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Diffstat (limited to 'docs/miscellaneous.rst')
| -rw-r--r-- | docs/miscellaneous.rst | 20 |
1 files changed, 19 insertions, 1 deletions
diff --git a/docs/miscellaneous.rst b/docs/miscellaneous.rst index a7d5c445..01154854 100644 --- a/docs/miscellaneous.rst +++ b/docs/miscellaneous.rst @@ -314,7 +314,7 @@ The following is the order of precedence for operators, listed in order of evalu Global Variables ================ -- ``block.blockhash(uint blockNumber) returns (bytes32)``: hash of the given block - only works for 256 most recent blocks +- ``block.blockhash(uint blockNumber) returns (bytes32)``: hash of the given block - only works for 256 most recent, excluding current, blocks - deprecated in version 0.4.22 and replaced by ``blockhash(uint blockNumber)``. - ``block.coinbase`` (``address``): current block miner's address - ``block.difficulty`` (``uint``): current block difficulty - ``block.gaslimit`` (``uint``): current block gaslimit @@ -331,6 +331,7 @@ Global Variables - ``assert(bool condition)``: abort execution and revert state changes if condition is ``false`` (use for internal error) - ``require(bool condition)``: abort execution and revert state changes if condition is ``false`` (use for malformed input or error in external component) - ``revert()``: abort execution and revert state changes +- ``blockhash(uint blockNumber) returns (bytes32)``: hash of the given block - only works for 256 most recent blocks - ``keccak256(...) returns (bytes32)``: compute the Ethereum-SHA-3 (Keccak-256) hash of the :ref:`(tightly packed) arguments <abi_packed_mode>` - ``sha3(...) returns (bytes32)``: an alias to ``keccak256`` - ``sha256(...) returns (bytes32)``: compute the SHA-256 hash of the :ref:`(tightly packed) arguments <abi_packed_mode>` @@ -346,6 +347,23 @@ Global Variables - ``<address>.send(uint256 amount) returns (bool)``: send given amount of Wei to :ref:`address`, returns ``false`` on failure - ``<address>.transfer(uint256 amount)``: send given amount of Wei to :ref:`address`, throws on failure +.. note:: + Do not rely on ``block.timestamp``, ``now`` and ``blockhash`` as a source of randomness, + unless you know what you are doing. + + Both the timestamp and the block hash can be influenced by miners to some degree. + Bad actors in the mining community can for example run a casino payout function on a chosen hash + and just retry a different hash if they did not receive any money. + + The current block timestamp must be strictly larger than the timestamp of the last block, + but the only guarantee is that it will be somewhere between the timestamps of two + consecutive blocks in the canonical chain. + +.. note:: + The block hashes are not available for all blocks for scalability reasons. + You can only access the hashes of the most recent 256 blocks, all other + values will be zero. + .. index:: visibility, public, private, external, internal Function Visibility Specifiers |