c - 具有默认值的 readline

Question

我可以使用 GNU readline 将用户输入限制为 5 个字符:

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

static int limit_rl(FILE *f)
{
    if (rl_end > 5) {
        return '\b';
    }
    return rl_getc(f);
}

int main(void)
{
    char *str;

    rl_getc_function = limit_rl;
    str = readline("> ");
    printf("%s\n", str);
    free(str);
    return 0;
}

但是，如何读取具有默认值(不是提示)的输入，例如:

> ummy
  ^ cursor here

如果用户输入 d 和 Enter 返回“dummy”

如果用户键入 DEL 和 Enter 返回“mmy”

Answer 1

在 readline 的主页上提到了一种可能的用法:

rl.c is an example program that uses Readline to read a line of input from a user and echo it to the standard output, suitable for use by shell scripts.

由于编辑现有条目很可能是其中的一部分，所以我决定查看其来源 (direct download link)。这确实展示了如何通过使用 Hook 函数将字符串插入 readline 在它出现在屏幕上之前使用的缓冲区中:

Variable: rl_hook_func_t * rl_startup_hook

If non-zero, this is the address of a function to call just before readline prints the first prompt.
(https://cnswww.cns.cwru.edu/php/chet/readline/readline.html#IDX223)

在钩子(Hook)函数中你可以直接操作内部缓冲区，例如插入文本:

Function: int rl_insert_text (const char *text)

Insert text into the line at the current cursor position. Returns the number of characters inserted.
(https://cnswww.cns.cwru.edu/php/chet/readline/readline.html#IDX295)

hook 函数只需要执行一次(在 readline_internal_setup 中每次 readline 调用只调用一次)，但显然 rl 的作者选择了腰带 -吊带接近并在使用后特别禁用它。

来自 rl.c 的相关片段，评论是我的:

/* a global char * to hold a default initial text */
static char *deftext;

/* the callback function. The argument is supposed to be 'void' per
   its declaration:
       typedef int rl_hook_func_t (void);
   so you cannot provide the default text here */
static int set_deftext ()
{
  if (deftext)
    {
      /* Apparently the "current cursor position" in which text is inserted
         is 0, when initially called */
      rl_insert_text (deftext);
      deftext = (char *)NULL;

      /* disable the global 'rl_startup_hook' function by setting it to NULL */
      rl_startup_hook = (rl_hook_func_t *)NULL;
    }
  return 0;
}

// ...
if (deftext && *deftext)
   rl_startup_hook = set_deftext;

temp = readline (prompt);