c – 具有默认值的readline

2023年12月1日 218次阅读

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,如何使用默认值(不是提示)读取输入,例如：

> ummy
  ^ cursor here

如果用户输入d并输入则返回“dummy”

如果用户键入DEL并且Enter返回“mmy”

最佳答案 在readline的主页上提到了一个可能的用途：

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.

因为编辑现有条目很可能是其中的一部分,所以我决定查看它的来源(direct download link).这确实展示了如何通过使用钩子函数将字符串插入到屏幕上显示的readline之前的缓冲区中：

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.
(07001)

在钩子函数内部,您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.
(07002)

钩子函数只需要执行一次(在readline_internal_setup中每个readline调用只调用一次),但显然rl的作者选择了腰带和吊带方法,并在使用后专门禁用它.

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);
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