+ ASSERT (errno == 0);
+ ASSERT (strstr (str, "nknown") == NULL);
+ ASSERT (strstr (str, "ndefined") == NULL);
+
+ /* POSIX requires strerror to produce a non-NULL result for all
+ inputs; as an extension, we also guarantee a non-empty reseult.
+ Reporting EINVAL is optional. */
+ errno = 0;