Handle SIGTTIN and SIGTTOU on Unix-like OSs with X11 graphics.

That makes it possible to push ngspice into the background while
plot windows are open: the windows remain responsive.
Discussion on ngspice-users, starting 2023-09-01,
title: "`quit` in Interactive Mode."
Also restore terminal state when forcing exit with SIGINT (control-C).

src/frontend/control.c

@@ -615,9 +615,10 @@ getcommand(char *string)
 #if !defined(HAVE_GNUREADLINE) && !defined(HAVE_BSDEDITLINE)
     /* set cp_altprompt for use by the lexer - see parser/lexical.c */
     cp_altprompt = get_alt_prompt();
+#else
+    cp_cwait = TRUE;
 #endif /* !defined(HAVE_GNUREADLINE) && !defined(HAVE_BSDEDITLINE) */
 
-    cp_cwait = TRUE;
     wlist = cp_parse(string);
     cp_cwait = FALSE;
     if (cp_debug) {

src/frontend/signal_handler.c

@@ -91,6 +91,7 @@ ft_sigintr(void)
 
     if (interrupt_counter >= 3) {
         fprintf(cp_err, "\nKilling, since %d interrupts have been requested\n\n", interrupt_counter);
+        cp_ccon(FALSE);
         controlled_exit(1);
     }
 
@@ -99,6 +100,7 @@ ft_sigintr(void)
     }
 
     /* here we jump to the start of command processing in main() after resetting everything.  */
+    cp_background = FALSE;
     LONGJMP(jbuf, 1);
 }
 
@@ -112,6 +114,32 @@ sigfloat(int code)
     LONGJMP(jbuf, 1);
 }
 
+/* Shared handler for SIGTTIN and SIGTTOU.  Restart event handling if caught
+ * attempting terminal IO as a background process.
+ */
+
+bool cp_background = FALSE;
+
+#ifdef SIGTTIN
+void
+sigttio(void)
+{
+    if (cp_cwait) {
+        /* Attempted command input/output on the terminal while in background.
+         * Set background flag and restart event loop.
+         */
+        cp_background = TRUE;
+        LONGJMP(jbuf, 1);
+    } else {
+        /* Non-command terminal IO in background. That should never happen.
+         * Stop.
+         */
+
+        (void) signal(SIGTSTP, SIG_DFL);
+        (void) kill(getpid(), SIGTSTP); /* This should stop us */
+    }
+}
+#endif
 
 /* This should give a new prompt if cshpar is waiting for input.  */
 
@@ -122,8 +150,10 @@ sigstop(void)
 {
     gr_clean();
     cp_ccon(FALSE);
-    (void) signal(SIGTSTP, SIG_DFL);
+    if (!cp_background) {
-    (void) kill(getpid(), SIGTSTP); /* This should stop us */
+        (void) signal(SIGTSTP, SIG_DFL);
+        (void) kill(getpid(), SIGTSTP); /* This should stop us */
+    }
 }
 
 

src/frontend/signal_handler.h

@@ -8,6 +8,7 @@
 
 void ft_sigintr(void);
 void sigfloat(int code);
+void sigttio(void);
 void sigstop(void);
 void sigcont(void);
 void sigill(void);

src/include/ngspice/cpextern.h

@@ -67,7 +67,8 @@ extern wordlist *cp_parse(char *string);
 
 /* control.c */
 
-extern bool cp_cwait;
+extern bool cp_cwait;        // Interactive and awaiting command input.
+extern bool cp_background;   // Running in background.
 extern bool cp_dounixcom;
 extern char *cp_csep;
 extern char * get_alt_prompt(void);

src/main.c

@@ -681,10 +681,32 @@ app_rl_readlines(void)
         history_set_pos(history_length);
 
         if (SETJMP(jbuf, 1)) { /* Set location to jump to after handling SIGINT (ctrl-C)  */
-            ft_sigintr_cleanup();
+            if (!cp_background)
+                ft_sigintr_cleanup();
         }
 
-        line = readline(prompt());
+#if defined(SIGTTIN) && !defined(X_DISPLAY_MISSING)
+        if (cp_background) {
+            /* This process is running in the background, so reading from
+             * the terminal will fail.  Instead, call the X11 input loop
+             * directly.  It will process X11 events until terminal input
+             * is available, then return. If the process is still in background
+             * readline() will then cause another SIGTTIN and this loop
+             * will restart at SETJMP().  Such polling sees to be the only way
+             * to detect a return to the foreground.
+             *
+             * Global cp_cwait is set early so that SIGTTOU from output
+             * caused by clicking in a plot window will not stop the program.
+             */
+
+            cp_cwait = TRUE;
+            app_event_func(); // Direct call to process X11 input.
+        }
+#endif
+        cp_cwait = TRUE;
+        line = readline(cp_background ? NULL : prompt());
+        cp_cwait = FALSE;
+        cp_background = FALSE;
 
         if (!line) {
             cp_evloop("quit");
@@ -706,7 +728,6 @@ app_rl_readlines(void)
             }
             tfree(expanded_line);
         }
-
         tfree(line);
     }
     /* History gets written in ../fte/misccoms.c com_quit */
@@ -1189,6 +1210,10 @@ int main(int argc, char **argv)
 
 #ifdef SIGTSTP
         signal(SIGTSTP, (SIGNAL_FUNCTION) sigstop);
+#endif
+#ifdef SIGTTIN
+        signal(SIGTTIN, (SIGNAL_FUNCTION) sigttio);
+        signal(SIGTTOU, (SIGNAL_FUNCTION) sigttio);
 #endif
     }