Lisp data types have very different representations from those used by conventional languages such as C. Since the system interfaces are designed for conventional languages, Lisp must translate objects to and from the Lisp representations. Many simple objects have a direct translation: integers, characters, strings and floating point numbers are translated to the corresponding Lisp object. A number of types, however, are implemented differently in Lisp for reasons of clarity and efficiency.
Instances of enumerated types are expressed as keywords in Lisp.
Records, arrays, and pointer types are implemented with the Alien
facility (see page 
.) Access functions are defined
for these types which convert fields of records, elements of arrays,
or data referenced by pointers into Lisp objects (possibly another
object to be referenced with another access function).
One should dispose of Alien objects created by constructor
functions or returned from remote procedure calls when they are no
longer of any use, freeing the virtual memory associated with that
object. Since Aliens contain pointers to non-Lisp data, the
garbage collector cannot do this itself. If the memory
was obtained from make-alien (page ) or from a foreign function call
to a routine that used malloc, then free-alien (page ) should
be used. If the Alien was created
using MACH memory allocation (e.g. vm_allocate), then the
storage should be freed using vm_deallocate.