6.5  Locations

It is also possible to define variables containing unboxed C data, so called locations. It should be noted that locations may only contain simple data, that is: everything that fits into a machine word, and double-precision floating point values.

[syntax] (define-location NAME TYPE [INIT])
Identical to (define-external NAME TYPE [INIT]), but the variable is not accessible from outside of the current compilation unit (it is declared static).

[syntax] (let-location ((NAME TYPE [INIT]) ...) BODY ...)
Defines a lexically bound location.

[syntax] (location NAME)
NAME should be an external variable defined by define-external or a location defined by define-location or let-location. This form returns a pointer object that contains the address of the variable NAME.

(define-external foo int)
((foreign-lambda* void (((pointer int) ip)) "*ip = 123;") 
  (location foo))
foo                                                                               ==> 123

This facility is especially useful in situations, where a C function returns more than one result value:

#>
#include <math.h>
<#

(define modf
  (foreign-lambda double "modf" double (pointer double)) )

(let-location ([i double])
  (let ([f (modf 1.99 (location i))])
    (print "i=" i ", f=" f) ) )

location returns a value of type c-pointer, when given the name of a callback-procedure defined with define-external.