node.js/ read 100 first bytes of a file

javascript, node.js

Solution

You're confusing the offset and position argument. From the docs:

`offset` is the offset in the buffer to start writing at.

`position` is an integer specifying where to begin reading from in the file. If position is null, data will be read from the current file position.

You should change your code to this:

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

Basically the `offset` is will be index that fs.read will write to the buffer. Let's say you have a buffer with length of 10 like this: `<Buffer 01 02 03 04 05 06 07 08 09 0a>` and you will read from `/dev/zero` which is basically only zeros, and set the offset to 3 and set the length to 4 then you will get this: `<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);
    });
});

Also to make things you might wanna try using `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);
});

Problem

Original source