### 7.8 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 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.