+STATIC void
+S_fixup_errno_string(pTHX_ SV* sv)
+{
+ /* Do what is necessary to fixup the non-empty string in 'sv' for return to
+ * Perl space. */
+
+ PERL_ARGS_ASSERT_FIXUP_ERRNO_STRING;
+
+ assert(SvOK(sv));
+
+ if(strEQ(SvPVX(sv), "")) {
+ sv_catpv(sv, UNKNOWN_ERRNO_MSG);
+ }
+ else {
+
+ /* In some locales the error string may come back as UTF-8, in which
+ * case we should turn on that flag. This didn't use to happen, and to
+ * avoid as many possible backward compatibility issues as possible, we
+ * don't turn on the flag unless we have to. So the flag stays off for
+ * an entirely invariant string. We assume that if the string looks
+ * like UTF-8 in a single script, it really is UTF-8: "text in any
+ * other encoding that uses bytes with the high bit set is extremely
+ * unlikely to pass a UTF-8 validity test"
+ * (http://en.wikipedia.org/wiki/Charset_detection). There is a
+ * potential that we will get it wrong however, especially on short
+ * error message text, so do an additional check. */
+ if ( ! IN_BYTES /* respect 'use bytes' */
+ && is_utf8_non_invariant_string((U8*) SvPVX_const(sv), SvCUR(sv))
+
+#ifdef USE_LOCALE_MESSAGES
+
+ && _is_cur_LC_category_utf8(LC_MESSAGES)
+
+#else /* If can't check directly, at least can see if script is consistent,
+ under UTF-8, which gives us an extra measure of confidence. */
+
+ && isSCRIPT_RUN((const U8 *) SvPVX_const(sv), (U8 *) SvEND(sv),
+ TRUE) /* Means assume UTF-8 */
+#endif
+
+ ) {
+ SvUTF8_on(sv);
+ }
+ }
+}
+
+/*
+=for apidoc sv_string_from_errnum
+
+Generates the message string describing an OS error and returns it as
+an SV. C<errnum> must be a value that C<errno> could take, identifying
+the type of error.
+
+If C<tgtsv> is non-null then the string will be written into that SV
+(overwriting existing content) and it will be returned. If C<tgtsv>
+is a null pointer then the string will be written into a new mortal SV
+which will be returned.
+
+The message will be taken from whatever locale would be used by C<$!>,
+and will be encoded in the SV in whatever manner would be used by C<$!>.
+The details of this process are subject to future change. Currently,
+the message is taken from the C locale by default (usually producing an
+English message), and from the currently selected locale when in the scope
+of the C<use locale> pragma. A heuristic attempt is made to decode the
+message from the locale's character encoding, but it will only be decoded
+as either UTF-8 or ISO-8859-1. It is always correctly decoded in a UTF-8
+locale, usually in an ISO-8859-1 locale, and never in any other locale.
+
+The SV is always returned containing an actual string, and with no other
+OK bits set. Unlike C<$!>, a message is even yielded for C<errnum> zero
+(meaning success), and if no useful message is available then a useless
+string (currently empty) is returned.
+
+=cut
+*/
+
+SV *
+Perl_sv_string_from_errnum(pTHX_ int errnum, SV *tgtsv)
+{
+ char const *errstr;
+ if(!tgtsv)
+ tgtsv = sv_newmortal();
+ errstr = my_strerror(errnum);
+ if(errstr) {
+ sv_setpv(tgtsv, errstr);
+ fixup_errno_string(tgtsv);
+ } else {
+ SvPVCLEAR(tgtsv);
+ }
+ return tgtsv;
+}
+