Pass Byte Array as std::vector<char> from Node.js to C++ Addon

Question

I have some constraints where the addon is built with nan.h and v8 (not the new node-addon-api).

The end function is a part of a library. It accepts std::vector<char> that represents the bytes of an image.

I tried creating an image buffer from Node.js:

const img = fs.readFileSync('./myImage.png');
myAddonFunction(Buffer.from(img));

I am not really sure how to continue from here. I tried creating a new vector with a buffer, like so:

std::vector<char> buffer(data);

But it seems like I need to give it a size, which I am unsure how to get. Regardless, even when I use the initial buffer size (from Node.js), the image fails to go through.

terminate called after throwing an instance of 'std::bad_alloc'
  what():  std::bad_alloc
[1]    16021 abort (core dumped)

However, when I read the image directly from C++, it all works fine:

std::ifstream ifs ("./myImage.png", std::ios::binary|std::ios::ate);
std::ifstream::pos_type pos = ifs.tellg();
std::vector<char>  buffer(pos);
ifs.seekg(0, std::ios::beg);
ifs.read(&buffer[0], pos);

// further below, I pass "buffer" to the function and it works just fine.

But of course, I need the image to come from Node.js. Maybe Buffer is not what I am looking for?

Maybe a tutorial on how to use Buffer from C++ is what you're looking for ;-) E.g.: community.risingstack.com/using-buffers-node-js-c-plus-plus — jmrk
– jmrk, Commented Nov 5, 2019 at 10:01
@jmrk I was actually searching for such a tutorial but could only find C++ to Node.js. Thank you! I'll take a look. — Ariel Weinberger
– Ariel Weinberger, Commented Nov 5, 2019 at 11:16

Satyan · Accepted Answer · 2019-11-13 23:57:45Z

Here is an example based on N-API; I would also encourage you to take a look similar implementation based on node-addon-api (it is an easy to use C++ wrapper on top of N-API)
https://github.com/nodejs/node-addon-examples/tree/master/array_buffer_to_native/node-addon-api

#include <assert.h>
#include "addon_api.h"
#include "stdio.h"

napi_value CArrayBuffSum(napi_env env, napi_callback_info info)
{
    napi_status status;
    const size_t MaxArgExpected = 1;
    napi_value args[MaxArgExpected];
    size_t argc = sizeof(args) / sizeof(napi_value);

    status = napi_get_cb_info(env, info, &argc, args, nullptr, nullptr);
    assert(status == napi_ok);
    if (argc < 1)
        napi_throw_error(env, "EINVAL", "Too few arguments");

    napi_value buff = args[0];
    napi_valuetype valuetype;
    status = napi_typeof(env, buff, &valuetype);
    assert(status == napi_ok);

    if (valuetype == napi_object)
    {
        bool isArrayBuff = 0;
        status = napi_is_arraybuffer(env, buff, &isArrayBuff);
        assert(status == napi_ok);

        if (isArrayBuff != true)
            napi_throw_error(env, "EINVAL", "Expected an ArrayBuffer");
    }

    int32_t *buff_data = NULL;
    size_t byte_length = 0;
    int32_t sum = 0;

    napi_get_arraybuffer_info(env, buff, (void **)&buff_data, &byte_length);
    assert(status == napi_ok);

    printf("\nC:  Int32Array size = %d,  (ie: bytes=%d)",
           (int)(byte_length / sizeof(int32_t)), (int)byte_length);
    for (int i = 0; i < byte_length / sizeof(int32_t); ++i)
    {
        sum += *(buff_data + i);
        printf("\nC:  Int32ArrayBuff[%d] = %d", i, *(buff_data + i));
    }

    napi_value rcValue;
    napi_create_int32(env, sum, &rcValue);

    return (rcValue);
}

The JavaScript code to call the addon

'use strict'
const myaddon = require('bindings')('mync1');

function test1() {
    const array = new Int32Array(10);

    for (let i = 0; i < 10; ++i)
        array[i] = i * 5;

    const sum = myaddon.ArrayBuffSum(array.buffer);
    console.log();
    console.log(`js: Sum of the array  = ${sum}`);
}

test1();

The Output of the code execution:

C:  Int32Array size = 10,  (ie: bytes=40)
C:  Int32ArrayBuff[0] = 0
C:  Int32ArrayBuff[1] = 5
C:  Int32ArrayBuff[2] = 10
C:  Int32ArrayBuff[3] = 15
C:  Int32ArrayBuff[4] = 20
C:  Int32ArrayBuff[5] = 25
C:  Int32ArrayBuff[6] = 30
C:  Int32ArrayBuff[7] = 35
C:  Int32ArrayBuff[8] = 40
C:  Int32ArrayBuff[9] = 45
js: Sum of the array  = 225

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