+.Pp
+If a new protocol family is defined,
+the socreate process is free to return any desired error code.
+The
+.Fn socket
+system call will pass this error code along
+(even if it is undefined).
+.Sh LEGACY SYNOPSIS
+.Fd #include <sys/types.h>
+.Fd #include <sys/socket.h>
+.Pp
+The include file
+.In sys/types.h
+is necessary.