c - GNU Readline：如何清除输入行？

Question

通过注册回调函数，我以“选择”方式使用 GNU Readline：

rl_callback_handler_install("", on_readline_input);

然后连接rl_callback_read_char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，我的程序将消息异步打印到屏幕上，有时会与用户的输入交错。“干净”的会话如下所示：

user input
SERVER OUTPUT
SERVER OUTPUT
user input
SERVER OUTPUT

但是，如果当服务器响应到达时用户正在排队呢？然后它变得丑陋：

user input
SERVER OUTPUT
user inSERVER OUTPUT
put
SERVER OUTPUT

如果用户输入了任何内容（这很容易通过检查来判断），我只需在服务器输出之前打印一个换行符即可解决此问题rl_line_buffer，然后rl_forced_update_display()在打印服务器输出后执行此操作。现在看起来像这样：

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，问题就出现了——然后它看起来像这样：

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

这很糟糕，因为在用户看来他们键入了三个命令（三个输入的三个响应与两个输入的三个响应一样可能，这就是实际发生的情况）。

一个讨厌的黑客（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') 字符而不是 来改进这一点" - INCOMPLETE"，但这在我的终端上似乎根本没有做任何事情（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行呢？要么通过某种方式打印退格（我可以弄清楚要打印多少 - 它是strlen(rl_line_buffer)），还是通过使用一些我还不知道的 Readline 工具？

Answer 1

经过大量黑客攻击后，我能够获得这种机制。我希望其他人会发现它有用。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

带空格？尝试"\b \b"为每个要“删除”的字符而不是单个'\b'.

---

编辑

它是如何工作
的 假设你写了“Hello, world!” 到显示设备，你想替换“世界！” 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，正如您的问题的答案中所描述的那样。

5.2.2节字符显示语义

> 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.

Answer 3

您可以做的一件事是使用\r跳转到服务器输出的行首。然后，您可以使用字段宽度说明符将输出右填充到行的其余部分。实际上，这将覆盖用户已经输入的任何内容。

fprintf (stdout, "\r%-20s\n", "SERVER OUTPUT");

在执行此操作之前，您可能希望fflush(stdout)确保缓冲区处于一致状态。

Answer 4

我试图用 ncurses 窗口分离服务器输出和用户输入。使用线程模拟服务器输出。程序一直运行，直到您输入以 'q' 开头的行。

#include <unistd.h> 
#include <curses.h> 
#include <pthread.h> 

WINDOW *top, *bottom;

int win_update( WINDOW *win, void *data ){
  wprintw(win,"%s", (char*)data ); wrefresh(win);
  return 0;
}

void *top_thread( void *data ){
  char buff[1024];
  int i=0;
  while(1){
    snprintf(buff, 1024, "SERVER OUTPUT: %i\n", i++ );
    use_window( top, win_update, (void*)buff );
    sleep(1);
  }
  return NULL;
}

int main(){
  initscr();
  int maxy, maxx;
  getmaxyx( stdscr, maxy, maxx );

  top = newwin(maxy-1,maxx,0,0);
  wsetscrreg(top,0,maxy-1); idlok(top,1); scrollok(top,1);
  pthread_t top_tid;
  pthread_create(&top_tid, NULL, top_thread, NULL);

  bottom = newwin(1,maxx,maxy-1,0);
  char buff[1024], input[maxx];
  do{
    werase(bottom); wmove(bottom,0,0);
    wprintw(bottom,"input> " ); wrefresh(bottom);
    wgetnstr(bottom,input,sizeof(input));
    snprintf(buff, 1024, "user input: '%s'\n", input );
    use_window( top, win_update, (void*)buff );
  }while( input[0] != 'q' );

  endwin();
}

Answer 5

这些功能有帮助吗？

• rl_reset_line_state()
• rl_clear_message()
• rl_delete_text()
• rl_kill_text()

此外，您能否调解服务器输出 - 控制服务器输出，使其仅在您希望的时间和地点出现，而不仅仅是覆盖用户输入的内容？例如，如果您的应用程序在 curses 模式下运行，您是否可以在一个为用户输入和其余输出（服务器输出和接受的用户输入）保留的子窗口的底部有一个拆分窗口，在底部有一两行上面有第二个子窗口吗？

Answer 6

这似乎也有效：

rl_clear_visible_line();
printf(...);
rl_reset_line_state();
rl_redisplay();