readline with a default value

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"

Look at the `rl` example – it's possible to set a startup hook (`rl_startup_hook`) that performs an `rl_insert_text` once and then disables itself. — Jongware, Oct 26 '15 at 11:14
@ameyCU, yes, it must be present in the `rl_line buffer` and in the screen — David Ranieri, Oct 26 '15 at 11:15
@Jongware, works like a charm: [code](http://pastebin.com/qmGLPx2t), thanks a million!! — David Ranieri, Oct 26 '15 at 11:25
@Jongware, your comment seems an answer to me, post your comment as answer and I'll accept it. — David Ranieri, Oct 26 '15 at 11:38
@AlterMann why are you `free(str)` ing that? Its not dynamically allocated so it doesn't need to be free'd — Magisch, Oct 26 '15 at 12:00
@Magisch, are you sure? valgrind returns a memory leak when `free(str);` is not used. — David Ranieri, Oct 26 '15 at 12:06
@Magisch: it's in the documentation: "The line `readline` returns is allocated with `malloc();` the caller should `free()` the line when it has finished with it." — Jongware, Oct 26 '15 at 12:50

score 2 · Accepted Answer · answered Oct 26 '15 at 12:45

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);

readline with a default value

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