How to inflate a file with zlib.NET?

Question

I'm using the zlib.NET library to try and inflate files that are compressed by zlib (on a Linux box, perhaps). Here's what I'm doing:

zlib.ZInputStream zinput =
    new zlib.ZInputStream(File.Open(path, FileMode.Open, FileAccess.Read));

while (stopByte != (data = zinput.ReadByte()))
{
  // check data here
}

zinput.Close();

The data bytes match the compressed data bytes, so I must be doing something wrong.

Answer 1

Other than failing to use a "using" statement to close the stream even in the face of an exception, that looks okay to me. Is the data definitely compressed? Are you able to decompress it with zlib on the linux box?

Having looked at the source code, it's pretty ghastly - a call to int Read(buffer, offset, length) will end up calling its internal int Read() method length times for example. Given that sort of shaky start, I'm not sure I'd trust the code particularly heavily, but I'd have expected it to work at least slightly! Have you tried using SharpZipLib?

Answer 2

It appears I made the mistake of assuming all virtual methods were overridden, which wasn't the case. I was using zlib.ZInputStream.ReadByte(), which is just the inherited Stream.ReadByte(), which doesn't do any inflate.

I used zlib.ZInputStream.Read() instead, and it worked like it should.

Answer 3

Skipping the zlib header (first two bytes, 78 9C) and then using the DeflateStream built into .net worked for me.

using(var input = File.OpenRead(...))
using(var output = File.Create(...))
{
    // if there are additional headers before the zlib header, you can skip them:
    // input.Seek(xxx, SeekOrigin.Current);

    if (input.ReadByte() != 0x78 || input.ReadByte() != 0x9C)//zlib header
        throw new Exception("Incorrect zlib header");

    using (var deflateStream = new DeflateStream(decryptedData, CompressionMode.Decompress, true))
    {
        deflateStream.CopyTo(output);
    }
}