mirror of
https://github.com/ethereum/solidity
synced 2023-10-03 13:03:40 +00:00
215 lines
11 KiB
ReStructuredText
215 lines
11 KiB
ReStructuredText
**************************************
|
|
Units and Globally Available Variables
|
|
**************************************
|
|
|
|
.. index:: wei, finney, szabo, ether
|
|
|
|
Ether Units
|
|
===========
|
|
|
|
A literal number can take a suffix of ``wei``, ``finney``, ``szabo`` or ``ether`` to convert between the subdenominations of Ether, where Ether currency numbers without a postfix are assumed to be Wei, e.g. ``2 ether == 2000 finney`` evaluates to ``true``.
|
|
|
|
.. index:: time, seconds, minutes, hours, days, weeks, years
|
|
|
|
Time Units
|
|
==========
|
|
|
|
Suffixes like ``seconds``, ``minutes``, ``hours``, ``days``, ``weeks`` and
|
|
``years`` after literal numbers can be used to convert between units of time where seconds are the base
|
|
unit and units are considered naively in the following way:
|
|
|
|
* ``1 == 1 seconds``
|
|
* ``1 minutes == 60 seconds``
|
|
* ``1 hours == 60 minutes``
|
|
* ``1 days == 24 hours``
|
|
* ``1 weeks == 7 days``
|
|
* ``1 years == 365 days``
|
|
|
|
Take care if you perform calendar calculations using these units, because
|
|
not every year equals 365 days and not even every day has 24 hours
|
|
because of `leap seconds <https://en.wikipedia.org/wiki/Leap_second>`_.
|
|
Due to the fact that leap seconds cannot be predicted, an exact calendar
|
|
library has to be updated by an external oracle.
|
|
|
|
.. note::
|
|
The suffix ``years`` has been deprecated due to the reasons above and cannot be used starting version 0.5.0.
|
|
|
|
These suffixes cannot be applied to variables. If you want to
|
|
interpret some input variable in e.g. days, you can do it in the following way::
|
|
|
|
function f(uint start, uint daysAfter) public {
|
|
if (now >= start + daysAfter * 1 days) {
|
|
// ...
|
|
}
|
|
}
|
|
|
|
Special Variables and Functions
|
|
===============================
|
|
|
|
There are special variables and functions which always exist in the global
|
|
namespace and are mainly used to provide information about the blockchain
|
|
or are general-use utility functions.
|
|
|
|
.. index:: abi, block, coinbase, difficulty, encode, number, block;number, timestamp, block;timestamp, msg, data, gas, sender, value, now, gas price, origin
|
|
|
|
|
|
Block and Transaction Properties
|
|
--------------------------------
|
|
|
|
- ``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
|
|
- ``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``): complete calldata
|
|
- ``msg.gas`` (``uint``): remaining gas - deprecated in version 0.4.21 and to be replaced by ``gasleft()``
|
|
- ``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
|
|
- ``now`` (``uint``): current block timestamp (alias for ``block.timestamp``)
|
|
- ``tx.gasprice`` (``uint``): gas price of the transaction
|
|
- ``tx.origin`` (``address``): sender of the transaction (full call chain)
|
|
|
|
.. note::
|
|
The values of all members of ``msg``, including ``msg.sender`` and
|
|
``msg.value`` can change for every **external** function call.
|
|
This includes calls to library functions.
|
|
|
|
.. 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:: abi, encoding, packed
|
|
|
|
ABI Encoding and Decoding Functions
|
|
-----------------------------------
|
|
|
|
- ``abi.decode(bytes encodedData, (...)) returns (...)``: ABI-decodes the given data, while the types are given in parentheses as second argument. Example: ``(uint a, uint[2] memory b, bytes memory c) = abi.decode(data, (uint, uint[2], bytes))``
|
|
- ``abi.encode(...) returns (bytes)``: ABI-encodes the given arguments
|
|
- ``abi.encodePacked(...) returns (bytes)``: Performs :ref:`packed encoding <abi_packed_mode>` of the given arguments
|
|
- ``abi.encodeWithSelector(bytes4 selector, ...) returns (bytes)``: ABI-encodes the given arguments starting from the second and prepends the given four-byte selector
|
|
- ``abi.encodeWithSignature(string signature, ...) returns (bytes)``: Equivalent to ``abi.encodeWithSelector(bytes4(keccak256(bytes(signature)), ...)```
|
|
|
|
.. note::
|
|
These encoding functions can be used to craft data for function calls without actually
|
|
calling a function. Furthermore, ``keccak256(abi.encodePacked(a, b))`` is a way
|
|
to compute the hash of structured data (although be aware that it is possible to
|
|
craft a "hash collision" using different inputs types).
|
|
|
|
See the documentation about the :ref:`ABI <ABI>` and the
|
|
:ref:`tightly packed encoding <abi_packed_mode>` for details about the encoding.
|
|
|
|
.. index:: assert, revert, require
|
|
|
|
Error Handling
|
|
--------------
|
|
|
|
``assert(bool condition)``:
|
|
invalidates the transaction if the condition is not met - to be used for internal errors.
|
|
``require(bool condition)``:
|
|
reverts if the condition is not met - to be used for errors in inputs or external components.
|
|
``require(bool condition, string message)``:
|
|
reverts if the condition is not met - to be used for errors in inputs or external components. Also provides an error message.
