Maps are used to bind a variable length external key to an internal id of type int. The size of the external key is given as an argument when a particular map is created. All external keys bound using this map are expected to be of this size. It's important that you use a zero-izing routine like bzero before assigning values to a structure that will be used with the map routines. The C language can have uninitialized data in the interstices of structures (i.e., padding areas), and these can cause structures that are ``equal'' (i.e., all fields have the same values) to fail to map to the same value in the x-kernel .