readline with a default value - c

Question

I am able to limit the user input to 5 characters using GNU readline:
#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;
}
But, how to read an input with a default value (not a prompt), e.g.:
> ummy
^ cursor here
if user types d and Enter return "dummy"
if user types DEL and Enter return "mmy"

Answer 1

On the homepage of readline a possible use is mentioned:
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.
and since editing an existing entry could well be part of this, I decided to take a look at its source (direct download link). This indeed shows how to insert a string into the buffer used by readline before it appears on the screen, through the use of a hook function:
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)
Inside the hook function you can directly manipulate the internal buffer, for example to insert text:
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)
The hook function only needs to do this once (it is called only once per readline call inside readline_internal_setup), but apparently rl's author went for the belt-and-suspenders approach and specifically disables it after it has been used.
Relevant snippets from rl.c, comments are mine:
/* 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);