foreign-slot-pointer Function

Summary

Returns a pointer to a specified slot of an object.

Package

fli

Signature

foreign-slot-pointer object slot-name &key type object-type => pointer

Arguments

Values

Description

The function foreign-slot-pointer returns a foreign pointer to the slot slot-name in object.

If type is supplied, then foreign-slot-pointer assumes that the slot contains an object of that type, overriding the type in the structure definition.

If object-type is supplied then foreign-slot-pointer assumes that object is of the that type and the compiler might be able to optimize the access to the slot. If object-type is not supplied, then the object type is determined dynamically from object.

Examples

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)

See also

2.2.3 Structures and unions
decf-pointer
incf-pointer
make-pointer
foreign-slot-value
foreign-slot-offset