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.

define-location

 [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).

let-location

 [syntax] (let-location ((NAME TYPE [INIT]) ...) BODY ...)

Defines a lexically bound location.

location

 [syntax] (location NAME)
 [syntax] (location X)

This form returns a pointer object that contains the address of the variable NAME. If the argument to location is not a location defined by define-location, define-external or let-location, then

 (location X)

is essentially equivalent to

 (make-locative X)

(See the manual chapter or locatives for more information about locatives.
Note that (location X) may be abbreviated as #$X.

(define-external foo int)
((foreign-lambda* void (((c-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 (c-pointer double)) )

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

See location and c-string* for a tip on returning a c-string* type.
location returns a value of type c-pointer, when given the name of a callback-procedure defined with define-external.

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