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Units and Globally Available Variables

Units and Globally Available Variables
Ether Units
A literal number can take a suffix of wei, finney, szabo or ether to specify a subdenomination of Ether, where Ether numbers without a postfix are assumed to be Wei.
assert(1 wei == 1);
assert(1 szabo == 1e12);
assert(1 finney == 1e15);
assert(1 ether == 1e18);
The only effect of the subdenomination suffix is a multiplication by a power of ten.
Time Units
Suffixes like seconds, minutes, hours, days and weeks after literal numbers can be used to specify 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
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.
The suffix years has been removed in version 0.5.0 due to the reasons above.
These suffixes cannot be applied to variables. For example, if you want to interpret a function parameter in days, you can 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
blockhash(uint blockNumber) returns (bytes32): hash of the given block - only works for 256 most recent, excluding current, blocks
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 payable): 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 payable): sender of the transaction (full call chain)
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.
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.
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.
The function blockhash was previously known as block.blockhash. It was deprecated in version 0.4.22 and removed in version 0.5.0.
The function gasleft was previously known as msg.gas. It was deprecated in version 0.4.21 and removed in version 0.5.0.
ABI Encoding and Decoding Functions
abi.decode(bytes memory 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 memory): ABI-encodes the given arguments
abi.encodePacked(...) returns (bytes memory): Performs packed encoding of the given arguments
abi.encodeWithSelector(bytes4 selector, ...) returns (bytes memory): ABI-encodes the given arguments starting from the second and prepends the given four-byte selector
abi.encodeWithSignature(string memory signature, ...) returns (bytes memory): Equivalent to abi.encodeWithSelector(bytes4(keccak256(bytes(signature))), ...)`
These encoding functions can be used to craft data for external function calls without actually calling an external 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 function parameter types).
See the documentation about the ABI and the tightly packed encoding for details about the encoding.
Error Handling
See the dedicated section on assert and require for more details on error handling and when to use which function.assert(bool condition):causes an invalid opcode and thus state change reversion 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 memory 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 changesrevert(string memory 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 Keccak-256 hash of the inputsha256(bytes memory) returns (bytes32):compute the SHA-256 hash of the inputripemd160(bytes memory) returns (bytes20):compute RIPEMD-160 hash of the inputecrecover(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)
Function ecrecover returns an address, and not an address payable. See address payable for conversion, in case you need to transfer funds to the recovered address.
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.
There used to be an alias for keccak256 called sha3, which was removed in version 0.5.0.
Members of Address Types
.balance (uint256):balance of the Address in Wei payable>.transfer(uint256 amount):send given amount of Wei to Address, reverts on failure, forwards 2300 gas stipend, not adjustable payable>.send(uint256 amount) returns (bool):send given amount of Wei to Address, returns false on failure, forwards 2300 gas stipend, not adjustable.call(bytes memory) returns (bool, bytes memory):issue low-level CALL with the given payload, returns success condition and return data, forwards all available gas, adjustable.delegatecall(bytes memory) returns (bool, bytes memory):issue low-level DELEGATECALL with the given payload, returns success condition and return data, forwards all available gas, adjustable.staticcall(bytes memory) returns (bool, bytes memory):issue low-level STATICCALL with the given payload, returns success condition and return data, forwards all available gas, adjustable
For more information, see the section on Address.
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.
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.
If state 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.
Prior to version 0.5.0, .call, .delegatecall and .staticcall only returned the success condition and not the return data.
Prior to version 0.5.0, there was a member called callcode with similar but slightly different semantics than delegatecall.
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