readline() internal buffer

Using GNU Readline:

The function readline() displays prompt and reads user's input.

Can I modify its internal buffer? and how to achieve that?

#include <readline/readline.h>
#include <readline/history.h>

int main()
{
    char* input;
        // Display prompt and read input 
        input = readline("please enter your name: ");

        // Check for EOF.
        if (!input)
            break;

        // Add input to history.
        add_history(input);

        // Do stuff...

        // Free input.
        free(input);
    }
}

Yes, one can modify readline's edit buffer, e.g. by using the function rl_insert_text(). In order to make this useful, I think you'll need to use readline's slightly more complicated "callback interface" instead of the all-singing and dancing readline() function in your example.

Readline comes with very good and complete documentation, therefore I just give a minimal example program to help you to get started :

/* compile with gcc -o test <this program>.c -lreadline */

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

void line_handler(char *line) { /* This function (callback) gets called by readline
                                   whenever rl_callback_read_char sees an ENTER */ 
  printf("You changed this into: '%s'\n", line);
  exit(0);
}

int main() {
  rl_callback_handler_install("Enter a line: ", &line_handler);
  rl_insert_text("Heheheh...");    /* insert some text into readline's edit buffer... */
  rl_redisplay ();                 /* Make sure we see it ... */

  while (1) {
    rl_callback_read_char();       /* read and process one character from stdin */
  }
}