with-dynamic-lisp-array-pointer

with-dynamic-lisp-array-pointer Macro

Summary

Creates a dynamic-extent foreign pointer which points to the data in a given Lisp array while the forms are executed.

Package

fli

Signature

with-dynamic-lisp-array-pointer (pointer-var lisp-array &key start type) &body body => last

Arguments

`pointer-var`⇩	A variable to be bound to the foreign pointer.
`lisp-array`⇩	A static or pinned Lisp array (a string or a byte/single-float/double-float array).
`start`⇩	An index into the Lisp array.
`type`⇩	A foreign type. The default is :void.
`body`⇩	A list of forms.

Values

last

The value of the last form in body.

Description

The macro with-dynamic-lisp-array-pointer enables the data in a Lisp array to be shared directly with foreign code, without making a copy. A dynamic-extent pointer to the array's data can be used within body wherever the :pointer foreign type allows.

with-dynamic-lisp-array-pointer creates a dynamic extent foreign pointer, with element type type, which is initialized to point to the element of lisp-array at index start. The default value of start is 0.

This foreign pointer is bound to pointer-var, the forms of body are executed and the value of the last form is returned.

Pointers created with this macro must be used with care. There are three restrictions:

lisp-array must be static or pinned, for example allocated as shown below.
The pointer has dynamic extent and lisp-array is guaranteed to be preserved only during the execution of body. If you keep the value of the pointer, you must also preserve lisp-array, that is you must ensure it is not garbage-collected.
Lisp strings and arrays are not null-terminated, therefore foreign code must only access the data of lisp-array up to its known length.

Examples

An example of using a static array:

(let ((vector 
       (make-array 3 :element-type '(unsigned-byte 8)
                   :initial-contents '(65 77 23)
                   :allocation :static)))
  (fli:with-dynamic-lisp-array-pointer 
      (ptr vector :start 1 :type '(:unsigned :byte))
    (fli:dereference ptr)))
=> 
77

An example of using a pinned array:

(let ((vector 
       (make-array 3 :element-type '(unsigned-byte 8)
                   :initial-contents '(65 77 23)
                   :allocation :pinnable)))
  (with-pinned-objects (vector)
    (fli:with-dynamic-lisp-array-pointer 
        (ptr vector :start 1 :type '(:unsigned :byte))
      (fli:dereference ptr))))
=> 
77