GNU Readline:如何清除输入行？

Question

我通过注册回调函数,以"选择"的方式使用GNU Readline:

rl_callback_handler_install("", on_readline_input);

然后挂钩rl_callback_read_char作为我的select()循环的回调STDIN_FILENO.这都是非常标准的东西,并且工作正常.

现在,我的程序异步地将消息打印到屏幕上,有时与用户的输入交错."干净"的会话看起来像这样:

user input
SERVER OUTPUT
SERVER OUTPUT
user input
SERVER OUTPUT

但是,如果用户在服务器响应到达时位于线路中间,该怎么办？然后它变得丑陋:

user input
SERVER OUTPUT
user inSERVER OUTPUT
put
SERVER OUTPUT

如果用户输入任何内容(通过检查很容易判断rl_line_buffer),然后rl_forced_update_display()在打印服务器输出后执 现在它看起来像这样:

user input
SERVER OUTPUT
user in
SERVER OUTPUT
user input
SERVER OUTPUT

这更好,但仍然不完美.当用户键入整行但尚未按Enter键时出现问题 - 那么它看起来像这样:

user input
SERVER OUTPUT
user input
SERVER OUTPUT
user input
SERVER OUTPUT

这很糟糕,因为用户看起来他们输入了三个命令(三个输入的三个响应与两个输入的三个响应一样可能,这实际上是发生的).

讨厌的黑客(有效)是这样做的:

user input
SERVER OUTPUT
user input - INCOMPLETE
SERVER OUTPUT
user input
SERVER OUTPUT

我想我可以通过打印退格('\ b')字符来改进这一点而不是" - INCOMPLETE",但这似乎在我的终端上没有做任何事情(Ubuntu Hardy上的gnome-terminal). printf("ABC\b");只是打印ABC,无论出于何种原因.

那么如何擦除不完整的输入线？要么以某种方式打印退格(我可以弄清楚要打印多少 - 它是strlen(rl_line_buffer)),或者使用一些我还不知道的Readline工具？

Answer 1

经过大量的黑客攻击，我能够获得这种机制。我希望其他人会觉得它有用。它甚至不使用 select()，但我希望你能明白这一点。

#include <readline/readline.h>
    #include <readline/history.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <stdlib.h>

    const char const* prompt = "PROMPT> ";

    void printlog(int c) {
        char* saved_line;
        int saved_point;
        saved_point = rl_point;
        saved_line = rl_copy_text(0, rl_end);
        rl_set_prompt("");
        rl_replace_line("", 0);
        rl_redisplay();
        printf("Message: %d\n", c);
        rl_set_prompt(prompt);
        rl_replace_line(saved_line, 0);
        rl_point = saved_point;
        rl_redisplay();
        free(saved_line);
    }


    void handle_line(char* ch) {
        printf("%s\n", ch);
        add_history(ch);
    }

    int main() {
        int c = 1;

        printf("Start.\n");
        rl_callback_handler_install(prompt, handle_line);

        while (1) {
            if (((++c) % 5) == 0) {
                printlog(c);
            }

            usleep(10);
            rl_callback_read_char();
        }
        rl_callback_handler_remove();
    }

Answer 2

有空格吗？尝试打印"\b \b"您想要“删除”的每个字符，而不是单个'\b'.

编辑

它是如何工作的
假设您写了“Hello, world!” 到显示设备并且您想要替换“world!” 与“吉姆”。

Hello, world!
             ^ /* active position */ /* now write "\b \b" */
               /* '\b' moves the active position back;
               // ' ' writes a space (erases the '!')
               // and another '\b' to go back again */
Hello, world
            ^ /* active position */ /* now write "\b \b" again */
Hello, worl
           ^ /* active position */ /* now write "\b \b" 4 times ... */
Hello, 
       ^ /* active position */ /* now write "Jim." */
Hello, Jim.
           ^ /* active position */

可移植性
我不确定，但标准具体描述了 '\b' 和 '\r' 的行为，正如您问题的答案中所描述的那样。

5.2.2 字符显示语义

> 1   The active position is that location on a display device where the next character output by
>     the fputc function would appear. The intent of writing a printing character (as defined
>     by the isprint function) to a display device is to display a graphic representation of
>     that character at the active position and then advance the active position to the next
>     position on the current line. The direction of writing is locale-specific. If the active
>     position is at the final position of a line (if there is one), the behavior of the display devic e
>     is unspecified.
>  
> 2   Alphabetic escape sequences representing nongraphic characters in the execution
>     character set are intended to produce actions on display devices as follows:
>     \a (alert) Produces an audible or visible alert without changing the active position.
>     \b (backspace) Moves the active position to the previous position on the current line. If
>        the active position is at the initial position of a line, the behavior of the display
>        device is unspecified.
>     \f ( form feed) Moves the active position to the initial position at the start of the next
>        logical page.
>     \n (new line) Moves the active position to the initial position of the next line.
>     \r (carriage return) Moves the active position to the initial position of the current line.
>     \t (horizontal tab) Moves the active position to the next horizontal tabulation position
>        on the current line. If the active position is at or past the last defined horizontal
>        tabulation position, the behavior of the display device is unspecified.
>     \v (vertical tab) Moves the active position to the initial position of the next vertical
>         tabulation position. If the active position is at or past the last defined vertical
>         tabulation position, the behavior of the display device is unspecified.
>  
> 3   Each of these escape sequences shall produce a unique implementation-defined value
>     which can be stored in a single char object. The external representations in a text file
>     need not be identical to the internal representations, and are outside the scope of this
>     International Standard.