serial: deliver EOF (0x04) on stdin end for headless console

read()==0 completes the external-clock transfer with SB=0x04 (EOT), so a gbos
program's readc() sees EOF instead of polling forever. Enables cat/wc-style
tools and lets the shell exit on Ctrl-D / pipe end.

1 files changed, 7 insertions, 1 deletions

diff --git a/src/gb.c b/src/gb.c
index 7e6771b..3caa986 100644
--- a/src/gb.c
+++ b/src/gb.c
@@ -134,11 +134,17 @@ void gb_write(GB *gb, u16 addr, u8 val) {
             // junk. Stays pending (bit7 set) while stdin has nothing.
             if (gb->serial_in_fd >= 0) {
                 unsigned char c;
-                if (read(gb->serial_in_fd, &c, 1) == 1) {
+                ssize_t n = read(gb->serial_in_fd, &c, 1);
+                if (n == 1) {
                     gb->sb = c;
                     gb->sc &= ~0x80;
                     gb_request_interrupt(gb, INT_SERIAL);
+                } else if (n == 0) {
+                    gb->sb = 0x04;         // EOF -> EOT (Ctrl-D)
+                    gb->sc &= ~0x80;
+                    gb_request_interrupt(gb, INT_SERIAL);
                 }
+                // n < 0 (EAGAIN on a live tty): stay pending, poll again
             }
         }
         return;