Converting node.js buffer into a ref-struct instance

Question

I am writing a program in both C and Javascript (on node.js), using ffi, ref, and a few other ref- packages.

I have the following code, which I compile into a library libfun.so:

fun.c

#include "fun.h"
#include <stdio.h>
#include <stdlib.h>


void fill_array(void **data_array, int length)
{
    int i;
    for (i = 0; i < length; i++) {
        data_array[i] = malloc(sizeof(data));
        ((data *)data_array[i])->id = 256;
        ((data *)data_array[i])->message = 512;
    }   
}

void print_array(void **data_array, int length)
{
    int i = 0;
    for (i = 0; i < length; i++) {
        printf("(%d, %d)\n", ((data *)data_array[i])->id, 
               ((data *)data_array[i])->message);
    }
}

fun.h

#ifndef fun_h__
#define fun_h__

typedef struct {
    int id;
    int message;
} data;

void fill_array(void **,int);
void print_array(void **,int);

#endif

fun.js

var ffi = require('ffi');
var Struct = require('ref-struct');
var ref = require('ref');
var ArrayType = require('ref-array');

// js analog of the data struct from fun.h
var Data = Struct({
  id: ref.types.int,
  message: ref.types.int,
});
// js analog of the type data *
var DataPointer = ref.refType(Data);

// pvoid is the js analog of void * in C
var pvoid = ref.refType(ref.types.void);
var PVoidArray = ArrayType(pvoid);

// set up our foreign functions from libfun
var libfun = ffi.Library('./libfun', {
    'fill_array' : ['void', [PVoidArray,ref.types.int]],
    'print_array' : ['void', [PVoidArray, ref.types.int]]
});
var myArray = new PVoidArray(10);
libfun.fill_array(myArray,10);
libfun.print_array(myArray,10); // this prints the array of structs correctly, from the C side

My question is: how can I print the array of structs from the Javascript side? I want to pass myArray in as a PVoidArray. I do not want to create an array of structs (i.e. create var DataArray = ArrayType(DataPointer), and use that instead of PVoidArray everywhere).

Let's start with myArray[0]. Can we use our variable Data to (in a flexible way) take myArray[0] and make a struct? Like some function bufferToArray(myArray[0],Data) == a Data instance containing the data of myArray[0].

I can't test this, but let me know if this works: ref.get(myArray, someIndex, DataPointer).deref() or ref.get(ref.get(myArray, someIndex), 0, Data) — Patrick Roberts
– Patrick Roberts, Commented Feb 16, 2018 at 23:26
ref.get(myArray.buffer,0,DataPointer).deref() works. Thanks. I'll be happy to accept this as an answer. — user9078927
– user9078927, Commented Feb 18, 2018 at 18:04

Patrick Roberts · Accepted Answer · 2018-02-18 19:05:29Z

1

+50

Looking at the documentation for ref.get(), you could use that:

ref.get(myArray.buffer, index, DataPointer).deref()

will return an instance of Data from index of myArray.

answered Feb 18, 2018 at 19:05

Patrick Roberts

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