Units and Globally Available Variables

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.

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. Due to the fact that leap seconds cannot be predicted, an exact calendar library has to be updated by an external oracle.

Catatan

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.

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)

Catatan

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.

Catatan

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.

Catatan

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.

ABI Encoding Functions

• abi.encode(...) returns (bytes): ABI-encodes the given arguments
• abi.encodePacked(...) returns (bytes): Performs packed encoding 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)), ...)`

Catatan

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 ABI and the tightly packed encoding for details about the encoding.

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

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)

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.

Address Related

<address>.balance (uint256):

balance of the Address in Wei

<address>.transfer(uint256 amount):

send given amount of Wei to Address, throws on failure, forwards 2300 gas stipend, not adjustable

<address>.send(uint256 amount) returns (bool):

send given amount of Wei to 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

For more information, see the section on Address.

Peringatan

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.

Catatan

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.

Catatan

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.

Catatan

The use of callcode is discouraged and will be removed in the future.

Contract Related

this (current contract's type):

the current contract, explicitly convertible to Address

selfdestruct(address recipient):

destroy the current contract, sending its funds to the given Address

suicide(address recipient):

deprecated alias to selfdestruct

Furthermore, all functions of the current contract are callable directly including the current function.