Function

Summary

Returns the value of a slot in a foreign object.

Package

fli

Signature

foreign-slot-value object
 slot-name
 &key type
 object-type
 copy-foreign-object
 => value

(setf foreign-slot-value) value
 object
 slot-name
 &key type
 object-type
 copy-foreign-object
 => value

Arguments

object

Either an instance of or a pointer to a FLI structure.

slot-name

A symbol or a list of symbols identifying the slot to be accessed.

type

The type of object . Specifying type makes accessing the object faster. If the specified type is different to the actual type, foreign-slot-value returns the value of the object in the format of type where possible.

object-type

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 .

copy-foreign-object

This option is only important when dealing with slots which are aggregate FLI types, and cannot be returned by value. The recognized values are t , nil and :error :

If copy-foreign-object is t , foreign-slot-value makes a copy of the aggregate slot of the object pointed to by pointer and returns the copy.

If copy-foreign-object is nil , foreign-slot-value returns the aggregate slot of the object directly.

If copy-foreign-object is :error then foreign-slot-value signals an error. This is the default value for copy-foreign-object .

Value

value

The value of the slot slot-name in the FLI object object is returned.

Description

The function foreign-slot-value returns the value of a slot in a specified object. An error is signaled if the slot is an aggregate type and copy-foreign-object is not set accordingly. Use foreign-slot-pointer to access such aggregate slots.

If slot-name is a symbol then it names the slot of object to be accessed. If slot-name is a list of symbols, then these symbols name slots in nested structures starting with the outermost structure object , as in the inner / middle / outer example below.

The function (setf foreign-slot-value) can be used to set the value of a slot in a structure, as shown in the example below.

Example

In the following example a foreign structure is defined, an instance of the structure is made with my-pos pointing to the instance, and foreign-slot-value is used to set the y slot of the object to 10.

(fli:define-c-struct POS

  (x :int)

  (y :int)

  (z :int))

(setq my-pos (fli:allocate-foreign-object :type 'POS)) (setf (fli:foreign-slot-value my-pos 'y) 10)

The next forms both return the value of the y slot at my-pos , which is 10.

(fli:foreign-slot-value my-pos 'y) (fli:foreign-slot-value my-pos 'y :object-type 'pos)

See the LispWorks User Guide and Reference Manual section "Optimizing your code" for an example showing how to inline foreign slot access.

This example accesses a slot in nested structures:

(fli:define-c-struct inner

  (v1 :int)

  (v2 :int))

(fli:define-c-struct middle

  (i1 (:struct inner))

  (i2 (:struct inner)))

(fli:define-c-struct outer

  (m1 (:struct middle))

  (m2 (:struct middle)))

(fli:with-dynamic-foreign-objects

    ((obj (:struct outer)))

  (setf (fli:foreign-slot-value obj '(m1 i2 v1)) 99))

See also

foreign-slot-pointer
foreign-slot-offset
dereference
with-foreign-slots