A foreign object, or a pointer to a foreign object.
A symbol or a list of symbols identifying the slot to be accessed, as described for foreign-slot-value.
The type of the slot slot-name .
The FLI structure type that contains slot-name . If this is passed, the compiler might be able to optimize the access to the slot. If this is omitted, the object type is determined dynamically from object .
The function
foreign-slot-pointer
takes a foreign object, a slot within the object, and optionally the type of the slot, and returns a pointer to the slot.
In the following example a structure type called
COMPASS
is defined. An instance of the structure is allocated using
allocate-foreign-object, pointed to by
point1
. Then
foreign-slot-pointer
is used to get a pointer, called
point2
, to the second slot of the foreign object.
(fli:define-c-struct COMPASS
(west :int)
(east :int))
(setq point1 (fli:allocate-foreign-object :type
'COMPASS))
(setq point2 (fli:foreign-slot-pointer point1 'east
:type :int))
The
:type
keyword can be used to return the value stored in the slot as a different type, providing the type is compatible. In the next example,
point3
is set to be a pointer to the same address as
point2
, but it expects the value stored there to be a boolean.
(setq point3 (fli:foreign-slot-pointer point1 'east
:type '(:boolean :int)))
Using dereference the value can be set as an integer using
point2
and read as a boolean using
point3
.
(setf (fli:dereference point2) 0)
(fli:dereference point3)
(setf (fli:dereference point2) 1)
(fli:dereference point3)