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7.7.1 The alien-funcall Primitive

— Function: sb-alien:alien-funcall alien-function &rest arguments

The alien-funcall function is the foreign function call primitive: alien-function is called with the supplied arguments and its C return value is returned as a Lisp value. The alien-function is an arbitrary run-time expression; to refer to a constant function, use extern-alien or a value defined by define-alien-routine.

The type of alien-function must be (alien (function ...)) or (alien (* (function ...))). The function type is used to determine how to call the function (as though it was declared with a prototype.) The type need not be known at compile time, but only known-type calls are efficiently compiled. Limitations:

Here is an example which allocates a (struct foo), calls a foreign function to initialize it, then returns a Lisp vector of all the (* (struct foo)) objects filled in by the foreign call:

     ;; Allocate a foo on the stack.
     (with-alien ((f (struct foo)))
       ;; Call some C function to fill in foo fields.
       (alien-funcall (extern-alien "mangle_foo" (function void (* foo)))
                      (addr f))
       ;; Find how many foos to use by getting the A field.
       (let* ((num (slot f 'a))
              (result (make-array num)))
         ;; Get a pointer to the array so that we don't have to keep extracting it:
         (with-alien ((a (* (array (* (struct foo)) 100)) (addr (slot f 'b))))
           ;; Loop over the first N elements and stash them in the result vector.
           (dotimes (i num)
             (setf (svref result i) (deref (deref a) i)))
           ;; Voila.
           result)))