Use setjmp and alarm to break out of options handling

src/nyancat.c

@@ -46,6 +46,7 @@
 #include <unistd.h>
 #include <signal.h>
 #include <time.h>
+#include <setjmp.h>
 #include "telnet.h"
 
 /* The animation frames are stored separately. */
@@ -89,6 +90,18 @@ void SIGINT_handler(int sig){
 	exit(0);
 }
 
+jmp_buf environment;
+
+/*
+ * Handle the alarm which breaks us off of options
+ * handling if we didn't receive a terminal
+ */
+void SIGALRM_handler(int sig) {
+	alarm(0);
+	longjmp(environment, 1);
+	/* Unreachable */
+}
+
 void newline(int n) {
 	int i = 0;
 	for (i = 0; i < n; ++i) {
@@ -200,7 +213,10 @@ int main(int argc, char ** argv) {
 				}
 			}
 
+			signal(SIGALRM, SIGALRM_handler);
 			/* Negotiate options */
+			if (!setjmp(environment)) {
+				alarm(1);
 				while (!feof(stdin) && !done) {
 					unsigned char i = getchar();
 					unsigned char opt = 0;
@@ -211,6 +227,7 @@ int main(int argc, char ** argv) {
 								/* End of extended option mode */
 								sb_mode = 0;
 								if (sb[0] == TTYPE) {
+									alarm(0);
 									strcpy(term, &sb[2]);
 									goto ready;
 								}
@@ -274,6 +291,7 @@ int main(int argc, char ** argv) {
 					}
 				}
 			}
+		}
 	} else {
 		/* Otherwise, we were run standalone, find the terminal type from the environement */
 		char * nterm = getenv("TERM");