[racket] how can I convert a list or vector to ctype array directly?

Instead through iteration like for or do which assign values to element of array, deliver list or vector directly to the ctype function imported by ffi.
Because I think that operating the list or vector with built-in functions is more conveniently than the ctype array.
In the reference, I find _array/list and _array/vector function, here is the description:

"Like _array, but the Racket representation is a list (or list of lists for a multidimensional array) of elements copied to and from an underlying C array."

and I test the two functions, after deliver their cpointers as arguments to ctype function, the list or vector doesn't change. here is my code:

#lang racket
(require ffi/unsafe
         ffi/unsafe/define)

(define-ffi-definer define-sm (ffi-lib "scalam"))

;;----------------------------------------------
 ; sm add element of array with 1.
;
; int sm(int m, int n, double * p)
; {
;     int i, j;
;     double *p_end;
;     p_end = p + (m * n) - 1;
;    for (;p <= p_end; p++)
;         (*p) = (* p) + 1;
;    
;     return 0;
;  }
;---------------------------------------------------------
(define-sm sm (_fun _int _int  _pointer -> _int)) 

(define t (_array/vector _double 3))
(define x (malloc t))
(define a (ptr-ref x t 0))

(for ((i '(0 1 2)))
    (vector-set! a i (+ i 1.0)))

(define info (sm 3 1 x))

(for ((i '(0 1 2)))
    (display (vector-ref a i))

;=====================
a doesn't change. or is there something wrong?

thank you.
------------------
lhk
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