# 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.

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.

### Block and Transaction Properties¶

• `block.blockhash(uint blockNumber) returns (bytes32)`: hash of the given block - only works for 256 most recent blocks excluding current
• `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
• `msg.data` (`bytes`): complete calldata
• `msg.gas` (`uint`): remaining gas
• `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 `block.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.

### Error Handling¶

`assert(bool condition)`:
throws if the condition is not met - to be used for internal errors.
`require(bool condition)`:
throws if the condition is not met - to be used for errors in inputs or external components.
`revert()`:
abort execution and revert state changes

### 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`.
`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`.
`keccak256(...) returns (bytes32)`:
compute the Ethereum-SHA-3 (Keccak-256) hash of the (tightly packed) arguments
`sha256(...) returns (bytes32)`:
compute the SHA-256 hash of the (tightly packed) arguments
`sha3(...) returns (bytes32)`:
alias to `keccak256`
`ripemd160(...) returns (bytes20)`:
compute RIPEMD-160 hash of the (tightly packed) arguments
`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)

In the above, “tightly packed” means that the arguments are concatenated without padding. This means that the following are all identical:

```keccak256("ab", "c")
keccak256("abc")
keccak256(0x616263)
keccak256(6382179)
keccak256(97, 98, 99)
```

If padding is needed, explicit type conversions can be used: `keccak256("\x00\x12")` is the same as `keccak256(uint16(0x12))`.

Note that constants will be packed using the minimum number of bytes required to store them. This means that, for example, `keccak256(0) == keccak256(uint8(0))` and `keccak256(0x12345678) == keccak256(uint32(0x12345678))`.

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.