Problems with copying Buffer in NodeJS

Question

I'm trying to create method to append buffers.

Here's code, which takes very strange result:

   var offset = 0
   var a = new Buffer(0)
   var b = new Buffer('test')

   offset = a.length
   a.length += b.length
   a.copy(b, offset)
   console.log(a.toString())
   // everything works normaly
   // returns: test

   b = new Buffer('hello')
   offset = a.length
   a.length += b.length
   a.copy(b, offset)
   console.log(a.toString())
   // code the same
   // but returns: test<Buff
   // nor: testhello

   // at the third time code doesn't works and raise error: targetStart out of bounds

What do I do wrong?

I'm not sure if it is advisable to alter the buffer length like this, I'm actually surprised that Node even allows it. — lanzz
– lanzz, Commented Jun 7, 2012 at 14:29

ShrekOverflow · Accepted Answer · 2012-06-07 14:53:01Z

5

:EDIT:

Got confused by the question. I thought he is going to handle buffers of same type.

Buffers cant be resized as they are defined by a fixed size in Node.js so the best option is to use a function like this

   function concat(buffers) {

    if (!Array.isArray(buffers)) {
      buffers = Array.prototype.slice.call(arguments);
    }

    var buffersToConcat = [], length = 0;
    buffers.forEach(function (buffer) {
        if (!Buffer.isBuffer(buffer)) {
          buffer = new Buffer(buffer);
        }
        length += buffer.length;
        buffersToConcat.push(buffer);
    });

    var concatinatedBuffer = new Buffer(length), index = 0;
    buffersToConcat.forEach(function (buffer) {
      buffer.copy(concatinatedBuffer, index, 0, buffer.length);
      index += buffer.length;
    });

    return concatinatedBuffer;
  }

What were you doing wrong? You were trying to manipulate the length of a fixed size allocation in memory by the += method. Hence it threw exception beacause += operator obviously didnt re-allocated memory

*What are we doing here *

Pretty simple we are just making a new buffer out of the argument buffers.

edited Jun 7, 2012 at 14:53

answered Jun 7, 2012 at 14:34

ShrekOverflow

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3 Comments

idzikovsky Over a year ago

As a result variable „a“ becomes string, nor Buffer. That's nice when you work with strings, but wrong when you work with binary Buffers.

ShrekOverflow Over a year ago

from your example i got confused >_>

Florian Margaine Over a year ago

Why the if (buffer) in the first forEach? I mean, you're in forEach, of course it's going to exist. Something I'm not seeing? :p

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