slcanpty: probe stdin capabilities at startup

/dev/null returns EOF therefore select() finishes immediately.
Now EOF is probed on start.

Other /dev/null as stdin workarounds:
- lscanpty ... < /dev/ptmx (dirty but works)
- cat </dev/null | slcanpty ... (doesn't work for me and even the patch
  is "disabled" then)

Signed-off-by: Janusz Uzycki <j.uzycki@elproma.com.pl>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>

slcanpty.c

@@ -386,6 +386,27 @@ int can2pty(int pty, int socket, int *tstamp)
 	return 0;
 }
 
+int check_select_stdin(void)
+{
+	fd_set rdfs;
+	struct timeval timeout;
+	int ret;
+
+	FD_ZERO(&rdfs);
+	FD_SET(0, &rdfs);
+	timeout.tv_sec = 0;
+	timeout.tv_usec = 0;
+
+	ret = select(1, &rdfs, NULL, NULL, &timeout);
+
+	if (ret < 0)
+		return 0; /* not selectable */
+
+	if (ret > 0 && getchar() == EOF)
+		return 0; /* EOF, eg. /dev/null */
+
+	return 1;
+}
 
 int main(int argc, char **argv)
 {
@@ -395,6 +416,7 @@ int main(int argc, char **argv)
 	struct sockaddr_can addr;
 	struct termios topts;
 	struct ifreq ifr;
+	int select_stdin = 0;
 	int running = 1;
 	int tstamp = 0;
 	int is_open = 0;
@@ -416,6 +438,8 @@ int main(int argc, char **argv)
 		return 1;
 	}
 
+	select_stdin = check_select_stdin();
+
 	/* open pty */
 	p = open(argv[1], O_RDWR);
 	if (p < 0) {
@@ -489,7 +513,10 @@ int main(int argc, char **argv)
 	while (running) {
 
 		FD_ZERO(&rdfs);
+
+		if (select_stdin)
 			FD_SET(0, &rdfs);
+
 		FD_SET(p, &rdfs);
 		FD_SET(s, &rdfs);