test.scm

;; this function is defined in vector_of_pairs.c and used in this file
(define vector_of_pairs (foreign-safe-lambda void "vector_of_pairs" int))

;; define the scheme callback -- note that this is extern'd in vector_of_pairs.c
(define-external (scm_cb (scheme-object x)) void
    (print x))

;; call C function to generate vector of pairs, passing N into it
(vector_of_pairs 10)

vector_of_pairs.c

#include "chicken.h"

/* This is defined in test.scm */
extern void scm_cb(C_word x);

void vector_of_pairs(int N) {
    int i;
    C_word *vecp, *v0;
    C_word *pairp;

    /* allocate memory for vector and pairs */
    vecp = C_alloc(C_SIZEOF_VECTOR(N));

    /* there are N pairs in the vector */
    pairp = C_alloc(C_SIZEOF_PAIR * N);

    /* v0 is a pointer to the start of the vector's allocated memory */
    v0 = vecp;

    /* first location sets up number of elements in the object */
    *(vecp++) = N;

    /* insert pairs of integers into the vector */
    for (i = 0; i < N; ++i) {
        /* vecp iterates over the memory starting at v0 and inserts pairs */
        *(vecp++) = C_pair(&pairp, C_fix(i), C_fix(N-i));
    }

    /* call the scheme callback with the newly-created vector stored at v0 */
    scm_cb((C_word)v0);
}