### 7.9 Yes-or-No Questions ¶

Some non GUI programs ask a yes-or-no question. If the messages (especially the questions) are translated into foreign languages, be sure that you localize the answers too. It would be very bad habit to ask a question in one language and request the answer in another, often English.

The GNU C Library contains `rpmatch` to give applications easy access to the corresponding locale definitions.

Function: `int` rpmatch `(const char *response)` ¶

  Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

  The function `rpmatch` checks the string in response for whether or not it is a correct yes-or-no answer and if yes, which one. The check uses the `YESEXPR` and `NOEXPR` data in the `LC_MESSAGES` category of the currently selected locale. The return value is as follows:

  `1`

    The user entered an affirmative answer.

  `0`

    The user entered a negative answer.

  `-1`

    The answer matched neither the `YESEXPR` nor the `NOEXPR` regular expression.

  This function is not standardized but available beside in the GNU C Library at least also in the IBM AIX library.

This function would normally be used like this:

      ...
      /* Use a safe default.  */
      _Bool doit = false;

      fputs (gettext ("Do you really want to do this? "), stdout);
      fflush (stdout);
      /* Prepare the getline call.  */
      line = NULL;
      len = 0;
      while (getline (&line, &len, stdin) >= 0)
        {
          /* Check the response.  */
          int res = rpmatch (line);
          if (res >= 0)
            {
              /* We got a definitive answer.  */
              if (res > 0)
                doit = true;
              break;
            }
        }
      /* Free what getline allocated.  */
      free (line);

Note that the loop continues until a read error is detected or until a definitive (positive or negative) answer is read.