GNU Readline: Is there a function to cancel readline input requests?

To do this, you must use an alternate (or "callback") interface to read lines. In fact, you don't need to cancel anything, you just (temporarily) break out rl_callback_read_charof the loop to do whatever you need to do. This may even happen before the user sends ENTER , but only after the keypress .

#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("%s? Hah!!\n", line);
}

int main() {
  rl_callback_handler_install("Ask a question: ", &line_handler);

  while (1) {
    rl_callback_read_char();
    if (strstr(rl_line_buffer, "you")) { /* They're asking about *me* =:-0 */
      printf("\nNo personal questions please! Goodbye!\n");
      break;
      /* or make a snarky remark and continue */
    }
  }
}

If you want to "cancel" without pressing a key, you have to use a signal to interrupt the read()system call rl_callback_read_char()(for example, by setting alarm()). Note, however, that readline installs its own signal handlers .

A slightly more complicated approach would be to insert the select()upper 2 file descriptors into the ring, stdinand for example pipe (in the self-pipe trick ) to use that second descriptor (and/or timeout) to "wake up" select(), and then Break out of the loop like in the example below.