Chicken » Locations

## 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

`(define-location NAME TYPE [INIT])` syntax

  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

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

  Defines a lexically bound location.

### location

`(location NAME)` syntax
`(location X)` syntax
`#$` read

  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 section on 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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