- * the Perl interpreter, but otherwise (for XS) the wrapping does
- * not take place. See L<perlxs/Thread-aware system interfaces>.
+ * the Perl interpreter. This is done automatically for the perl core and
+ * extensions, but not generally for XS modules unless they
+ * #define PERL_REENTRANT
+ * See L<perlxs/Thread-aware system interfaces>.
+ *
+ * For a function 'foo', use the compile-time directive
+ * #ifdef PERL_REENTR_USING_FOO_R
+ * to test if the function actually did get replaced by the reentrant version.
+ * (If it isn't getting replaced, it might mean it uses a different prototype
+ * on the given platform than any we are expecting. To fix that, add the
+ * prototype to the __DATA__ section of regen/reentr.pl.)