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As following answer for How can I find out what the highest block is? guided: we can get the latest mined block's number and its hash inside geth as follows.

> eth.syncing.currentBlock
569367
> eth.getBlock(eth.syncing.currentBlock).hash
"0xac51e27c3fe38af1a0aeb2867a741ba6aa16780e435a586d934ccea3a71874b1"

[Q] Inside a smart contract, is it possible to get the latest mined block's number and its hash? or should I keep an array inside my contract and when new block is mined, externally (my_contract.addNewHash(<some_hash>)) should I put its hash into my array for my contract to read from?

I have tried following keywords from Solidity's website: https://github.com/iurimatias/embark-framework/wiki/Solidity on Solidity Browser and Populus, for example: block.number, block.blockhash, but I do face with following error:

Untitled:6:17: Error: Expected identifier, got 'Semicolon'
    block.number;

Thank you for your valuable time and help.

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1 Answer 1

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16

Transactions are always executed in the block being mined. So, at any time, block.number will return the block height in which the transaction is being mined.

However, the hash of the block is not known until it is mined, and the output of transactions is part of the hash calculation, so the transaction has no access to the hash of the block being mined.

If this is any consolation, you can get the hash of the latest mined block, i.e. the one right before this block being mined with block.blockhash(block.number - 1).

Try this:

contract Test {
    uint public blockNumber;
    bytes32 public blockHashNow;
    bytes32 public blockHashPrevious;

    function setValues() {
        blockNumber = block.number;
        blockHashNow = block.blockhash(blockNumber);
        blockHashPrevious = block.blockhash(blockNumber - 1);
    }    
}

Make a transaction to setValues() then you will see that:

  • blockNumber is set at that number it was mined in
  • blockHashNow is set at 0x
  • blockHashPrevious is set at the previous block hash indeed
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  • I don't know why but when I tried in Solidity Browser, function does not see the public variables. Error: Undeclared identifier. blockHashPrevious. But I understand the idea.
    – alper
    Commented Jan 17, 2017 at 11:55
  • Sorry, typo fixed.
    – Xavier Leprêtre B9lab
    Commented Jan 17, 2017 at 11:59
  • why is the current block hash zero?
    – Jonathan Lindgren
    Commented Oct 16, 2017 at 5:31
  • Not sure where you got that info.
    – Xavier Leprêtre B9lab
    Commented Oct 16, 2017 at 20:54
  • The current block hash is zero because the block is yet to be mined, so a hash for all the transactions in the block cannot be calculated yet. This is why accessing the previous blocks hash works. The block.number is the current block that is executing the function (setValues() in this case).
    – sabhiram
    Commented Nov 1, 2021 at 21:59

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