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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 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:


The user entered an affirmative answer.


The user entered a negative answer.


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;
  /* 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.

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