Why do gzip of Java and Go get different results?

Question

Firstly, my Java version:

string str = \"helloworld\";
ByteArrayOutputStream localByteArrayOutputStream = new ByteArrayOutputStream(str.length());
GZIPOutput         


        

Answer 1

From RFC 1952, the GZip file header is structured as:

+---+---+---+---+---+---+---+---+---+---+
|ID1|ID2|CM |FLG|     MTIME     |XFL|OS | (more-->)
+---+---+---+---+---+---+---+---+---+---+

Looking at the output you've provided, we have:

                          |    Java |          Go
ID1                       |      31 |          31
ID2                       |     139 |         139
CM (compression method)   |       8 |           8
FLG (flags)               |       0 |           0
MTIME (modification time) | 0 0 0 0 | 0 9 110 136
XFL (extra flags)         |       0 |           0
OS (operating system)     |       0 |         255

So we can see that Go is setting the modification time field of the header, and setting the operating system to 255 (unknown) rather than 0 (FAT file system). In other respects they indicate that the file is compressed in the same way.

In general these sorts of differences are harmless. If you want to determine if two compressed files are the same, then you should really compare the decompressed versions of the files though.