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## def-foreign-type

Arguments: name definition

Defines name to be a user-defined foreign class with the given definition. name must either be a symbol or a list beginning with a symbol and followed by attributes (see below). definition is not evaluated and must be a foreign type description as described in The Syntax for Foreign Types in ftype.htm.

Like defclass, def-foreign-type returns the class metaobject. The class metaobject can also be retrieved in the usual way by find-class.

The def-foreign-type macro immediately defines the given class in the current Lisp. It also expands into a form that causes that type definition to be made when the resulting form is evaluated.

Note that when a new foreign type is defined in terms of one or more named previously defined foreign types, the new definition is expressed in terms of the current primitive definition of the named types. If the named types are subsequently re-defined, the new type definition will not change. Thus:

       (def-foreign-type one (:struct (nil :int)))
       (def-foreign-type two one)
       (def-foreign-type one (:struct (nil (:array :int 4))))

The foreign type `two` is not affected by the redefinition of the foreign type `one`.

Only one attribute can be specified. It is:

Attribute | Type | What
:pack | integer | worst case alignment needed by data objects.

For example:

    (def-foreign-type (foostruct (:pack 1)) (:struct (x :char) (y :int)))

Would pack the integer right next to the character, without the normal 3 bytes of padding.

When no `:pack` attribute is specified, the general rule is that a structure field that is N bytes long will always be aligned so that its offset from the beginning of the structure is a multiple of N bytes. Extra unused bytes will be added to the structure just before the field if needed to conform to that rule. If a `:pack` attribute of K is specified, then the forced alignment will never be to a multiple of more than K bytes. (Only powers of 2 make sense for the `:pack` attribute.) Consider the following example:

    (def-foreign-type barstruct
      (:struct (one :long)(two :char)(three :long)))

A `:long` is four bytes long, while a `:char` is one byte long. This would cause three bytes of padding to be inserted before field three, so that field three is aligned at a multiple of four bytes from the beginning of the structure. Now consider this variation:

    (def-foreign-type (barstruct (:pack 2))
      (:struct (one :long)(two :char)(three :long)))

This would limit the alignment of any field to a multiple of 2 bytes at the most, and so only one unused byte would be inserted before field three to align it to a multiple of two bytes from the beginning of the structure.

Note that the native word size of the machine can affect how structure fields are packed, if a version of Allegro that exploits the native word size of the machine is used. Consider this example:

    (def-foreign-type pointerstruct
      (:struct (one :nat)(two :long)(three :nat)))

The `:nat` type indicates the "natural" word length of the machine. When running a 32-bit lisp on a 64-bit machine, however, a `:nat` will still be 32 bits. Therefore, when running a 32-bit lisp on any machine, all three fields of that structure will be four bytes long, and no padding bytes will be inserted. But when running a 64-bit lisp on a 64-bit machine, the `:nat` fields will be eight bytes long while the :long field will still be four. This will cause four padding bytes to be inserted before field three to align it to a multiple of eight bytes, unlike in a 32-bit lisp. Using a `:pack` attribute of 4, on the other hand, would pack the fields in a 64-bit lisp just as it would in a 32-bit lisp.

See ftype.htm for information on foreign types in Allegro CL and foreign-functions.htm for general information on foreign functions in Allegro CL. See in particular the list of types in The Syntax for Foreign Types in that documents.

Copyright (c) 1998-2022, Franz Inc. Lafayette, CA., USA. All rights reserved.
This page has had moderate revisions compared to the 10.0 page.
Created 2019.8.20.

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