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# Blockchain

## Topics Covered

* Solidity
* Block 101
* Custom ERC20
* Contract

## Working on a block

### Block and Transaction Properties

* `blockhash(uint blockNumber) returns (bytes32)`: hash of the given block - only works for 256 most recent, excluding current, blocks
* `block.chainid` (`uint`): current chain id
* `block.coinbase` (`address payable`): current block miner’s address
* `block.difficulty` (`uint`): current block difficulty
* `block.gaslimit` (`uint`): current block gaslimit
* `block.number` (`uint`): current block number
* `block.timestamp` (`uint`): current block timestamp as seconds since unix epoch
* `gasleft() returns (uint256)`: remaining gas
* `msg.data` (`bytes calldata`): complete calldata
* `msg.sender` (`address`): sender of the message (current call)
* `msg.sig` (`bytes4`): first four bytes of the calldata (i.e. function identifier)
* `msg.value` (`uint`): number of wei sent with the message
* `tx.gasprice` (`uint`): gas price of the transaction
* `tx.origin` (`address`): sender of the transaction (full call chain)