|
|
``revert()``:
|
|
abort execution and revert state changes
|
|
``revert(string reason)``:
|
|
abort execution and revert state changes, providing an explanatory string
|
|
|
|
.. index:: keccak256, ripemd160, sha256, ecrecover, addmod, mulmod, cryptography,
|
|
|
|
Mathematical and Cryptographic Functions
|
|
----------------------------------------
|
|
|
|
``addmod(uint x, uint y, uint k) returns (uint)``:
|
|
compute ``(x + y) % k`` where the addition is performed with arbitrary precision and does not wrap around at ``2**256``. Assert that ``k != 0`` starting from version 0.5.0.
|
|
``mulmod(uint x, uint y, uint k) returns (uint)``:
|
|
compute ``(x * y) % k`` where the multiplication is performed with arbitrary precision and does not wrap around at ``2**256``. Assert that ``k != 0`` starting from version 0.5.0.
|
|
``keccak256(bytes memory) returns (bytes32)``:
|
|
compute the Ethereum-SHA-3 (Keccak-256) hash of the input
|
|
``sha256(bytes memory) returns (bytes32)``:
|
|
compute the SHA-256 hash of the input
|
|
``sha3(bytes memory) returns (bytes32)``:
|
|
alias to ``keccak256``
|
|
``ripemd160(bytes memory) returns (bytes20)``:
|
|
compute RIPEMD-160 hash of the input
|
|
``ecrecover(bytes32 hash, uint8 v, bytes32 r, bytes32 s) returns (address)``:
|
|
recover the address associated with the public key from elliptic curve signature or return zero on error
|
|
(`example usage <https://ethereum.stackexchange.com/q/1777/222>`_)
|
|
|
|
It might be that you run into Out-of-Gas for ``sha256``, ``ripemd160`` or ``ecrecover`` on a *private blockchain*. The reason for this is that those are implemented as so-called precompiled contracts and these contracts only really exist after they received the first message (although their contract code is hardcoded). Messages to non-existing contracts are more expensive and thus the execution runs into an Out-of-Gas error. A workaround for this problem is to first send e.g. 1 Wei to each of the contracts before you use them in your actual contracts. This is not an issue on the official or test net.
|
|
|
|
.. index:: balance, send, transfer, call, callcode, delegatecall, staticcall
|
|
.. _address_related:
|
|
|
|
Address Related
|
|
---------------
|
|
|
|
``<address>.balance`` (``uint256``):
|
|
balance of the :ref:`address` in Wei
|
|
``<address>.transfer(uint256 amount)``:
|
|
send given amount of Wei to :ref:`address`, throws on failure, forwards 2300 gas stipend, not adjustable
|
|
``<address>.send(uint256 amount) returns (bool)``:
|
|
send given amount of Wei to :ref:`address`, returns ``false`` on failure, forwards 2300 gas stipend, not adjustable
|
|
``<address>.call(bytes memory) returns (bool)``:
|
|
issue low-level ``CALL`` with the given payload, returns ``false`` on failure, forwards all available gas, adjustable
|
|
``<address>.callcode(bytes memory) returns (bool)``:
|
|
issue low-level ``CALLCODE`` with the given payload, returns ``false`` on failure, forwards all available gas, adjustable
|
|
``<address>.delegatecall(bytes memory) returns (bool)``:
|
|
issue low-level ``DELEGATECALL`` with the given payload, returns ``false`` on failure, forwards all available gas, adjustable
|
|
``<address>.staticcall(bytes memory) returns (bool)``:
|
|
issue low-level ``STATICCALL`` with the given payload, returns ``false`` on failure, forwards all available gas, adjustable
|
|
|
|
For more information, see the section on :ref:`address`.
|
|
|
|
.. warning::
|
|
There are some dangers in using ``send``: The transfer fails if the call stack depth is at 1024
|
|
(this can always be forced by the caller) and it also fails if the recipient runs out of gas. So in order
|
|
to make safe Ether transfers, always check the return value of ``send``, use ``transfer`` or even better:
|
|
Use a pattern where the recipient withdraws the money.
|
|
|
|
.. note::
|
|
Prior to version 0.5.0, Solidity allowed address members to be accessed by a contract instance, for example ``this.balance``.
|
|
This is now forbidden and an explicit conversion to address must be done: ``address(this).balance``.
|
|
|
|
.. note::
|
|
If storage variables are accessed via a low-level delegatecall, the storage layout of the two contracts
|
|
must align in order for the called contract to correctly access the storage variables of the calling contract by name.
|
|
This is of course not the case if storage pointers are passed as function arguments as in the case for
|
|
the high-level libraries.
|
|
|
|
|
|
.. note::
|
|
The use of ``callcode`` is discouraged and will be removed in the future.
|
|
|
|
.. index:: this, selfdestruct
|
|
|
|
Contract Related
|
|
----------------
|
|
|
|
``this`` (current contract's type):
|
|
the current contract, explicitly convertible to :ref:`address`
|
|
|
|
``selfdestruct(address recipient)``:
|
|
destroy the current contract, sending its funds to the given :ref:`address`
|
|
|
|
``suicide(address recipient)``:
|
|
deprecated alias to ``selfdestruct``
|
|
|
|
Furthermore, all functions of the current contract are callable directly including the current function.
|
|
|