Gzipstream header and suffix

Question

How do I know the size of my compressed file used GzipStream? I know that it has a header and suffix. First 10 bytes - it's header, second 8 bytes - suffix. How do I know the size file in the suffix?

It goes without saying, but I'll say it anyway because none of the answers do: the last 4 bytes is the original length MOD 2^32. — Ronnie Overby, Apr 09 '19 at 19:57

WizzKidd · Answer 1 · 2019-01-05T23:05:47.890

1

I see that you down voted my previous answer most likely because it was an example using Java. The principle is still the same, so the answer to your question would be that the last 4 bytes contain the information you require. Hopefully this answer is more what you are after.

Here is a C# Decompress function example of decompressing the GZip inclusive of getting the size of the compressed file used by GZipStream:

static public byte[] Decompress(byte[] b)
{
    MemoryStream ms = new MemoryStream(b.length);
    ms.Write(b, 0, b.Length);
    //last 4 bytes of GZipStream = length of decompressed data
    ms.Seek(-4, SeekOrigin.Current);
    byte[] lb = new byte[4];
    ms.Read(lb, 0, 4);
    int len = BitConverter.ToInt32(lb, 0);
    ms.Seek(0, SeekOrigin.Begin);
    byte[] ob = new byte[len];
    GZipStream zs = new GZipStream(ms, CompressionMode.Decompress);
    zs.Read(ob, 0, len);
    returen ob;
}

edited Jan 05 '19 at 23:05

answered Nov 20 '16 at 23:08

WizzKidd

196
8

It's Java, not C#. In some cases C# and Java can be very similar and acceptable to answer to java with c# or c# with java, but in this case the code is completely different. – Gusman Nov 22 '16 at 03:21
1

If you edit your answer and place the other response in this I will remove the negative vote, I can't remove unless you modify the answer, S.O. doesn't allows it. – Gusman Nov 22 '16 at 03:30
Hi @gusman I have edited my post as you requested in order to remove the down bite (albeit this was 3 years ago, lol) – WizzKidd Jan 05 '19 at 23:07

score 1 · Answer 2 · answered Nov 22 '16 at 03:27

1

Something a bit better written:

public int GetUncompressedSize(string FileName)
{
   using(BinaryReader br = new BinaryReader(File.OpenRead(pathToFile))
   {
        br.BaseStream.Seek(SeekOrigin.End, -4);
        return br.ReadInt32();
   }
}

answered Nov 22 '16 at 03:27

Gusman

14,905
2
34
50

score 0 · Answer 3 · answered Nov 21 '16 at 10:59

I see that you down voted my previous answer most likely because it was an example using Java. The principle is still the same, so the answer to your question would be that the last 4 bytes contain the information you require. Hopefully this answer is more what you are after.

Here is a C# Decompress function example of decompressing the GZip inclusive of getting the size of the compressed file used by GZipStream:

static public byte[] Decompress(byte[] b)
{
    MemoryStream ms = new MemoryStream(b.length);
    ms.Write(b, 0, b.Length);
    //last 4 bytes of GZipStream = length of decompressed data
    ms.Seek(-4, SeekOrigin.Current);
    byte[] lb = new byte[4];
    ms.Read(lb, 0, 4);
    int len = BitConverter.ToInt32(lb, 0);
    ms.Seek(0, SeekOrigin.Begin);
    byte[] ob = new byte[len];
    GZipStream zs = new GZipStream(ms, CompressionMode.Decompress);
    zs.Read(ob, 0, len);
    returen ob;
}

Gzipstream header and suffix

3 Answers3