Bitcoin Forum

I am using ubuntu and trying to get into blockchain development. I found a post on stackoverflow where a person performed the hashing of a block header and compared it with traget value. I know how to find target value from nBits but I cannot figure out how to find block header of any block so that I can also compare both to figure out whether the PoW is correct or not.

I am using Ubuntu.

Assuming that you are running Bitcoin Core, you can use the getblockheader command to get the block header for a given block hash. To get the raw block header so you can hash it, you would do

I made some changes in chainparams.h of bitcoin core and I am trying to create genesis block. I have genesis block hash and merkel root but debug.log says hash too high

What I am actually trying to do is compare the target value with hash of genesis block header.

On running 'getblockheader' it says command not found.

You have to actually mine the genesis block. You can do that with a while loop in chainparams.cpp that calls CheckProofofWork (with some modification). Or just use a script like https://github.com/lhartikk/GenesisH0 to do it for you.

I already generated genesis hash and merkle root using this repository but after compiling and running the core it gave "assertion failed".

Then I used          printf("genesis.GetHash = %s\n", genesis.GetHash().ToString().c_str());

and hash generated from it was correct but it genrated a debug.log file which had error :-


    2018-03-13 16:48:44 ERROR: ReadBlockFromDisk: Errors in block header at CBlockDiskPos(nFile=0, nPos=8)
    2018-03-13 16:48:44 *** Failed to read block
    2018-03-13 16:48:44 Error: Error: A fatal internal error occurred, see debug.log for details

Tracing back to ReadBlockFromDisk I reached to file validation.h and found this function :-


bool ReadBlockFromDisk(CBlock& block, const CDiskBlockPos& pos, const Consensus::Params& consensusParams)
{
    block.SetNull();

    // Open history file to read
    CAutoFile filein(OpenBlockFile(pos, true), SER_DISK, CLIENT_VERSION);
    if (filein.IsNull())
        return error("ReadBlockFromDisk: OpenBlockFile failed for %s", pos.ToString());

    // Read block
    try {
        filein >> block;
    }
    catch (const std::exception& e) {
        return error("%s: Deserialize or I/O error - %s at %s", __func__, e.what(), pos.ToString());
    }

    // Check the header
    if (!CheckProofOfWork(block.GetPoWHash(), block.nBits, consensusParams))
        return error("ReadBlockFromDisk: Errors in block header at %s", pos.ToString());

    return true;
}

while tracing back to CBlockDiskPos I reached this funtion in chain.h file :-


struct CDiskBlockPos
{
    int nFile;
    unsigned int nPos;

    ADD_SERIALIZE_METHODS;

    template <typename Stream, typename Operation>
    inline void SerializationOp(Stream& s, Operation ser_action) {
        READWRITE(VARINT(nFile));
        READWRITE(VARINT(nPos));
    }

    CDiskBlockPos() {
        SetNull();
    }

    CDiskBlockPos(int nFileIn, unsigned int nPosIn) {
        nFile = nFileIn;
        nPos = nPosIn;
    }

    friend bool operator==(const CDiskBlockPos &a, const CDiskBlockPos &b) {
        return ([Suspicious link removed]ile == b.nFile && a.nPos == b.nPos);
    }

    friend bool operator!=(const CDiskBlockPos &a, const CDiskBlockPos &b) {
        return !(a == b);
    }

    void SetNull() { nFile = -1; nPos = 0; }
    bool IsNull() const { return (nFile == -1); }

    std::string ToString() const
    {
        return strprintf("CBlockDiskPos(nFile=%i, nPos=%i)", nFile, nPos);
    }

};

Now I am trying to figure out what to do next?

Did you delete your blocks directory after modify code ?
If you are trying to read old blocks using old rules with new rules, you'll hit a lot of those errors. The best is to clean up everything at each try.