headless: interactive TTY mode (raw stdin) for driving a guest OS live

When stdin is a terminal, --headless now disables canonical mode + local echo
so keystrokes reach the guest immediately and only the guest echoes (no doubled
chars). Piped/redirected stdin is untouched (scripted input still works). Keep
ISIG so Ctrl-C quits; restore termios on exit. VMIN=1 so a no-data non-blocking
read is EAGAIN, not a 0-byte read the serial layer would misread as EOF.

1 files changed, 17 insertions, 0 deletions

diff --git a/src/main.c b/src/main.c
index f0709c2..648ded2 100644
--- a/src/main.c
+++ b/src/main.c
@@ -9,6 +9,7 @@
 #include <signal.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <termios.h>
 
 static volatile sig_atomic_t running = 1;
 static void on_sig(int s) { (void)s; running = 0; }
@@ -100,6 +101,21 @@ int main(int argc, char **argv) {
         if (fl != -1) fcntl(STDIN_FILENO, F_SETFL, fl | O_NONBLOCK);
         signal(SIGINT, on_sig);
         signal(SIGTERM, on_sig);
+        // Interactive: if stdin is a terminal, drop line-buffering + local echo
+        // so keystrokes reach the guest immediately and only the guest echoes.
+        // (Piped/redirected stdin is left alone, for scripted input.)
+        struct termios saved_tio;
+        bool raw_tty = isatty(STDIN_FILENO) && tcgetattr(STDIN_FILENO, &saved_tio) == 0;
+        if (raw_tty) {
+            struct termios t = saved_tio;
+            t.c_lflag &= ~(ICANON | ECHO);   // char-at-a-time, no echo (keep ISIG)
+            // VMIN=1 (not 0): with O_NONBLOCK, "no data" is EAGAIN, not a 0-byte
+            // read that we'd otherwise misread as EOF.
+            t.c_cc[VMIN] = 1;
+            t.c_cc[VTIME] = 0;
+            tcsetattr(STDIN_FILENO, TCSANOW, &t);
+            fprintf(stderr, "[gbc: interactive serial console - Ctrl-C to quit]\n");
+        }
         bool uncapped = (target_fps <= 0);
         double start_t = now_sec();
         u64 start_cyc = gb->cycles;
@@ -118,6 +134,7 @@ int main(int argc, char **argv) {
                 }
             }
         }
+        if (raw_tty) tcsetattr(STDIN_FILENO, TCSANOW, &saved_tio);
         cart_free(&gb->cart); free(gb); return 0;
     }