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