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-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.