header checksum

[compdecomp7]

I have a compressed block that was compressed as per the lz4 algorithm. But it doesn't have the Frame/Header at the beginning or the footer that contains the content checksum at the end. I have the original file that was compressed.

 

I would like to use the "./lz4 -d" command to check the correctness of the compression. My questions are

 

1. Does the lz4 command have any option so that I can use the "./lz4 -d " command to decompress a file that doesn't have the Frame/footer at the beginning or the content checksum at the end?
2. My plan is to create the Frame/header with the content checksum (C.Checksum) disabled as per the Frame descriptor but I am not very familiar with the xxhash so I don't know how to create the HC/header checksum for that Frame descriptor. Is there any command line utility that I can use to find the header checksum if I provide the frame in a file? I am looking for some thing like    @xxhash header_in_a_file >header_checksum

Thanks in advance
   

[Cyan]

> 1. Does the lz4 command have any option so that I can use the "./lz4 -d " command to decompress a file that doesn't have the Frame/footer at the beginning

Nope. Frame header is compulsory for the lz4 command.

> 2. Is there any command line utility that I can use to find the header checksum if I provide the frame in a file? 

Yes. The xxhsum utility, provided as part of xxhash repository, can give this information.

Select the 32-bits variant for lz4.

[compdecomp7]

Thanks a lot Yann, for pointing to the xxhsum.

 

 But I have some other confusion now. Following is the xxd of a compressed file that I compressed using ./lz4 command.

 

0000000: 0422 4D18 6440 A747  ."M.d@.G

0000008: 0000 8030 3031 3132  ...00112

0000010: 3233 3334 3435 3536  23344556

0000018: 3637 3738 3861 6162  67788aab

0000020: 6263 6364 6465 6566  bccddeef

0000028: 6667 6768 6869 696A  fgghhiij

0000030: 6A6B 6B6C 6C6D 6D6E  jkkllmmn

0000038: 6E6F 6F70 7071 7172  nooppqqr

0000040: 7273 7374 7475 7576  rssttuuv

0000048: 7677 7778 7879 797A  vwwxxyyz

0000050: 7A0A 0000 0000 1864  z......d

0000058: AB40                 .@

 

I believe A7 is the header checksum for this file.

 

To see if I am using the xxhsum correctly , I grabbed the first 6 bytes of the compressed .lz4 file and dumped it into a file

 . So I now have the frame descriptor only without the header checksum.  Now when I run the xxhsum on this 6 byte file this is what I see

 

%xxhsum -H0 myfile.1.lz4.frame.no_HC

7c920c9b  myfile.1.lz4.frame.no_HC

 

The second byte should be a7 but it is 0c . Am I doing something wrong?

 

Regards.

[compdecomp7]

Here's the xxd of the 6 byte header file without the HC

0000000: 0422 4D18 6440       ."M.d@

[Cyan]

The header checksum is the "one-byte checksum of combined descriptor fields"
descriptor fields do not include the initial magic number.

In your example, the checksum should be calculated on the last two bytes only.

[compdecomp7]

Thanks again, Yann. I see the expected results now for both the header and the content checksum.