[gentoo-user] Re: How to copy gzip data from bytestream?

[Grant Edwards]

On 2022-02-22, Felix Kuperjans <felix@××××××××××××××.com> wrote:

> you could use gzip to tell you the compressed size of the file and then
> use another method to copy just those bytes (dd for example):
>
> gzip -clt </dev/sdX
>
> Should print the compressed size in bytes, although by reading through
> the entire stream once.

That doesn't work. It shows the size of the drive as the
"uncompressed" size and 0 as compressed:

# gzip -clt </dev/sdd
        compressed        uncompressed  ratio uncompressed_name
        31658606592                   0   0.0% stdout

The actual size of the compressed data is about 1/3 the value shown
above.

It's not reading through the stream. It's seeking to the end and
looking at what it thinks is the trailer info. I thought that maybe
using a pipe instead of a file would make it read through the data,
but that doesn't work either:

    $ ls > foo
    $ ls -l foo
    -rw-r--r-- 1 grante users 12923 Feb 22 07:51 foo
    
    $ gzip foo
    $ ls -l foo.gz
    -rw-r--r-- 1 grante users 6083 Feb 22 07:51 foo.gz
    
    $ gzip -clt <foo.gz
             compressed        uncompressed  ratio uncompressed_name
                   6083               12923  53.1% stdout
    
    $ echo asdf >> foo.gz
    
    $ gzip -clt <foo.gz
             compressed        uncompressed  ratio uncompressed_name
                   6088           174482547 100.0% stdout
    
    $ cat foo.gz | gzip -clt
             compressed        uncompressed  ratio uncompressed_name
                     -1                  -1   0.0% stdout
    
    
    
Here's relevent portion of the strace for the 'gzip -clt <foo.gz'
where it seeks to end-8 and reads what it thinks is the uncompressed
length and the CRC:

    lseek(0, -8, SEEK_END)                  = 6080
    read(0, "2\0\0asdf\n", 8)               = 8
    write(1, "               6088           17"..., 54) = 54
    close(0)                                = 0
    close(1)                                = 0
    exit_group(0)                           = ?