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(libc.info)Yes-or-No Questions

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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)
     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 GNU libc
     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, stdout) >= 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 an read error is detected or
until a definitive (positive or negative) answer is read.