c++ - 将字节数组作为 std::vector<char> 从 Node.js 传递到 C++ Addon

Question

我有一些限制，其中插件是使用 nan.h 和 v8(不是新的 node-addon-api)构建的。

结束函数是库的一部分。它接受 std::vector<char>表示图像的字节。

我尝试从 Node.js 创建图像缓冲区:

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

我不太确定如何从这里继续。我尝试创建一个带有缓冲区的新 vector ，如下所示:

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

但似乎我需要给它一个尺寸，但我不确定如何获得。无论如何，即使我使用初始缓冲区大小(来自 Node.js)，图像也无法通过。

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

但是，当我直接从 C++ 读取图像时，一切正常:

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. 

当然，我需要图像来自 Node.js。也许 Buffer 不是我要找的？

Answer 1

这里是一个基于N-API的例子；我还鼓励您看一下基于 node-addon-api 的类似实现(它是在 N-API 之上易于使用的 C++ 包装器)
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);
}

调用插件的 JavaScript 代码

'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();

代码执行的输出:

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