ffi/unsafe: add `array-length'

Closes PR 12576

collects/ffi/unsafe.rkt

@@ -973,7 +973,7 @@
 
 ;; (_array <type> <len> ...+)
 (provide _array
-         array? array-ptr
+         array? array-length array-ptr
          (protect-out array-ref array-set!))
 
 (define _array
@@ -985,11 +985,11 @@
    [(t n . ns)
     (_array (apply _array t ns) n)]))
 
-(define-struct array (ptr type len))
+(define-struct array (ptr type length))
 (define array-ref
   (case-lambda
    [(a i) 
-    (let ([len (array-len a)])
+    (let ([len (array-length a)])
       (if (< -1 i len)
           (ptr-ref (array-ptr a) (array-type a) i)
           (raise-mismatch-error 'array-ref "index out of bounds: " i)))]
@@ -1001,7 +1001,7 @@
 (define array-set!
   (case-lambda
    [(a i v)
-    (let ([len (array-len a)])
+    (let ([len (array-length a)])
       (if (< -1 i len)
           (ptr-set! (array-ptr a) (array-type a) i v)
           (raise-mismatch-error 'array-ref "index out of bounds: " i)))]

collects/scribblings/foreign/types.scrbl

@@ -1137,6 +1137,13 @@ sub-array).}
 Extracts the pointer for an array's storage.}
 
 
+@defproc[(array-length [a array?]) exact-nonnegative-integer?]{
+
+Extracts the length of an array. For a multidimensional array, the
+result is still a single number; extract an element to get
+a sub-array to get the length of the next dimension, and so on.}
+
+
 @defproc[(_array/list [type ctype?] [count exact-nonnegative-integer?] ...+)
          ctype?]{
 

collects/tests/racket/foreign-test.rktl

@@ -223,6 +223,7 @@
     (test (for/list ([i 7]) (+ i 10)) cast p _pointer (_list o _byte 7))
     (t (for/list ([i 7]) (+ i 11)) 'increment_c_array (_fun _pointer -> (_list o _byte 7)) p)
     (let ([a (ptr-ref p (_array _byte 7))])
+      (test 7 array-length a)
       (test 12 array-ref a 1)
       (ptr-set! p _byte 1 17)
       (test 17 array-ref a 1)))