The def-alien-routine Macro

[Macro]
alien: def-alien-routine name result-type (aname atype style)

This macro is a convenience for automatically generating Lisp interfaces to simple foreign functions. The primary feature is the parameter style specification, which translates the C pass-by-reference idiom into additional return values.

name is usually a string external symbol, but may also be a symbol Lisp name or a list of the foreign name and the Lisp name. If only one name is specified, the other is automatically derived, (see section 8.4.2.)

result-type is the Alien type of the return value. Each remaining subform specifies an argument to the foreign function. aname is the symbol name of the argument to the constructed function (for documentation) and atype is the Alien type of corresponding foreign argument. The semantics of the actual call are the same as for  alien-funcall (page ). style should be one of the following:

:in

specifies that the argument is passed by value. This is the default. :in arguments have no corresponding return value from the Lisp function.

:out

specifies a pass-by-reference output value. The type of the argument must be a pointer to a fixed sized object (such as an integer or pointer). :out and :in-out cannot be used with pointers to arrays, records or functions. An object of the correct size is allocated, and its address is passed to the foreign function. When the function returns, the contents of this location are returned as one of the values of the Lisp function.

:copy

is similar to :in, but the argument is copied to a pre-allocated object and a pointer to this object is passed to the foreign routine.

:in-out

is a combination of :copy and :out. The argument is copied to a pre-allocated object and a pointer to this object is passed to the foreign routine. On return, the contents of this location is returned as an additional value.

Any efficiency-critical foreign interface function should be inline expanded by preceding def-alien-routine with:

(declaim (inline lisp-name))
  

In addition to avoiding the Lisp call overhead, this allows pointers, word-integers and floats to be passed using non-descriptor representations, avoiding consing (see section 5.11.2.)