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From: Jeff Solomon <>
Date: Fri, 9 Apr 1999 10:13:27 -0700 (PDT)
Subject: new readline example
Message-ID: <14094.12094.527305.199695@mrclean.Stanford.EDU>
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.
#if defined (HAVE_CONFIG_H)
#include <config.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <termios.h> /* xxx - should make this more general */
# include "readline.h"
# include <readline/readline.h>
/* 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;
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 ) {
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 ) {
rl_add_defun("change-prompt", change_prompt, CTRL('t'));
rl_callback_handler_install(get_prompt(), process_line);
while(1) {
FD_SET(fileno(stdin), &fds);
if( select(FD_SETSIZE, &fds, NULL, NULL, NULL) < 0) {
if( FD_ISSET(fileno(stdin), &fds) ) {
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 ) {
if( strcmp(line, "sleep") == 0 ) {
} else {
fprintf(stderr, "|%s|\n", line);
free (line);
/* 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 */
/* redraw the current line - this is an undocumented function. It invokes the
* redraw-current-line command.
rl_refresh_line(0, 0);
char *
/* The prompts can even be different lengths! */
sprintf(prompt_buf, "%s",
prompt ? "Hit ctrl-t to toggle prompt> " : "Pretty cool huh?> ");
return prompt_buf;