Some Alien type names are Common Lisp symbols, but the names are still exported from the alien package, so it is legal to say alien:single-float. These are the basic Alien type specifiers:
[ Alien type]
* type
A pointer to an object of the specified type. If type is t, then it means a pointer to anything, similar to ``void *'' in ANSI C. Currently, the only way to detect a null pointer is:
See section 6.5(zerop (sap-int (alien-sap ptr)))
[ Alien type]
array type 
dimension
An array of the specified dimensions, holding elements of type type. Note that (* int) and (array int) are considered to be different types when type checking is done; pointer and array types must be explicitly coerced using cast.
Arrays are accessed using deref, passing the indices as
additional arguments. Elements are stored in column-major order (as
in C), so the first dimension determines only the size of the memory
block, and not the layout of the higher dimensions. An array whose
first dimension is variable may be specified by using nil as the
first dimension. Fixed-size arrays can be allocated as array
elements, structure slots or with-alien variables. Dynamic
arrays can only be allocated using make-alien (page 
).
[ Alien type]
struct name
(field type bits
)
A structure type with the specified name and fields. Fields are allocated at the same positions used by the implementation's C compiler. bits is intended for C-like bit field support, but is currently unused. If name is nil, then the type is anonymous.
If a named Alien struct specifier is passed to
def-alien-type (page ) or with-alien (page ), then this defines,
respectively, a new global or local Alien structure type. If no
fields are specified, then the fields are taken from the
current (local or global) Alien structure type definition of
name.
[ Alien type]
union name
(field type bits)
Similar to struct, but defines a union type. All fields are allocated at the same offset, and the size of the union is the size of the largest field. The programmer must determine which field is active from context.
[ Alien type]
enum name spec
An enumeration type that maps between integer values and keywords. If name is nil, then the type is anonymous. Each spec is either a keyword, or a list (keyword value). If integer is not supplied, then it defaults to one greater than the value for the preceding spec (or to zero if it is the first spec.)
[ Alien type]
signed bits
A signed integer with the specified number of bits precision. The
upper limit on integer precision is determined by the machine's word
size. If no size is specified, the maximum size will be used.
[ Alien type]
integer bits
Identical to signed--the distinction between signed
and integer is purely stylistic.
[ Alien type]
unsigned bits
Like signed, but specifies an unsigned integer.
[ Alien type]
boolean bits
Similar to an enumeration type that maps 0 to nil and
all other values to t. bits determines the amount of
storage allocated to hold the truth value.
[ Alien type]
single-float
A floating-point number in IEEE single format.
[ Alien type]
double-float
A floating-point number in IEEE double format.
[ Alien type]
function result-type arg-type
A Alien function that takes arguments of the specified
arg-types and returns a result of type result-type.
Note that the only context where a function type is directly
specified is in the argument to alien-funcall (see section
alien-funcall (page ).) In all other contexts, functions are
represented by function pointer types: (* (function ...)).
[ Alien type]
system-area-pointer
A pointer which is represented in Lisp as a
system-area-pointer object (see section 6.5.)