+Returns the maximum @code{errno} value for which a corresponding
+symbolic name or message is available. Note that in the case where we
+use the @code{sys_errlist} supplied by the system, it is possible for
+there to be more symbolic names than messages, or vice versa. In
+fact, the manual page for @code{perror(3C)} explicitly warns that one
+should check the size of the table (@code{sys_nerr}) before indexing
+it, since new error codes may be added to the system before they are
+added to the table. Thus @code{sys_nerr} might be smaller than value
+implied by the largest @code{errno} value defined in @code{<errno.h>}.