node.js /读取文件的100个第一个字节

Question

我试图读取部分文件:前100个字节然后再打开..我正在尝试读取/npm文件的前100个字节:

app.post('/random', function(req, res) {
    var start = req.body.start;
    var fileName = './npm';
    var contentLength = req.body.contentlength;
    var file = randomAccessFile(fileName + 'read');
    console.log("Start is: " + start);
    console.log("ContentLength is: " + contentLength);
    fs.open(fileName, 'r', function(status, fd) {
        if (status) {
            console.log(status.message);
            return;
        }
        var buffer = new Buffer(contentLength);
        fs.read(fd, buffer, start, contentLength, 0, function(err, num) {
            console.log(buffer.toString('utf-8', 0, num));
        });
    });

输出是:

Start is: 0
ContentLength is: 100

和下一个错误:

fs.js:457
  binding.read(fd, buffer, offset, length, position, wrapper);
          ^
Error: Length extends beyond buffer
    at Object.fs.read (fs.js:457:11)
    at C:\NodeInst\node\FileSys.js:132:12
    at Object.oncomplete (fs.js:107:15)

可能是什么原因？

Answer 1

你混淆了偏移量和位置参数.来自文档:

offset 是缓冲区中开始写入的偏移量.

position是一个整数,指定从文件中开始读取的位置.如果position为null,则将从当前文件位置读取数据.

您应该将代码更改为:

    fs.read(fd, buffer, 0, contentLength, start, function(err, num) {
        console.log(buffer.toString('utf-8', 0, num));
    });

基本上,offsetis将是fs.read将写入缓冲区的索引.假设你有一个长度为10的缓冲区:<Buffer 01 02 03 04 05 06 07 08 09 0a>你将从中读取/dev/zero基本上只有零,并将偏移量设置为3并将长度设置为4然后你会得到这个:<Buffer 01 02 03 00 00 00 00 08 09 0a>.

fs.open('/dev/zero', 'r', function(status, fd) {
    if (status) {
        console.log(status.message);
        return;
    }
    var buffer = new Buffer([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
    fs.read(fd, buffer, 3, 4, 0, function(err, num) {
        console.log(buffer);
    });
});

也可以尝试使用以下内容fs.createStream:

app.post('/random', function(req, res) {
    var start = req.body.start;
    var fileName = './npm';
    var contentLength = req.body.contentlength;
    fs.createReadStream(fileName, { start : start, end: contentLength - 1 })
        .pipe(res);
});

Answer 2

由于节点 10 是实验性的Readable[Symbol.asyncIterator]（在节点 v12 中不再是实验性的）。

'use strict';

const fs = require('fs');

async function run() {
  const file = 'hello.csv';
  const stream = fs.createReadStream(file, { encoding: 'utf8', start: 0, end: 100 });
  for await (const chunk of stream) {
    console.log(`${file} >>> ${chunk}`);
  }

  // or if you don't want the for-await-loop
  const stream = fs.createReadStream(file, { encoding: 'utf8', start: 0, end: 100 });
  const firstByte = await stream[Symbol.asyncIterator]().next();
  console.log(`${file} >>> ${firstByte.value}`);
}

run();

将打印出第一口