+loop()
+{
+	char buf[BUFSIZ];
+	fd_set fds, fds1;
+	register int i, cc, max;
+
+	FD_ZERO(&fds);
+	FD_SET(0, &fds);
+	FD_SET(target_fd, &fds);
+	max = target_fd + 1;
+	for (;;) {
+		bcopy(&fds, &fds1, sizeof(fd_set));
+		i = select(max, &fds1, NULL, NULL, NULL);
+		if (i < 0) {
+			if (errno == EINTR)
+				continue;
+			tcsetattr(0, TCSAFLUSH, &saved_termios);
+			perror("select");
+			exit(1);
+		}
+		if (FD_ISSET(0, &fds1)) {
+			cc = read(0, buf, sizeof buf);
+			if (cc <= 0)
+				return;
+			if (cc == 1 && buf[0] == 0x1C)
+				return;
+			write(target_fd, buf, cc);
+		}
+		if (FD_ISSET(target_fd, &fds1)) {
+			cc = read(target_fd, buf, sizeof buf);
+			if (cc <= 0) {
+				tcsetattr(0, TCSAFLUSH, &saved_termios);
+				fprintf(stderr, "EOF/error on target tty\n");
+				exit(1);
+			}
+			safe_output(buf, cc);
+		}
+	}
+}
+
+tty_passthru()
+{
+	static int zero = 0;
+
+	ioctl(target_fd, FIONBIO, &zero);
+
+	tcgetattr(0, &saved_termios);
+	bcopy(&saved_termios, &my_termios, sizeof(struct termios));
+	cfmakeraw(&my_termios);
+	my_termios.c_cc[VMIN] = 1;
+	my_termios.c_cc[VTIME] = 0;
+	tcsetattr(0, TCSAFLUSH, &my_termios);
+
+	printf("Entering tty pass-thru; type ^\\ to exit\r\n\n");
+	loop();
+	tcsetattr(0, TCSAFLUSH, &saved_termios);
+	return 0;
+}