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+/*
+From: Jeff Solomon <jsolomon@stanford.edu>
+Date: Fri,  9 Apr 1999 10:13:27 -0700 (PDT)
+To: chet@po.cwru.edu
+Subject: new readline example
+Message-ID: <14094.12094.527305.199695@mrclean.Stanford.EDU>
+
+Chet,
+
+I've been using readline 4.0. Specifically, I've been using the perl
+version Term::ReadLine::Gnu. It works great.
+
+Anyway, I've been playing around the alternate interface and I wanted
+to contribute a little C program, callback.c, to you that you could
+use as an example of the alternate interface in the /examples
+directory of the readline distribution.
+
+My example shows how, using the alternate interface, you can
+interactively change the prompt (which is very nice imo). Also, I
+point out that you must roll your own terminal setting when using the
+alternate interface because readline depreps (using your parlance) the
+terminal while in the user callback. I try to demostrate what I mean
+with an example. I've included the program below.
+
+To compile, I just put the program in the examples directory and made
+the appropriate changes to the EXECUTABLES and OBJECTS line and added
+an additional target 'callback'.
+
+I compiled on my Sun Solaris2.6 box using Sun's cc.
+
+Let me know what you think.
+
+Jeff
+*/
+/*
+Copyright (C) 1999 Jeff Solomon
+*/
+
+#if defined (HAVE_CONFIG_H)
+#include <config.h>
+#endif
+
+#include <sys/types.h>
+
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+#include <stdlib.h>
+
+#include <stdio.h>
+#include <termios.h>	/* xxx - should make this more general */
+
+#ifdef READLINE_LIBRARY
+#  include "readline.h"
+#else
+#  include <readline/readline.h>
+#endif
+
+#ifndef STDIN_FILENO
+#  define STDIN_FILENO 0
+#endif
+
+/* This little examples demonstrates the alternate interface to using readline.
+ * In the alternate interface, the user maintains control over program flow and
+ * only calls readline when STDIN is readable. Using the alternate interface,
+ * you can do anything else while still using readline (like talking to a
+ * network or another program) without blocking.
+ *
+ * Specifically, this program highlights two importants features of the
+ * alternate interface. The first is the ability to interactively change the
+ * prompt, which can't be done using the regular interface since rl_prompt is
+ * read-only.
+ * 
+ * The second feature really highlights a subtle point when using the alternate
+ * interface. That is, readline will not alter the terminal when inside your
+ * callback handler. So let's so, your callback executes a user command that
+ * takes a non-trivial amount of time to complete (seconds). While your
+ * executing the command, the user continues to type keystrokes and expects them
+ * to be re-echoed on the new prompt when it returns. Unfortunately, the default
+ * terminal configuration doesn't do this. After the prompt returns, the user
+ * must hit one additional keystroke and then will see all of his previous
+ * keystrokes. To illustrate this, compile and run this program. Type "sleep" at
+ * the prompt and then type "bar" before the prompt returns (you have 3
+ * seconds). Notice how "bar" is re-echoed on the prompt after the prompt
+ * returns? This is what you expect to happen. Now comment out the 4 lines below
+ * the line that says COMMENT LINE BELOW. Recompile and rerun the program and do
+ * the same thing. When the prompt returns, you should not see "bar". Now type
+ * "f", see how "barf" magically appears? This behavior is un-expected and not
+ * desired.
+ */
+
+void process_line(char *line);
+int  change_prompt(void);
+char *get_prompt(void);
+
+int prompt = 1;
+char prompt_buf[40], line_buf[256];
+tcflag_t old_lflag;
+cc_t     old_vtime;
+struct termios term;
+
+int 
+main()
+{
+    fd_set fds;
+
+    /* Adjust the terminal slightly before the handler is installed. Disable
+     * canonical mode processing and set the input character time flag to be
+     * non-blocking.
+     */
+    if( tcgetattr(STDIN_FILENO, &term) < 0 ) {
+        perror("tcgetattr");
+        exit(1);
+    }
+    old_lflag = term.c_lflag;
+    old_vtime = term.c_cc[VTIME];
+    term.c_lflag &= ~ICANON;
+    term.c_cc[VTIME] = 1;
+    /* COMMENT LINE BELOW - see above */
+    if( tcsetattr(STDIN_FILENO, TCSANOW, &term) < 0 ) {
+        perror("tcsetattr");
+        exit(1);
+    }
+
+    rl_add_defun("change-prompt", change_prompt, CTRL('t'));
+    rl_callback_handler_install(get_prompt(), process_line);
+
+    while(1) {
+      FD_ZERO(&fds);
+      FD_SET(fileno(stdin), &fds);
+
+      if( select(FD_SETSIZE, &fds, NULL, NULL, NULL) < 0) {
+        perror("select");
+        exit(1);
+      }
+
+      if( FD_ISSET(fileno(stdin), &fds) ) {
+        rl_callback_read_char();
+      }
+    }
+}
+
+void
+process_line(char *line)
+{
+  if( line == NULL ) {
+    fprintf(stderr, "\n", line);
+
+    /* reset the old terminal setting before exiting */
+    term.c_lflag     = old_lflag;
+    term.c_cc[VTIME] = old_vtime;
+    if( tcsetattr(STDIN_FILENO, TCSANOW, &term) < 0 ) {
+        perror("tcsetattr");
+        exit(1);
+    }
+    exit(0);
+  }
+
+  if( strcmp(line, "sleep") == 0 ) {
+    sleep(3);
+  } else {
+    fprintf(stderr, "|%s|\n", line);
+  }
+
+  free (line);
+}
+
+int
+change_prompt(void)
+{
+  /* toggle the prompt variable */
+  prompt = !prompt;
+
+  /* save away the current contents of the line */
+  strcpy(line_buf, rl_line_buffer);
+
+  /* install a new handler which will change the prompt and erase the current line */
+  rl_callback_handler_install(get_prompt(), process_line);
+
+  /* insert the old text on the new line */
+  rl_insert_text(line_buf);
+
+  /* redraw the current line - this is an undocumented function. It invokes the
+   * redraw-current-line command.
+   */
+  rl_refresh_line(0, 0);
+}
+
+char *
+get_prompt(void)
+{
+  /* The prompts can even be different lengths! */
+  sprintf(prompt_buf, "%s", 
+    prompt ? "Hit ctrl-t to toggle prompt> " : "Pretty cool huh?> ");
+  return prompt_buf;
+}